AUXETIC WEDGE OR FIELD STRUCTURE AND USE

Description

[0001] The invention relates to an auxetic bridge or field structure comprising island structure surfaces and connections between the individual island structure surfaces, wherein open spaces are present between the individual island structure surfaces and the connections with the island structure surfaces form a bridge or field structure and the connections are designed to be stretchable.

[0002] Such structures can also be described as auxetic metamechanical materials with improved extensibility and compressibility.

[0003] Three recent review articles, namely: Prawoto, Yunan. "Seeing auxetic materials from the mechanics point of view: a structural review on the negative Poisson's ratio." Computational Materials Science 58 (2012): 140-153; Ren, Xin, et al. "Auxetic metamaterials and structures: a review." Smart materials and structures 27.2 (2018): 023001. Saxena, Krishna Kumar, Raj Das, and Emilio P. Calius. "Three decades of auxetics research materials with negative Poisson's ratio: a review." Advanced Engineering Materials 18.11 (2016): 1847-1870 and Wang, Zhenwei, et al. "Progress in Auxetic Mechanical Metamaterials: Structures, Characteristics, Manufacturing Methods, and Applications." Advanced Engineering Materials 22.10 (2020): 2000312 summarizes the extensive progress made in auxetic structures over the last three decades from more than 300 research papers relating to modeling, fabrication and characterization of auxetic structures.

[0004] The use of flexible joints to improve the mechanical elongation properties of auxetic structures is not mentioned in these review articles.

[0005] The publication US 2018 / 0311833A1 describes auxetic structures in "Non-planar shearing auxetic structures, devices, and methods" as "A non-planar shearing auxetic structure, comprising each unit cell is defined by a plurality of elements and by an internal angle between two of the plurality of elements that are connected together by a pivot joint", where the pivot joint disclosed and described therein is in principle the location of the modified stretchable connection, which is also disclosed here.

[0006] Hassanin, Hany et al., in "4D printing of NiTi auxetic structure with improved ballistic performance" Micromachines 11.8 (2020): 745, reveal the development of a NiTi structure with a negative Poisson's ratio and superelasticity / shape memory for improved ballistic applications. A 4D printing process is used to fabricate the optimized auxetic NiTi structure.

[0007] The only auxetic design that appears similar to the interconnection geometries currently used in stretchable electronics is the rotachiral design. It resembles the "2D wavy network constructed with horseshoe building blocks, configured into a triangle lattice geometry," proposed by KI Jang et al. in "Soft network composite materials with deterministic and bio-inspired designs," Nature Communications 6, 6566 (2015). The authors note that at low strains, a negative effect of the Poisson's ratio of the structure is observed; however, the auxetic behavior of the stretchable network with a triangular lattice geometry is not investigated. No negative Poisson's ratio is observed for the square lattice geometry in this publication. Furthermore, the proposed wavy mesh designs do not provide a large island space available for potential functionalization of electronic components.

[0008] Electronic 2D thin-film devices intended to serve as interfaces to complex 3D biological surfaces must be able to withstand large macroscopic stresses without compromising the device's functional components. In the field of flexible and stretchable electronics, numerous solutions to this problem have been found by reconfiguring the geometry of the entire electronic circuit. One solution involves transforming the electronic circuit into a wave-like 2D thin-film metal mesh. Another popular technique is the "island bridge," in which a series of periodic islands are structured and connected in the center by stretchable junctions. The stresses on the islands remain low even under deformation because the stresses and strains are concentrated at the stretchable junctions.Examples of such connections known from the prior art include serpentine structures, fractal patterns, 2D wave lattices, and, more recently, Archimedean spiral structures connected at the center of the islands. These "island-bridge" structures are designed to stretch under biaxial loading. These solutions enable many impressive devices. However, there are some limitations to traditional stretchable designs, including a positive (or zero) Poisson's ratio. Furthermore, it is usually necessary to integrate conventional stretchable electronics onto an elastomeric substrate to promote elastic recovery.There is a need to configure 2D electronic components into freestanding structural geometries that allow the component to elastically achieve large macroscopic strains, conformally integrate into 3D surfaces, and achieve a high areal density for functional electronic components.

[0009] In US patent 8,552,299 B2, electronic circuits based on the island-bridge layout are shown. This layout consists of an array of periodic islands connected by serpentines. The structures shown have no auxetic properties and also exhibit a Poisson's ratio of zero or positive. In some cases, the fabricated devices mimic similar mechanical behavior to auxetic structures (i.e., the ability to bend over a dome-shaped curvature, expansion under biaxial (x- and y-directions) loading). However, in most cases, the serpentine connections bend out of plane at equilibrium, meaning that the connections are not in conformal contact with the underlying substrate during bending deformation. High-performance bioelectrodes (i.e., skin sensors) require complete conformal contact (i.e.,a perfect interface) with the skin to maximize signal detection.

[0010] Document US 10,192,830 B2 also proposes useful 2D designs of self-similar and fractal designs for developing stretchable connections based on serpentine structures. It has been shown that these fractal connections improve the maximum strain that circuits can achieve when arranged in an island-bridge layout. The use of fractal designs in the plane likely improves the conformity problem caused by kinked serpentine structures outside the plane. The use of fractal designs for auxetic structures is not mentioned. Additionally, 2D crosslinking materials with a zero or positive Poisson number, based on the serpentine shape and arranged in a fractal pattern, are demonstrated.It is not mentioned that these mesh materials exhibit auxetic behavior, and there are no periodic large-area islands as part of the structure for component integration.

[0011] The mechanical properties of auxetic materials make them ideal platforms for stretchable electronics that can be integrated into the body. The problem with conventional auxetic structures is that the maximum achievable strain is relatively small and limited by the stiffness of the connection. In Grima, Joseph N., et al., "Auxetic behavior from stretching connected squares," Journal of Materials Science 43.17 (2008): 5962-5971, the strain behavior in rotating square models is investigated. The squares are connected at the corners by hinged joints, which, if assumed to be perfectly rigid, can simply rotate relative to each other to exhibit auxetic behavior under applied deformation. These squares can also generate auxetic behavior by lengthening (or shortening) the sides of the squares through the stretching mechanism. In Grima, Joseph N.Elaine Manicaro and Daphne Attard, "Auxetic Behavior of Connected Squares and Rectangles of Different Sizes", Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 467.2126 (2011): 439-458, demonstrate that rotating squares and rectangles of different sizes can be connected and still retain auxetic behavior.

[0012] Some examples of traditional auxetic structures that could be modified to have improved extensibility / compressibility are honeycombs, chiral (circle, square, hexagon, etc.), triangular and star structures, rhombic grids (square and oblong), origami-based metamaterials, kirigami-based metamaterials, key-bricked structures, slot-perforated structures, rotating rigid structures, rotating semi-rigid structures, and hierarchical auxetic systems.

[0013] Document US 8,883,287 B2 discloses structures similar to auxetic "rotating square" structures. In this document, the rigid connection points between the squares are replaced by connection points with a strain relief function, which have the same shape as a serpentine connection, similar to the "island bridge" structures known from stretchable electronics. However, the structures presented here are not described as auxetic or assigned a negative Poisson's ratio. Furthermore, the structures must be stretched biaxially in both the x and y directions, and not just uniaxially, to achieve expansion. In other words, this means that the structures presented in the document are not auxetic. The "island connection" structures known from classical stretchable electronics can also only expand under biaxial strain.

[0014] Auxetic framework structures are known from the prior art as flexible substrates for functional components, particularly for electronic components such as solar cells, antennas, batteries, and sensors. Reference is made, for example, to US 2010 / 330338 A1, US 2018 / 061743 A1, and US 2020 / 144431 A1. The aforementioned documents disclose auxetic substrates made of conventional substrate materials such as the semiconductor silicon or a polymer plastic, which themselves do not exhibit significant electrical conductivity. US 2020 / 144431 A1 discloses an auxetic bridge or field structure comprising: Island structure surfaces and connections between the individual island structure surfaces, wherein open spaces exist between the individual island structure surfaces and the connections with the island structure surfaces form a web or field structure; wherein the connections intersect the island structure surfaces at the same location and angle as the previous rigid connectors intersect the auxetic framework; the island structure surfaces are not or only slightly deformable under force, wherein the stresses on the island structure surfaces are at least one order of magnitude lower than those of the ductile connection and / or are not or only insignificantly variable in magnitude, and the Poisson's ratio of the structure is negative under uniaxial deformation.

[0015] Furthermore, it is well known in the prior art to manufacture implants, especially stents, from SMA based on NiTi alloys.

[0016] It is also widely known in the prior art to functionalize the surfaces of such medical implants with coatings.

[0017] Conventional auxetic structures can be modified in various ways to meet the requirements of a medical implant. Large-area islands can be periodically integrated into the framework of conventional auxetic geometries known in the art. Furthermore, the serrated hinges of large-area auxetic geometries (e.g., rotating polygons) can be modified by more compliant structures (e.g., serpentines) to allow for greater deformations.

[0018] In their article "Design of self-expanding auxetic stents using topology optimization" (Frontiers in Bioengineering and Biotechnology 8 (2020): 736), Xue et al. discuss the relationship between stent structures and clinical outcomes in the treatment of coronary artery disease. First, they develop a method for simultaneous topological optimization to systematically find the best material distribution within the design space. For topology optimization, they propose an extended parametric level-set method with shell elements to ensure the accuracy and efficiency of the calculations. Subsequently, they introduce a negative Poisson's ratio auxetic metamaterial in self-expanding stents. The auxetics can improve the mechanical properties of the structure, e.g.,The fracture toughness, identification and shear strength, and vibration energy absorption are all evaluated, which helps to mitigate the disadvantages resulting from mechanical failures. Finally, the optimized self-expanding stent is numerically validated using the commercial software ANSYS and then manufactured as a prototype using additive manufacturing techniques.

[0019] Document US 2017 / 0007400 A1 discloses a composite biomaterial comprising a continuous metal sheet with curved elements defining a first window pattern and a polymer layer over at least one surface of the continuous metal sheet. The curved elements are elastically stretchable, allowing the continuous metal sheet to bend in more than one axis without buckling or folding.

[0020] Modern stretchable microelectronic mechanical systems (MEMS) are typically based on wave-like meshed thin-film metals and island-bridge layouts with thin-film serpentine interconnects. Thanks to the serpentine and wave structures, extremely large uniaxial global strains can be achieved. The major drawback of modern stretchable electronics is that the stretchable structures have a positive or zero Poisson's ratio, meaning that under uniaxial tensile stress, the structure either contracts or remains unchanged in the perpendicular direction.

[0021] Next-generation electronic devices intended for integration into biological tissue must be minimally invasive and ultra-conformal, enabling the co-integration of complex microelectronic circuits. Electronics designed for integration onto various body surfaces, such as veins, blood vessels, skin, heart valves, etc., must withstand the extreme stretching, bending, and torsional deformations associated with the body's natural curvature. For example, electronic tattoos worn directly on the skin—that is, skin sensors—and medical implants placed inside the body should ideally conform to their environment to optimize device performance. It is also desirable for the structure to have large low-stress areas to accommodate and electrically connect various MEMS components. An additional requirement for most medical implants, e.g.,The requirement for stents is that they must also be able to compress and expand by at least ~4 times their volume in order to be squeezed down to a sufficiently small diameter for a microcatheter to be used for non-invasive endovascular treatments.

[0022] The major drawback of state-of-the-art auxetic structures is that the extensibility and compressibility of the entire structure is limited by the stiffness of the connecting hinges (or pivot joints). The stiffness of traditional auxetic structures can restrict their practical use as implantable medical devices or other extensible electronic applications, as the volume expansion / compression requirements cannot be met with traditional designs.

[0023] In the prior art, electronic components have already been connected to SMA films, and stents with auxetic framework structures have been proposed, which can obviously also be formed from an SMA material.

[0024] The problem with the devices known in the prior art is that the required implantable structure with electronic components in the form described below is not known, but such a structure is needed.

[0025] The biggest challenge in functionalizing conventional stent designs (i.e., lattice structures, traditional auxetic structures, synthetic composite structures) is the limited surface area available for integrating electronic devices. The next leap in quality of medical care requires long-term, non-invasive, unobtrusive, continuous monitoring of vital biomedical signs. Electronic sensors, which can be integrated onto various body surfaces, such as blood veins / vessels / arteries, heart valves, skin, bones, etc., must conform to these complex surfaces and withstand the extreme strain, bending, and torsional deformations associated with the body's natural curvature. The main problem with conventional electronic components is that they are rigid and typically cannot adapt to curved surfaces.

[0026] The next generation of medical implants will be functionalized to wirelessly transmit patient data to physicians in real time, thus enabling improved treatment quality. Conventional stents, based on woven, braided, and laser-cut shape-memory alloys, are somewhat rigid and cannot fully conform to the shape of the vein. Furthermore, conventional stents do not offer the high proportion of stress-free areas that active or passive functional components could accommodate. This feature is essential for future miniaturized, functionalized medical implants to allow the integration of wireless transmission antennas, energy harvesting devices, energy storage, and other types of sensor or actuator components.

[0027] The publication US 10,660,200 B2 discloses that Archimedean spiral designs are significantly more stretchable than serpentine and fractal designs currently used for stretchable electronics. Archimedean spirals exhibit considerably lower stresses than serpentine connections. They can thus achieve large uniaxial displacements through buckling and out-of-plane twisting. The structures disclosed in the publication connect the spirals via a known island-bridge layout, in which the Archimedean spiral connects the islands directly at the center of the surface. This connection point at the center of the islands' surfaces is the reason why the overall structure exhibits a positive or zero Poisson's ratio.

[0028] The problem of the reduced extensibility of auxetic structures described above can be solved by modifying the rigid hinge or pivot joint of an auxetic structure with extensible connections. A first example of this is US Patent 2020 / 0144431 A1, which demonstrates that the mechanical properties of auxetic structures can be improved by using serpentine connections. The patent discloses a deformable array of semiconductor devices and a method for fabricating such a deformable array. The deformable array comprises a plurality of islands, each containing at least one semiconductor device, and the plurality of islands are arranged in an auxetic geometry. The connections between the islands are, as mentioned, configured as serpentines, and each serpentine also contains a semiconductor device.The structure revealed here is capable of expanding under uniaxial stress.

[0029] The object of the present invention is to combine the advantageous mechanical properties of auxetic structures and stretchable intermediate connections in such a way as to create an auxetic structure that is particularly stretchable and at the same time provides sufficiently large areas that are subject to only minor stresses.

[0030] The present invention solves the aforementioned technical problems by introducing a mechanically stretchable auxetic structure. This structure utilizes the superior mechanical properties offered by auxetic metamechanical materials (hereinafter referred to as auxetic) and improves their stretchability and compressibility by integrating intermediate connections into the auxetic framework.

[0031] Auxetic materials are well-known periodic structures characterized by a negative Poisson's ratio, meaning they expand laterally when stretched longitudinally. Under uniaxial tensile stress, these structures expand or contract in the direction perpendicular to the applied force. These structures exhibit impressive material properties, including synclastic bending, a high stiffness-to-weight ratio, high shear strength, high hardness, high indentation / impact strength, reduced fatigue crack propagation, greater toughness and modulus stability, and vibration damping.In particular, synclastic bending offers unique possibilities for applications in wearable electronics and medical technology, as it enables conformal bending around concave, convex, dome-shaped curves and other complex, nonlinear 3D geometries, allowing two-dimensional auxetic structures to adapt to any three-dimensional surface.

[0032] The auxetic structure presented here is specifically designed to meet the expansion / compression requirements of applications such as functionalized medical implants. The aim is to provide a flexible, often bendable substrate for electronic components that is biocompatible and implantable, allowing for body movements without damaging or destroying the substrate, its structure, or the electronic components mounted on it.

[0033] A further task can be seen as the complementary realization or use of the combination of shape memory alloys as the backbone of the auxetic structure for a new design for medical devices that can be used in non-invasive endovascular treatments in the heart and brain.

[0034] These problems are solved with an auxetic bridge or field structure according to main claim 1.

[0035] The auxetic web or field structure has island structure surfaces and connections between the individual island structure surfaces, wherein open spaces exist between the individual island structure surfaces and the connections with the island structure surfaces form a web or field structure, characterized in that the connections are stretchable and are designed as Archimedean spiral connections and / or self-similar fractal design connections, the connections intersect the island structure surfaces at the same place and at the same angle as the previous rigid connectors intersect the auxetic framework, the island structure surfaces are not or only slightly bendable under the influence of force, wherein the stresses on the island structure surfaces are at least one order of magnitude lower than those of the stretchable connection, and / or are not or only insignificantly variable in their magnitude, and the Poisson's ratio of the structure is negative under uniaxial deformation.

[0036] Thus, stretchable components are provided by combining auxetic geometries from the literature with stretchable connections. The auxetic structures are characterized by a negative Poisson's ratio, which means that the structure stretches under uniaxial tensile stress in the direction perpendicular to the applied force.

[0037] Similarly, under uniaxial compressive stress, the structures presented here also contract in the vertical direction due to the negative Poisson's ratio.

[0038] Each thin-film material incorporated into the structures disclosed in this invention exhibits auxetic properties with improved extensibility and compressibility.

[0039] The invention enables even greater extensibility than disclosed in US 2020 / 0144431 A1, since the serpentine connections described in that disclosure are replaced by connections known to have greater extensibility, namely self-similar and / or fractal design connections and / or Archimedean spiral connections. Theoretically, the mechanical properties of many other auxetic structures could be improved by replacing their rigid connections with extensible ones.

[0040] The Archimedean spiral structure exhibits the greatest elastic elongation of up to 200%, while the regular serpentine and the self-similar serpentine exhibit elastic elongation of 112% and 98%, respectively. Therefore, stretchable auxetic structures with Archimedean connections can achieve enormous elongation and greater volume and area expansion compared to auxetic structures with stretchable serpentine connections, fractal connections, and conventional rigid hinges. The desired properties (such as elongation, etc.) of the auxetic web or field structure according to the invention are further enhanced by the fact that the connections intersect the island structure surfaces at the same location and angle as the previous rigid connectors, thus further improving the auxetic framework.

[0041] The present invention achieves greater radial expansion and less radial compression compared to conventional auxetic structures. This is achieved by replacing the rigid connection points of the island structure surfaces with flexible / stretchable Archimedean spiral connections or self-similar fractal design connections. Stretchable auxetic structures could be formed by island structure surfaces of varying sizes in combination with stretchable intermediate connections of different sizes / shapes.

[0042] Any material processed into this type of stretchable auxetic structure can benefit from improved material properties. Due to their negative Poisson's ratio, auxetic structures can conform to virtually any concave or convex surface. This property results in synclastic bending, enabling auxetic structures to adapt to nonlinear surfaces. By replacing the traditional "rotary joints" or rigid connection points of traditional auxetic structures with helical strut connections, specifically self-similar fractal design connections and / or Archimedean spirals, the stretchability and compressibility of the auxetic structure are improved by several orders of magnitude.

[0043] The connections can additionally have the following properties, at least in sections: arc-shaped and / or kink-free and / or with bending radii and / or bifilar and / or helical.

[0044] Archimedean-inspired designs for elastic joints are highly advantageous, as they have already been shown to achieve elongation exceeding 1020% when designed with a high amplitude-to-width ratio. It is assumed that a uniform and small curvature contributes to the greater elongation of such structures. Furthermore, complex spiral shapes, incorporating at least parts of known spirals, would also be conceivable.

[0045] In particular, the individual island structure surfaces and / or the individual connections of the auxetic bridge or field structure can vary in size and shape. This allows for an overall design of the auxetic structure adapted to the requirements.

[0046] The auxetic bridge or field structure can additionally be uniform and / or periodic, at least in sections, over areas.

[0047] The island structure surfaces can also be uniformly shaped. These island structures can cover a large area, while the interconnections allow for stress-free expansion. The island structures can be scaled to accommodate any type of conventional rigid electronic MEMS component. The stretchable auxetic structure can be designed to strike a balance between the density of the unit cell area and the maximum volume expansion of the entire structure.

[0048] In particular, the self-similar fractal design connections can include Koch lines, Peano lines, Hilbert lines, Moore loops, Vicsek loops and branched networks such as the Greek cross.

[0049] The auxetic bridge or field structure can be fabricated planarly (2D) and subsequently adapted to 3D surfaces while maintaining 3D auxetic behavior. This is particularly possible when the 2D fabricated structure is formed from an amorphous shape memory alloy.

[0050] The open spaces between the island structure surfaces and the connections can be mesh-like and potentially serve as space for further functionalization of the device or simply as a flexible connection.

[0051] Preferably, the auxetic bridge or field structure can have at least individual island structure surfaces for receiving electronics or individual island structure surfaces equipped with electronics.

[0052] The electronics located on the island structure surfaces can be electrically connected to each other via at least one conductor track electrically insulated from the auxetic bridge or field structure on at least one connection of the auxetic bridge or field structure.

[0053] In particular, the auxetic bridge or field structure according to the invention can be used as an implantable structure made of a biocompatible material, wherein the biocompatible material is a metallic shape memory alloy (SMA) and the auxetic bridge or field structure is metallic.

[0054] The implantable structure can have a self-expanding implant shape with a cylindrical and / or spherical and / or hemispherical and / or tubular and / or curved tube structure, at least in sections.

[0055] The auxetic bridge and field structure according to the invention thus provides an extremely deformable metallic auxetic substrate with large areas, which is suitable for the integration of functional electronic devices.

[0056] The electronic devices can also be brittle, e.g., containing ceramics, and yet remain intact and adhere to the island structure surfaces even when the framework is bent.

[0057] Electrically conductive metal connections between different islands or island structure surfaces of a framework must be precisely applied to the connections between the island structure surfaces, which becomes increasingly costly and complex with smaller structure dimensions. Electronic components or devices are generally at least two-terminal networks that generate electrical voltages or respond to voltage signals. It is therefore desirable, and practically unavoidable when a large portion of the island structure surfaces are occupied by components, that each connection of a framework structure carries at least two metallic, but electrically insulated, paths to the signal conductor. The insulation of the two paths must not be too thin, as even low voltages can otherwise lead to locally high field strengths and thus electrical breakdowns.Especially in scaffold structures intended for implantation in a patient's blood vessels, the structural dimensions must be small, and the widths of the connections / bridges must be in the range of a few tens of micrometers. If narrow bridges are occupied by more than one metallic conductor, the voltage signals that can be transmitted over the network are severely limited. For safety reasons, this can lead to restrictions in the selection of electronic components and / or in the design of the network architecture.

[0058] By manufacturing circuits on auxetically structured materials

[0059] Shape memory alloy substrates enable extreme flexibility within the entire electronic system. The advantageous material properties of the shape memory alloy (SMA), coupled with the superior mechanical properties of the auxetic structures, allow the substrates and all components manufactured on them to adapt to the various surfaces of the body. Medical implants, such as those intended for insertion into a cerebral vessel, are meant to last a lifetime; therefore, all materials and device components must be designed to withstand millions of stress cycles. NiTi SMAs are biocompatible, meaning they do not degrade in the patient's body or release harmful substances. Furthermore, some SMAs can be deformed superelastically without fatigue.

[0060] The only prior art document that describes a metamechanical auxetic design with island structure surfaces and connections, corresponding to the small, electrically interconnected island structure surfaces claimed here for functional device integration, is US 2020 / 0144431 A1. In contrast to the aforementioned document, the auxetic bridge or field structure according to the invention features Archimedean spiral connections and / or self-similar fractal design connections. The document uses only serpentine connections. In the auxetic bridge or field structure according to the invention, the connections intersect the island structure surfaces at the same location and angle as the previous rigid connectors intersect the auxetic framework. The aforementioned document provides no information regarding the angle of intersection. Therefore, it appears to have only a minor or even no significance there.Nevertheless, the angle of intersection is of central importance for the auxetic bridge or field structure according to the invention.

[0061] The auxetic bridge or field structure according to the invention overcomes the prejudice that self-supporting metal connections are not mechanically robust enough to be freestanding bridge structures by using a shape memory alloy. Furthermore, an array of various functional components can be produced, distributed across a multitude of island structure surfaces, which in turn are electrically connected to each other via the bridges.

[0062] Each uniquely modified auxetic geometry offers a compromise between mechanical flexibility, extensibility, deformability and areal density, and overall provides a wealth of possible adaptable shapes that could be used for the design of next-generation medical devices.

[0063] In particular, the shape memory alloy can be made of NiTi. SMAs are used here as auxetic backbones for medical implants, as some biocompatible NiTi-based SMAs have already demonstrated in trials that they can meet the cycle requirements. The SMA can also be one of the NiTi alloys such as TiNiCu, TiNiCuCo, TiNiHf, and similar alloys.

[0064] Furthermore, the electronic components can be selected from various elements, such as piezoelectric elements, energy storage devices, antenna structures, and prefabricated microprocessors. Examples of active electronics that could be integrated onto the island structure surfaces and / or the backbone of the auxetic structure include: electrodes, sensors (chemical, biological, electrical, magnetic, mechanical), actuators, interconnects, capacitors, LEDs, batteries and solar cells for energy storage, energy harvesters, and antennas. These components can be fabricated from virtually any material currently compatible with modern micromachining and deposition techniques. Examples of materials include metals, oxides, dielectrics, perovskites, semiconductors, polymers, ceramics, piezoelectric, pyroelectric, magnetic, ferroelectric, and magnetostrictive materials.

[0065] Additionally, auxetic structures made of shape memory alloys could be functionalized with other types of actuator devices made of shape memory alloys.

[0066] In addition to auxetic SMAs, which offer patient-specific design for medical implants, functionalization provides a novel feature by enabling the wireless transmission of vital patient data to the outside.

[0067] Auxetic shape memory alloys coupled with another shape memory alloy with a different transition temperature could be used to create self-folding auxetic structures.

[0068] Another characteristic is that the shape memory alloys used here are electrically conductive materials whose conductivity is only about one order of magnitude lower than that of conventional metals such as platinum, copper, and gold. The electrical conductivity of SMAs is similar to that of liquid metals, which are considered among the most ductile metals. The biggest challenge with liquid metals and conventional thin-film metals is that they must be integrated into an elastomeric substrate. The relatively high conductivity of the shape memory alloy, coupled with its mechanical robustness, allows the freestanding substrate to serve as a ground electrode for connecting all active electronic components. The components can be fabricated on individual island structures and then electrically connected via a metallic SMA substrate.The combined strength of the SMA material and the auxetic geometric structure results in low stresses under applied load throughout the entire auxetic structure. Therefore, the entire auxetic backbone structure could be designed to accommodate other thin-film material traces and even potentially complete devices.

[0069] It is therefore also possible to design the auxetic bridge or field structure as a common ground electrode to improve the state of the art.

[0070] Alternatively or additionally, at least two conductor tracks can be spaced apart next to each other and / or one above the other, each with an insulating layer between the metallic auxetic structure and the conductor track.

[0071] The immediate advantage lies in the fact that a second metallic path can be isolated from the ground electrode via a bridge in virtually any desired manner, and in particular, spaced apart, for example, by a comparatively thicker dielectric layer between the two. In other words, a plurality of highly conductive paths for voltage signals can be provided via the individual connections / bridges of the framework structure, with at least one path existing that can withstand a predefined maximum voltage without damaging the implant. This expands the possibilities for designing such a framework structure and its electronic components, ultimately improving the final functionality of a flexible electronic implant.

[0072] In one embodiment, at least above the electrical connection, a covering insulating layer and / or further insulating layers may be provided.

[0073] Furthermore, the insulating layer and / or further insulating layers of insulated conductor tracks can preferably consist of a non-conductive oxide. Such a material can also be applied using the methods described in this disclosure.

[0074] Furthermore, in a preferred variant, the implantable structure can also have at least a section of an outer layer made of a biocompatible polymer.

[0075] Particularly preferred is the application of a particularly biocompatible layer to the layers and components provided on the auxetic metallic SMA structure. In one embodiment, the auxetic metallic SMA structure can remain free of any covering material, since biocompatible material is used for this purpose.

[0076] Preferably, the structure aspect ratio, i.e., structure width to structure thickness, of the conductor track can be in the range of 0.5 to 1, at least in the area of ​​the webs or the web structure.

[0077] Furthermore, the conductor track thickness can range from approximately 10 nm to 200 µm. For medical implants, the target thickness of the overall structure could well be in the range of approximately 30–50 µm. However, for a wearable e-tattoo, conductor tracks up to 200 µm would be possible, as there is no spatial requirement regarding the component size.

[0078] One possible manufacturing process for an implantable structure made from a biocompatible material involves the following steps: Manufacturing an auxetic bridge or field structure with island structure surfaces and connections, wherein open spaces exist between the individual island structure surfaces and the connections with the island structure surfaces form a bridge or field structure, wherein at least individual island structure surfaces are designed to accommodate electronics or individual island structure surfaces equipped with electronics, and at least these island structure surfaces are not or not significantly bendable and / or are not or only insignificantly changeable in size under the influence of force on the auxetic bridge or field structure, wherein the biocompatible material is selected from a metallic shape memory alloy and the auxetic structure is formed from a metallic auxetic structure.Manufacturing electronics and / or electronic components distributed across at least two island structure surfaces and electrically interconnected, and an electrical connection between the electronics and / or electronic components, wherein this electrical connection is achieved by manufacturing at least one conductive trace electrically insulated from the metallic auxetic structure on at least one web structure of the metallic auxetic structure.

[0079] The at least one conductor track and / or the at least one insulating layer and / or the electronics and / or the electronic components on the auxetic structure can particularly preferably be sputtered and / or also preferably applied using a further thin-film technique, sol-gel technique or a chemical or physical deposition process.

[0080] Furthermore, at least one conductive trace and / or at least one insulating layer can be applied to the auxetic structure together with the electronics and / or electronic components. It is entirely possible to perform the corresponding steps simultaneously and thus achieve synergistic advantages in design and manufacturing.

[0081] The modification of traditional auxetic designs according to the invention enables the integration of low-stress island structure areas into the basic framework of the auxetic structures. These island structure areas are subjected to low stresses under mechanical loads, e.g., by crimping or stretching. The island structure areas can be scaled to the appropriate size to accommodate any type of traditional rigid electronic MEMS or NEMS components.

[0082] The use of shape memory alloys offers a major advantage in feature combination, as the auxetic structure can expand itself to extremely large volumes through a stress- or temperature-induced solid-to-solid phase transformation.

[0083] The extent of radial volume expansion is further increased by replacing the rigid connection points of the island structure surfaces with extensible intermediate connections, i.e., Archimedean spiral connections and / or self-similar fractal design connections.

[0084] The implementation of shape memory alloys as the backbone of the auxetic structure offers improved mechanical properties for the device, resulting in a longer lifespan. The shape memory effect is key to enabling the modified auxetic substrate to withstand high residual stress, thus ensuring the medical implant's resistance to deformation. Adding functionalized island structure surfaces enhances the device's capabilities by integrating elements such as pressure and strain sensors. Furthermore, the body's inflammatory response to device implantation could be reduced through direct local drug release in one design variant. Such a medical implant thus allows for the direct local delivery of medication to the intended site in the body, generally minimizing the side effects of medications administered via conventional methods (e.g., via subcutaneous injections).B. orally) is reduced.

[0085] In principle, spinal cord angioplasties (SMAs) can be crimped, i.e., folded into a small volume (without bending the island structure surfaces), and unfold themselves at body temperature inside the patient. This allows, among other things, minimally invasive positioning during endoscopic procedures.

[0086] A preferred manufacturing process is based on fabricating all device components using micro- / nanoelectromechanical systems (MEMS / NEMS) processing techniques on an amorphous auxetic SMA substrate, employing a process published in US 8,758,636 B2 (also known as Lima et al.). The entire composite material is then shaped into a complex 3D form by annealing (> 450 °C) to crystallize the SMA. Useful shapes envisaged here are a cylindrical 3D tube for stents and a 3D sphere or hemisphere for intrasaccular aneurysm devices, which can be compressed to sufficiently small dimensions for delivery via a microcatheter. This feature would allow the devices to be used through endovascular treatments, which are non-invasive and safer for the patient, i.e., shorter operating time, less exposure to X-ray and MRI radiation.This method can be used to structure amorphous shape memory alloy materials into the desired 2D auxetic structures. This process enables high geometric precision with feature sizes down to 10 micrometers using photolithography, sputtering, and chemical wet-etch release processes. Because the SMAs can be deposited amorphously with low surface roughness, they can be further processed at temperatures below 800 °C, making the SMA substrate compatible with most modern MEMS processes. State-of-the-art substrates currently used in implants and stretchable components, such as polymers and elastomers, cannot withstand these high temperatures (below 200 °C).

[0087] While existing commercial stents can contract and expand within the body, most designs contain a somewhat rigid backbone that cannot adapt to the shape of the artery. Auxetic stent structures would be superior to existing stents because they can be patient-specific and adapt to any complex geometric surface, which is necessary since each patient's cerebral vessels, arteries, and aneurysms have different shapes and sizes. According to current technology, auxetic stent structures exhibit higher radial forces at the stent circumference, up to 10 times higher than current stent designs. Furthermore, current stent designs do not offer large island structure areas that could be integrated with or even functionalized with active and passive devices.

[0088] As previously described, the invention described here can serve as a platform for the functionalization of any type of electronic device on virtually any type of complex geometric surface (from the macro to the nanoscale) using, in particular, auxetic shape memory alloy substrates.

[0089] Each uniquely modified auxetic geometry offers a compromise between mechanical flexibility, stretchability, deformability, and areal density, providing a wealth of possible adaptable shapes that could be useful for designing the next generation of stretchable electronics. Most stretchable applications share similar goals: maximizing stretchability and maximizing area coverage, i.e., enabling more functional areas for component integration.

[0090] A major advantage of the novel stretchable auxetic design is that, under applied tensile / compressive stress, all designs offer large, low-stress island structure areas suitable for accommodating virtually any type of MEMS component. This key feature could theoretically enable the co-integration of various electronic components, such as batteries, antennas, sensors, actuators, solar cells, and the like, through the substrate or a more conductive electrode deposited on the stretchable auxetic substrate. Semiconductor devices could be directly patterned into the self-similar and / or fractal and / or Archimedean interconnected form to create deformable semiconductor arrays using methods similar to those described in US 2020 / 0144431 A1.

[0091] The large island structure areas in the designs shown here are subject to low stresses under applied strains, which can accommodate electronic components for piezoelectric energy harvesters and semiconductor devices such as displays and solar cells, and for medical implants.

[0092] In a preferred embodiment, the structure may furthermore have a self-similar fractal design in its entirety or in sections to improve the compressibility and extensibility of the auxetic structure. In another embodiment, the arranged (or web-shaped) auxetic structure may have Archimedean spiral connections in its entirety or in sections.

[0093] The Poisson's ratio of the entire modified auxetic structure is negative, meaning that the structure stretches under uniaxial tensile stress in the perpendicular direction. The negative Poisson's ratio of auxetic structures is advantageous for stretchable electronics because the structures can conform to any curved surface, regardless of material or thickness. Modern stretchable microelectronic mechanical systems (MEMS) are typically based on wave-linked thin-film metals and island-bridge layouts with thin-film serpentine interconnects. Furthermore, the stretchable electronics are designed to maintain a high areal density for component integration while stretching.

[0094] Other improved mechanical properties generally offered by auxetic structures include high penetration / impact strength, reduced fatigue crack propagation, greater toughness and modulus, and vibration damping. Due to these properties, the stretchable auxetic structures disclosed here are potentially of interest for military and civilian applications such as bulletproof vests and impact-resistant vehicles and ships.

[0095] Electronic devices intended for mounting on the exterior of architectural structures, vehicles, ships, aircraft, or inside missile ships also require extremely high resistance to deformation. Many of these applications would benefit from stretchable auxetic substrates, as the structures presented here can be fabricated in a 2D film and bend around 3D curved surfaces such as spheres, curved tubes, and cylinders.

[0096] A key difference and feature of the invention lies in the fact that conventional stretchable electronic structures configured in an island connection format have a Poisson's ratio of zero, since they connect at the center of the island, as in the Archimedean spiral connection in Figure 1A and in the fractal horseshoe serpentine connection in Figure 1B is shown.

[0097] A significant difference and a key feature of the invention can also be seen in the fact that prior art designs do not connect island structure surfaces with Archimedean spiral connections and / or self-similar fractal design connections in order to create sufficient space and area for future innovations through MEMS sensor / actuator integration via the large island structure surfaces. As in Figure 1CAs can be seen, current wave-shaped mesh constructions do not offer enough space for the integration of sensors.

[0098] Another aspect of the present invention is the improvement of the radial expandability of the auxetic structure by replacing the rigid hinge connection points of the rotating auxetic polygon structures or unit cells with classical Archimedean spirals and / or self-similar fractal design connections. This concept is made clear by the introduction of a new type of representation, as shown in the figures with examples of Archimedean spiral connections. By mechanically designing the geometric properties of the auxetic structure, i.e., surface density, Archimedean spiral radius, etc., the flexibility / extensibility of the structure, as well as the bulk modulus, shear modulus, elastic modulus, and Poisson's ratio of the auxetic substrate, can be adjusted.The freedom of geometric design also enables the development of stretchable auxetic substrates with high surface coverage, which could enable devices with higher efficiency in the future.

[0099] Several auxetic mechanical metamaterials are described in the literature that can be modified to incorporate low-stress island structure surfaces. Examples include honeycomb, chiral, re-entrant, origami-based metamaterials, kirigami-based metamaterials, keyed-brick structures, slot-perforated structures, rotating rigid structures, and hierarchical auxetic systems.

[0100] The auxetic structure according to the invention requires a significant modification of traditional auxetic structures from the literature, in that their rigid or rotating connection points are replaced by stretchable connections. The meandering, wave-like connections (winding track structures) are formed from known stretchable connections, with one embodiment of the invention showing auxetic structures with stretchable connections made from Archimedean spiral structures. A second embodiment shows auxetic structures with stretchable fractal connections, and a third embodiment shows an auxetic wave-like mesh design. To ensure that the modified structure retains its auxetic behavior, the stretchable connection must intersect the auxetic framework at the same location and angle as the previous rigid connectors.Replacing rigid connections with flexible connections significantly improves the mechanical properties of the entire auxetic structure and allows for extreme extensibility and compressibility of the overall structure.

[0101] The invention is achieved by creating an arrangement of periodic island structure surfaces that are auxetically arranged, and then connecting the island structure surfaces with periodic stretchable intermediate connections (e.g., meandering fractal design connections or Archimedean spirals). The resulting stretchable auxetic structures can enable any material to become extremely resistant to applied deformation.

[0102] The island structure surfaces within the structure experience low stresses under applied strain, making them suitable for the large-area integration of functional electronic MEMS components. This property is necessary to enable the integration of wireless transmission antennas, energy harvesting devices, energy storage systems, and virtually any type of sensor or actuator component into medical devices and wearables.

[0103] Designing the geometry of the stretchable junctions (i.e., amplitude, wavelength, thickness, and width) allows for a compromise between maximum areal density and stretchability. Large-area, conformal, stretchable auxetic substrates would enable MEMS / NEMS devices with higher efficiency.

[0104] All of the above-mentioned applications specifically require that the substrate material is compatible with traditional 2D MEMS wafer-based processing techniques.

[0105] The exposed stretchable auxetic structures will have numerous applications as substrates / backbone structures in the fields of stretchable electronics, wearable electronics, impact resistance, energy-absorbing materials, architecture, space exploration, and implantable medical technologies. When the structure is used as a substrate, any material coated onto a stretchable auxetic substrate will benefit from the enhanced mechanical properties provided by the underlying structure.

[0106] The archetypal design presented here allows for extreme flexibility and stretchability within any type of device, which will be of great importance for next-generation medical applications such as smart stents, intrasaccular aneurysm devices, drug-delivering implants, and wearable biosensors.

[0107] Shape memory alloys (SMAs) structured in auxetic geometries offer superior mechanical properties that can enable the design of high-performance novel medical implants with increased mechanical flexibility and ductility. SMAs can achieve large intrinsic strains of up to 8% through thermal or stress-induced phase transformation. Furthermore, SMAs are used in this invention because they can be microstructurally designed to undergo (thermally) stress-induced (mechanical) phase transformation for 10 million cycles without altering their functional fatigue properties. These properties can increase the lifetime of a medical device, thereby making the device safer for patients overall.

[0108] The invention also has potential applications in the field of wearable electronics. For example, electronic tattoos and other substrates worn directly on the skin must conform naturally to the skin and be reversibly stretchable over very large distances for thousands or millions of cycles. Thanks to synclastic bending, auxetic SMAs can conform to the surface of the skin or substrate better than current stretchable designs, while simultaneously offering greater area coverage and better mechanical performance than current serpentine lattice designs. This could be useful in applications requiring bioelectrodes (skin sensors), such as electrocardiogram and magnetocardiogram sensors, which need a perfect interface with the skin. These applications benefit from the subtle electronic conductivity offered by metallic SMAs.The superior mechanical properties of the SMA material and its auxetic geometries would benefit many applications related to stretchable and wearable electronics and soft robotics.

[0109] The invention is described below with reference to the accompanying illustrations. Image description described, whereby this is intended to explain the invention and is not necessarily to be considered limiting. It shows: Fig. 1 Overview of extensible geometries known from the prior art with similarities to the present invention; Fig. 2 Rotating rigid auxetic structures according to the prior art ( Fig. 2A ) and embodiments of rotating Archimedean polygons according to the invention ( Fig. 2B-2G ); Fig. 3 Finite element simulations showing the improved extensibility and compressibility of an exemplary rotating Archimedean square structure according to the invention ( Fig. 3A-3G); Fig. 4 an embodiment of a rotating Archimedean square arrangement according to the invention (section - a single unit of the Archimedean spiral connection); Fig. 5 an example of a rotating Archimedean rectangular structure according to the invention designed as a 2D plate ( Fig. 5A ) and as 3D cylinders ( Fig. 5B ); Fig. 6 an experimental implementation of a rotating Archimedean rectangular structure according to the invention in the form of a 3D cylinder with a diameter of 5 mm and a length of 9 mm from different perspectives ( Fig. 6A-6C ); Fig. 7 an experimental implementation of a rotating Archimedean rectangular structure according to the invention, shaped into a 3D hemisphere with a radius of 2.5 mm from different perspectives ( Fig. 7A-7C); Fig. 8 Finite element simulations and experimental comparison of a first exemplary rotating Archimedean rectangular unit cell according to the invention, which shows clear auxetic behavior in uniaxial tensile and compression tests; Fig. 9 Finite element simulations and experimental comparison of a second embodiment of a rotating Archimedean rectangular structure, which shows clear auxetic behavior; Fig. 10 A demonstration of the elastic and superelastic recovery from enormous tensile and compressive loads of the in Fig. 8 and Fig. 9 The illustrated embodiments are based on rotating Archimedean auxetic rectangular structures according to the invention; Fig. 11 Finite element simulations of the enhanced compression in an exemplary rotating Archimedean auxetic rectangular structure according to the invention ( Figs. 11A-11C); Fig. 12 Finite element simulations of the extraordinary extensibility in an exemplary rotating Archimedean rectangular structure according to the invention ( Figs. 12A-12D ); Fig. 13 an embodiment of a rotating rectangular auxetic structure with second-order fractal horseshoe serpentine connections as a 2D surface ( Fig. 13A ), as well as the rotating rectangular auxetic structure with second-order fractal horseshoe serpentine connections in cylindrical 3D form ( Fig. 13B ); Fig. 14 an embodiment of a rotating rectangular auxetic structure with second-order fractal horseshoe serpentine connections and island structure surfaces of different sizes as a 2D surface ( Fig. 14A ) as well as the rotating rectangular auxetic structure with second-order fractal horseshoe serpentine connections and island structure surfaces of different sizes formed into a cylindrical 3D geometry ( Fig. 14B); Fig. 15 Finite element simulations that demonstrate the auxetic behavior of the in Fig. 13 The exemplary structure shown according to the invention is based on a rotating rectangular auxetic structure with second-order fractal horseshoe serpentines during expansion and compression ( Figs. 15A-15D ); Fig. 16 an example of a 2D grid design with a square grid geometry using second-order fractal horseshoe serpentine connections and square island structure surfaces as a 2D surface ( Fig. 16A ), as well as the 2D grid design with a square grid geometry using second-order fractal horseshoe serpentines and square island structure surfaces formed into a cylindrical 3D geometry ( Fig. 16B ); Fig. 17 Finite element simulations showing the auxetic behavior with enhanced expansion and compression in the in Fig. 16The illustrated embodiment of a structure in 2D grid design using second-order fractal horseshoe serpentine connections and square island structure surfaces shows ( Figs. 17A-17D ); Fig. 18 Finite element simulations that demonstrate the auxetic behavior with extraordinary flexibility for the in Fig. 16 The illustrated embodiment shows a uniaxial strain applied from one side of the structure and the other boundary is held as a fixed constraint ( Figs. 18A-18E ); Fig. 19 Finite element simulations showing the auxetic behavior with increased compressibility for the in Fig. 16Figure 20 shows an embodiment in which a uniaxial compressive load is applied from one side of the structure and the other boundary is held as a fixed constraint; Figure 20 shows an exemplary flowchart with steps a) - d) to illustrate the 3D fabrication of a functionalized shape memory alloy implant structure according to the invention; Figure 21 shows an exemplary schematic flowchart with the device cross-section steps a) - e) for the functionalization of an implantable auxetic structure according to the invention; Figure 22 shows an example accordingly. Fig. 21 Section d), supplemented by at least one further electrode; Fig. 23 a first embodiment of a functionalized implantable structure (smart stent) according to the invention in 3D tube form ( Fig. 23A ), in the excerpt the unit cell of the functional auxetic structure with three different device components ( Fig. 23B) and embedded in a blood vessel or artery ( Fig. 23C ); Fig. 24 a second embodiment of an "S-shaped island structure surface" auxetic geometry, modified so that island structure surfaces are available for the integration of devices from conventional "S-shaped" auxetic structures, shown as a 2D surface ( Fig. 24A ), as an excerpt ( Fig. 24B ) and as a 3D intelligent stent ( Fig. 24C ); Fig. 25 Finite element simulations of the exemplary embodiment of an "S-shaped island structure surface" auxetic structure made of Fig. 24 with the structure in equilibrium ( Fig. 25A ) and under uniaxial tensile force ( Fig. 25B ) and Fig. 26Representation of the synclastic bending behavior of an "S-shaped island structure surface" around a 3D sphere, showing that this could be used as an intrasaccular device.

[0110] Fig. 1 shows examples of the state of the art in stretchable electronics. Fig. 1Ais a traditional island connection structure with Archimedean connections according to Jiang et al. ARCHIMEDEAN SPIRAL DESIGN FOR DEFORMABLE ELECTRONICS US 10,660,200 B2 (2020). Fig. 1B shows a traditional island connection structure with second-order fractal horseshoe serpentine connections and Fig. 1C Shows a 2D wave mesh with horseshoe-shaped building blocks arranged in a square grid geometry. Designs according to Figs. 1B and 1C are known from the publication Rogers et al. Self-similar and fractal design for stretchable electronics US 10,192,830 B2 (2019).

[0111] As previously described, state-of-the-art ductile island connections are typically joined at the center of the islands, resulting in a Poisson's ratio of zero under uniaxial loading. The structures must be stretched biaxially (in the x and y directions) to achieve elongation or compressed biaxially to obtain a fully compressed structure. In uniaxial tensile tests (in the y direction only), the structures exhibit a positive or zero Poisson's ratio. The in Figure 1C The proposed 2D wave network is known to exhibit auxetic properties under uniaxial strain. The major drawback of this design is the lack of large-area islands for integrating functional components.

[0112] Fig. 2 shows in: A) Examples of traditional auxetic structures known in the prior art, based on rotating rigid structures, are known, for example, from Saxena, Krishna Kumar, Raj Das, and Emilio P. Calius. "Three decades of auxetics research- materials with negative Poisson's ratio: a review." Advanced Engineering Materials 18.11 (2016): 1847-1870 and further in B) to G) embodiments of the auxetic structures according to the invention with increased extensibility using Archimedean connections, namely: B) rotating Archimedean triangle C) rotating Archimedean parallelogram D) rotating Archimedean square E) rotating Archimedean rectangle variant 1 F) rotating Archimedean rectangle variant 2 G) rotating Archimedean rectangle variant 3.

[0113] The structures presented in this invention offer an extremely compliant and deformable substrate, enabling any material to become flexible, stretchable, and adaptable. In the Figure 2A Prior art examples of traditional rotating rigid polygonal auxetic geometries are shown. Figures 2B-2G show modifications of the rotating rigid auxetic structures according to the invention with Archimedean spiral connections to enable improved extensibility and compressibility. Here, the extensible Archimedean spiral is connected to the island structure surfaces at the same angle at which the island structure surfaces were previously connected in a traditional rotating polygon structure.

[0114] Wearable devices intended to be worn directly on the skin must be able to adapt to extreme movements and the resulting changes in the body's radius of curvature. Stretchable auxetic structures must be able to expand and contract freely to compensate for abrupt changes in deformation.

[0115] The auxetic behavior is shown in the following figures for stretchable auxetic structures based on rotating squares of the same size and rotating rectangles of different sizes.

[0116] Finite element analysis (FEM) with COMSOL Multiphysics 5.6 is used in some of the following figures to illustrate the auxetic behavior of the structures according to the invention under a uniaxial compressive force (compression) and a uniaxial tensile force (extension). The depicted structure according to the invention exhibits a negative Poisson's ratio, as shown by the volume expansion in the perpendicular direction during uniaxial displacement. The maximum elongation of the unit cell depends on the design of the exemplary Archimedean spiral joint.

[0117] For all presented FEM simulation results, copper was used as the material, assuming an elastic modulus of E= 119 GPa, a Poisson's ratio of 0.34 and a density of 8940 kg / m3.

[0118] Analysis of the von Mises stress distribution after applying strain to the unit cell confirms low stresses on the large periodic island structure surfaces, as the stress is concentrated and distributed throughout the ductile compound for all modeled structures.

[0119] Fig. 3 Figure 1 shows FEM results illustrating the extent of improved compressibility and ductility for rotating square auxetic structures with Archimedean connections according to the invention. The island structure surfaces are 3.2 mm x 3.2 mm. The Archimedean spiral connections have a tip-to-tip amplitude of 5 mm, a wavelength of 1.25 mm, a width of 50 µm, and a thickness of 50 µm. The macroscopic uniaxial strain is applied to the bottom edge of the structure in the direction indicated by the arrow.

[0120] Fig. 3AThe figure shows the structure undeformed in equilibrium with a neutral end-to-end distance = I. The magnitude of the macroscopic compressive or tensile stress is defined as the change in length (ΔI) / end-to-end distance in equilibrium (macroscopic stress % = ΔI / I * 100%).

[0121] Fig. 3B The figure shows a black rectangle with a white area on one side to represent a "fixed constraint" on one edge of the square, while a uniaxial deflection is applied to the opposite square. The large arrow indicates that the direction of the applied force is a compressive force (in the direction of the fixed constraint). This figure shows the biaxial compressive behavior of the structure under a uniaxial compressive load of -30%. The magnitude of the macroscopic compressive or tensile load is defined as (ΔI / I).

[0122] Fig. 3Cshows that the rotating square Archimedean structure deforms out of the plane after an applied compressive strain of -60%, so that the island structure surfaces of the structure almost stack on top of each other due to the deformation out of the plane.

[0123] The Fig. 3D-3G Figure 1 shows the auxetic structure according to the invention at various macroscopic uniaxial strains (~10% - 300%). As indicated by the arrow pointing away from the fixed clamping point, one end of the auxetic structure is pulled under uniaxial tension. The structure clearly exhibits a negative Poisson's ratio, which is represented by an expansion in the perpendicular direction of the applied tensile force.

[0124] Fig. 3G shows the expansion of the structure to four times its original area (300% of the total expansion of the original length).

[0125] The development of an array based on the presented exemplary structure according to the invention would enable a highly deformable electronic 2D device that can adapt to essentially any type of concave or convex 3D surface.

[0126] In Fig. 4 An example of a rotating square Archimedean arrangement is shown. The section depicts a single unit of the Archimedean spiral rod.

[0127] An array of a rotating square Archimedean polygon structure is suitable as a substrate for an implantable medical device because it would allow for improved extensibility and compressibility of the structure, meeting crimp requirements for insertion into the body via a microcatheter. For a given stent circumference, traditional auxetic stent structures offer 10 times higher radial forces than current designs (as reported, for example, in Dolla, William Jacob S., Brian A. Fricke, and Bryan R. Becker. "Structural and drug diffusion models of conventional and auxetic drug-eluting stents." (2007): 47-55). The addition of coiled web structures / connectors, also known as stretchable interconnects, to the framework of auxetic structures reduces the bending stiffness of the overall structure, resulting in crushing and straining even under small applied forces.This means that medical devices manufactured on the basis of stretchable auxetic structures are easier to crimp, maneuver, and deploy through a microcatheter.

[0128] Fig. 5 shows a different arrangement of the rotating Archimedean rectangular structure with rectangles of different sizes and an Archimedean spiral with a smaller tip-to-tip amplitude (2.2 mm) compared to the one in Fig. 4 shown spiral.

[0129] A major advantage of the stretchable auxetic structures presented here is that they can be fabricated planarly in 2D and then adapted to 3D surfaces. If the structure consists of an amorphous shape memory alloy, it is possible, for example, to fabricate a medical implant entirely using 2D MEMS techniques and then to transform the shape memory material into a complex 3D geometry through crystallization of the shape memory alloy. This concept is further developed in Figure 5B illustrates where the 2D plate was transformed into a tubular (or cylindrical) 3D shape.

[0130] Fig. 6Figure 1 shows an experimental implementation of a rotating Archimedean auxetic SMA rectangular structure according to the invention, made of TiNiCu (thickness = 27 µm), which was formed into a tubular (or cylindrical) 3D shape with a radius of curvature of 2.5 mm and a length of 9 mm. The structure could be used as a smart stent for flow diversion, with large island structure areas available for electronic integration.

[0131] In Fig. 7 An experimental implementation of a rotating Archimedean auxetic SMA rectangular structure according to the invention, made of TiNiCuCo (thickness = 53 µm), is presented in a 3D hemispherical geometry with a radius of curvature of 2.5 mm. This structure could be used as a functionalized intrasaccular aneurysm device.

[0132] Fig. 8demonstrates the improved extensibility and compressibility under uniaxial extension of a unit cell with rotating Archimedean rectangular geometry (exemplar in Figure 5 A uniaxial strain was applied to the top edge of the structure in the direction indicated by the arrow, while the bottom edge of the structure was held as a fixed restraint (indicated by a striped rectangle). The left column shows the results of the finite element modeling of the auxetic behavior using the material properties of copper ( Fig. 8A , D, F, H). The middle column shows experimental results of a rotating Archimedean rectangle made from the superelastic shape memory alloy TiNiCuCo ( Fig. 8B , E, G, I).

[0133] Shape memory alloys (SMAs) are known to have superior mechanical properties compared to conventional metals such as copper. SMAs are the preferred substrate material for the structures presented in this invention. The shape memory alloy NiTi is biocompatible and is already used in medical devices such as stents. The alloys TiNiCu and TiNiCuCo are not biocompatible but have already proven to be extremely low-fatigue SMAs capable of cycling between 0% and 2% intrinsic strain for more than 10 million cycles (Chluba, Christoph, et al. "Ultralow-fatigue shape memory alloy films". Science 348.6238 (2015): 1004-1007). Novel stretchable auxetic structures were fabricated from extremely low fatigue TiNiCuCo thin films (thickness = 53 µm) to demonstrate the concept.

[0134] In the Fig. 8A, B ) show the finite element simulation ( Fig. 8A ) and the experiment ( Fig. 8BFigure 1 shows an exemplary rotating Archimedean rectangular unit cell in equilibrium (end-to-end distance (I) is 4 mm). The inset in the right-hand column denotes the dimensions of the island structure surfaces (1.0 mm x 1.25 mm) and the Archimedean spiral (wavelength (λ) = 1.25 mm, tip-to-tip amplitude (A) = 2.2 mm, width (w) = 50 µm, thickness 53 µm) of the structure in equilibrium. Both the simulations and the experiments assume that the outermost edges of the geometry are 4 mm apart in equilibrium. The distance between the innermost edges of the rectangles is used as the starting point for calculating the Poisson's ratio (xequilibrium = 3.0 mm, yequilibrium = 2.6 mm).

[0135] In the Fig. 8D , E a uniaxial total pressure strain of -10% is applied to the top edge of the structure, resulting in contraction island structure surfaces in the vertical direction.

[0136] The Fig. 8F , G show a uniaxial tensile strain of 26.5% on the uppermost edge of the structure, resulting in an expansion of the island structure surfaces in the vertical direction.

[0137] Furthermore, the Fig. 8H A uniaxial tensile load of 90% is applied to the top edge of the structure, resulting in an expansion of the island structure surfaces in the vertical direction. With a displacement of 7.6 mm in the y-direction, the extensibility (total strain) of the entire unit cell structure is 90%. The inset shows that the value of the total strain is determined from the vertical distance between the outer edges of the top and bottom squares. The distance between the innermost edges of the rectangles is used as the starting value for calculating Poisson's ratio (xstretched = 3.7 mm, ystretched = 6.3 mm).

[0138] In the Fig. 8CThe auxetic nature of this rotating Archimedean rectangular structure is demonstrated by the respective expansion and compression in the perpendicular direction of the applied force. The structure was experimentally verified as auxetic with a Poisson's ratio of -0.16, calculated by dividing the change in axial strain ((3.7 mm - 3.0 mm) / 3.0 mm) by the change in transverse strain ((6.3 mm - 2.6 mm) / 2.0 mm). The values ​​used in this calculation employ the internal spacing between the island structure faces, as the strain of the spirals is responsible for the expansion / compression of the entire structure. During the strain experiments, uneven strain was observed in the four Archimedean spirals of the unit cell, resulting in strains ranging from 101% to 189%.The uneven behavior in the Archimedean spirals could be due to 1) the connection angle of the spiral with the island structure surfaces and 2) a misalignment of the structure in the x-axis when carrying out tensile tests parallel to the y-axis.

[0139] Alone Fig. 8 The dimensions and values ​​shown are exemplary for the arrangement of the rotating Archimedean rectangular structure shown and are not intended to be limiting for differently designed structures according to the invention.

[0140] Fig. 9 This paper presents finite element (FEM) simulations and an experimental comparison of a second embodiment of a rotating Archimedean rectangular array, where the array is both auxetic and highly extensible. With the exception of the thickness (t = 53 µm), all dimensions of the array were determined using the following methods: Fig. 8The unit cell shown is reduced by a factor of 2 (the island structure surfaces are 0.5 mm x 0.625 mm) and provided with an Archimedean spiral (wavelength (λ) = 0.625 mm, peak-to-peak amplitude (A) = 1.1 mm, width (w) = 25 µm, thickness 53 µm) and then arranged into an array (3 x 2) to represent the in Fig. 9 to generate the array structure shown. The simulation results in the left column use the material properties of copper, while the experimental results in the middle column use the shape memory alloy TiNiCuCo.

[0141] The lower edge is a fixed constraint, while the upper edge undergoes a prescribed uniaxial displacement in the direction of the arrow.

[0142] Fig. 9A ) shows the manufactured structure in equilibrium with indication of all relevant dimensions for the island structure surfaces (0.5 mm x 0.625 mm) and the Archimedean spiral (w = 25 µm, t = 53 µm, λ = 0.625 mm, A = 1.1 mm).

[0143] In Fig. 9B ) Finite element simulation and experiment of the structure in equilibrium (0% applied strain) are presented.

[0144] Fig. 9C ) shows FEM and experiment of the structure under a uniaxial 62% tensile load from the upper edge. Furthermore, it shows Fig. 9D ) FEM and experiment of the structure under a uniaxial tensile load of 102% from the top edge.

[0145] Alone Fig. 9 The dimensions and values ​​shown are exemplary for the arrangement of the rotating Archimedean rectangular structure shown and are not intended to be limiting for differently designed structures according to the invention.

[0146] Fig. 10 demonstrates the experimental elastic recovery of giant strains and compressions of the in Fig. 8 (rotating Archimedean rectangular unit cell) and Fig. 9(rotating Archimedean rectangular array) geometries shown. The elastic recovery of such large total strains is due to the combination of the Archimedean spiral and the superelastic effect known for TiNiCuCo shape memory alloys.

[0147] Fig. 10A) shows the rotating Archimedean rectangular unit cell structure (left column) and the arranged structure (right column) in equilibrium (0% applied strain).

[0148] In Fig. 10B) both structures are shown at maximum tested compression of -47% (unit cell) and -67% (array).

[0149] Figure 10C shows both structures at a maximum tested strain of 265% (unit cell) and 330% (array). At this applied total strain, the maximum strain in the Archimedean spirals is 454% (unit cell) and 780% (array).

[0150] A superelastic recovery of elastic strain after removal of the applied force with extremely low total plastic strain on the structures of 3.75% (unit cell) and 5% (array) is shown in Fig. 10D).

[0151] The macroscopic yield strength of copper in ductile auxetic geometries is determined when the flow stress exceeds 200 MPa or the first principal strain exceeds 2%. According to the simulation results, copper thin films with the same dimensions as those in Fig. 10Experimentally proven unit cell structures show that the unit cell can only be elastically stretched uniaxially by 15% (displacement of 0.6 mm), whereas TiNiCuCo can be elastically stretched by 265%. Similarly, copper in the lattice structure can elastically stretch up to a maximum of 112.5% ​​(displacement of 4.08 mm), while TiNiCuCo can elastically stretch by 330%. Exceptional ductility was achieved in the shape memory alloy Archimedean spiral with almost complete elastic recovery from a strain of 780%. These significant results support the assumption that shape memory alloys are a preferred material for stretchable electronic substrates fabricated using the structures disclosed in this invention.

[0152] Alone Fig. 10The dimensions and values ​​shown are exemplary for the arrangement of the rotating Archimedean rectangular structure shown and are not intended to be limiting for differently designed structures according to the invention.

[0153] For the presentation in the Figs. 11 and 12 will be in Fig. 9 The structure shown according to the invention is arranged again (1x3) to create a larger rotating Archimedean rectangular structure.

[0154] In Fig. 11 Exemplary FEM models are derived from enhanced compression in a rotating rectangle with an Archimedean auxetic structure ( Fig. 11A (Equilibrium)-9C (highly compressed)) is shown. For this purpose, a uniaxial compressive load is applied to the top and bottom edges of the structure in the directions indicated by the arrows.

[0155] Fig. 11A ) shows the structure in equilibrium (0% total strain).

[0156] In Fig. 11B ) the structure is shown under 10% compressive load from the upper edge and 10% compressive load from the lower edge, 20% uniaxial compressive load in total.

[0157] In Fig. 11C Figure 1 shows the structure under 16.66% compressive stress from the upper edge and 16.66% compressive stress from the lower edge. This results in a total uniaxial compressive load of 33.33%.

[0158] Fig. 12 This shows the simulation results of the auxetic behavior of the structure under uniaxial stress. The uniaxial tensile load at the top and bottom edges of the structure is depicted in the directions indicated by the arrows.

[0159] Fig. 12A ) shows the structure in equilibrium (0% applied strain).

[0160] In Fig. 12B The structure is subjected to 10% compressive strain from the top edge and 10% compressive strain from the bottom edge. This results in a total uniaxial compressive strain of 20%.

[0161] In Fig. 12C The structure is shown at 50% extension from the upper edge and 50% extension from the lower edge, resulting in a total 100% uniaxial tensile strain.

[0162] Fig. 12D ) again shows the structure under 100% extension from the top edge and 100% extension from the bottom edge, i.e. with 200% uniaxial tensile strain in total.

[0163] In Fig. 13 An embodiment is shown that features a rotating rectangular auxetic structure with second-order fractal horseshoe serpentine connections ( Fig. 13A ), as well as the rotating rectangular auxetic structure with second-order fractal horseshoe serpentine connections in cylindrical 3D form ( Fig. 13B ) shown.

[0164] Furthermore, in Fig. 14an embodiment comprising a rotating rectangular auxetic structure with second-order fractal horseshoe serpentine connections and island structure surfaces of different sizes ( Fig. 14A ), as well as the rotating rectangular auxetic structure with second-order fractal horseshoe serpentine connections and island structure surfaces of different sizes formed into a cylindrical 3D geometry, shown ( Fig. 14B ). In the middle of the structure, the island structure surfaces have a different size and shape than those at the edge of the structure or as shown in the preceding illustrations.

[0165] Fig. 15 shows a possible embodiment of the rotating rectangular auxetic structure with second-order fractal horseshoe serpentine connections made of Fig. 13 The bottom edge is held as a fixed constraint, while a uniaxial force is exerted on the top edge. Fig. 15A) shows the structure in equilibrium. Fig. 15B ) shows the structure under 100% pressure. In Fig. 15C ) is the structure under 40% uniaxial stress and in Fig. 15D ) under 75% uniaxial stress, with the structure exhibiting auxetic behavior.

[0166] Examples of a wave-like 2D mesh design with a square grid geometry using second-order fractal horseshoe serpentine connections and square island structure surfaces, and a 2D wave-like mesh design with a square grid geometry using second-order fractal horseshoe serpentines and square island structure surfaces formed into a cylindrical 3D geometry, are presented in Fig. 16 (Fig. 16A) - 16B )) shown.

[0167] In Fig. 17The auxetic behavior of a structure in a 2D wave lattice design with square lattice geometry is investigated using second-order fractal horseshoe serpentine connections and square island structure surfaces according to Fig. 14 The diagram shows increased expansion and compression. Uniaxial strain is applied to both the left and right edges of the structure in the direction indicated by the arrow. The following are shown: 17A) Structure under 30% compressive stress from the top edge and 30% compressive stress from the bottom edge. Total uniaxial compressive strain of 60%. 17B) Structure under 20% compressive stress from the top edge and 20% compressive stress from the bottom edge. Total uniaxial compressive strain of 40%. 17C) Structure under 10% strain from the top edge and 10% strain from the bottom edge. Total uniaxial tensile strain of 20%. 17D) Structure under 30% strain from the top edge and 30% strain from the bottom edge. Total uniaxial tensile strain of 60%.

[0168] In Fig. 18The auxetic behavior with increased strainability in a 2D wave lattice with square lattice geometry is demonstrated using second-order fractal horseshoe serpentine connections and square island structure faces. The black inset shows the innermost unit cell of the lattice structure, while the gray inset shows the boundary unit cell closest to the direction of the applied force on the lattice structure under strain. The top edge of the structure is held as a fixed constraint, while the bottom edges of the structure are uniaxially strained in the direction indicated by the arrow. Non-uniform strain behavior and out-of-plane bending are observed. 18A) Structure at equilibrium 18B) Structure at ~5% extension 18C) Structure at ~10% extension 18D) Structure at ~20% extension 18E) Structure at ~100% extension

[0169] Fig. 19This presents a finite element simulation of the auxetic behavior with increased compressibility in a 2D wave lattice with square lattice geometry, using second-order fractal horseshoe serpentine connections and square island structure surfaces. A uniaxial compressive load is applied to the lowest edges of the web structure in the direction indicated by the arrow. Non-uniform strain behavior is observed. The following are shown: 19A) Structure in equilibrium (0% load) 19B) Structure under ~10% compression 19C) Structure under ~20% compression 19D) Structure under ~40% compression.

[0170] Thin films of auxetic shape memory alloys will be the preferred substrate material for future medical implants. Shape memory alloys such as TiNi are currently used in passive medical devices like stents because they are biocompatible and can be made small enough to be delivered into the body's arteries via a microcatheter. The main advantage of using shape memory alloys as a functionalized auxetic scaffold for medical implants is that they provide the implant with additional functionality (i.e., self-deployment, shape recovery, superelasticity). Large strains of 6–8% are achieved through thermal and stress-induced phase transformations; therefore, auxetic SMAs can expand or deconvolve to much larger volumes than conventional auxetic metals (e.g., steel).Furthermore, non-biocompatible NiTi-based shape memory alloys such as TiNiCu and TiNiCuCo can be engineered to exhibit extremely low fatigue behavior, enabling them to undergo ~10 million transition cycles with minimal change in mechanical or thermal behavior, making them attractive for wearable devices.

[0171] In Fig. 20 The fabrication of a functionalized smart stent using an auxetic substrate made of a shape memory alloy is described. Fig. 20aFigure 1 shows that the starting material is an amorphous auxetic thin-film SMA structure, which is fabricated in a flat 2D state by a prior art fabrication method (e.g., photolithography or laser cutting). The typical reentrant auxetic geometry was modified to exhibit arrow-shaped, low-stress island structures, which we call "reentrant arrow-island structures". Fig. 20b ) shows that the amorphous sheet can be brought into a complex 3D shape by crystallization at high temperature (400°C-800°C), e.g. by rapid thermal annealing around a stainless steel object in the desired shape. Fig. 20cThe figure shows that the crystallized auxetic 3D structure can be returned to a flat 2D state by applying mechanical force (i.e., by clamping the component onto a flat substrate). At this point, the substrate is ready for typical advanced micromachining to directly fabricate and integrate MEMS and NEMS components onto the island structure surfaces of the auxetic structure.

[0172] Because the SMA backbone is conductive, both sides of the SMA substrate can be functionalized, increasing the available surface area for component integration. Since the SMA can serve as a common ground electrode, it can function as a "circuit board." Examples of components suitable for functionalization include integrated biosensors and readout components, antennas, piezoelectric devices, batteries, cameras, LEDs, actuators, and the like. Fig. 20dFigure 1 shows that after the fabrication of all functional components, the mechanical clamp is removed, allowing the functionalized auxetic shape memory substrate to elastically regain its 3D shape if it was fabricated with a superelastic SMA. Since only low stresses occur on the island structure surfaces during bending, stretching, twisting, and folding movements, the integrated circuits structured on the island structure surfaces of the SMA substrate also benefit from low deformation stresses. If the austenite's final temperature is above room temperature, the functionalized medical implant must be heated to fully regain its original shape.Since the phase transformation is reversible, the shape memory effect and the superelastic effect can be used to compress the component to the required small volume of the microcatheter and then bring it into its final position by heating it to body temperature (~37°C).

[0173] Fig. 21 shows the section-by-section construction of an auxetic bridge or field structure according to the invention with connections 13 and free spaces and island structure surfaces 11, 12 as an implantable structure made of a biocompatible material in cross-section.

[0174] This is an example of a series of manufacturing steps that can be performed to produce and electrically insulate two different components on adjacent island structure surfaces after the auxetic SMA framework has been mechanically clamped flat, as in Fig. 20c ) described.

[0175] In step a), the auxetic base structure, consisting of a crystallized shape memory alloy (SMA), is first produced, forming island structure surfaces 11 and 12 and connections 13, with the connections linking the island structure surfaces together. This process diagram, or quasi-production scheme, shows only two island structure surfaces 11 and 12 as examples, which are then functionalized. Some island structure surfaces may already be functionalized or may be functionalized subsequently.

[0176] In step b), a first electrical insulation layer 2 is applied to at least those areas of the structure where electrical connections and / or electrical / electronic components are to be installed. A complete coating of the structure is also possible.

[0177] In step c), a first electrode 3 is applied so that at least the necessary areas are coated with a suitable conductive material. This means that at least the areas of the connections and those island structure surfaces that require electrical connectivity are covered with the first electrode 3.

[0178] This configuration already provides an electrical structure, since the basic structure, namely the metallic auxetic SMA structure (with the shape memory alloy) 11, 12, 13, serves as a common ground electrode, and the newly applied electrode 3 represents the additional conductor. The SMA auxetic backbone itself then acts as the common ground electrode for connecting electrical components.

[0179] In step d) the electronic components 41, 42 are then assembled, which can be of a different nature and can in particular be interconnected in combination to form a large system.

[0180] In step e), the final step in this example involves coating the entire structure with a biocompatible material. For example, this could be a biocompatible polymer that protects the human body from electronics. The area of ​​the metallic auxetic SMA structure can be left out, as it is biocompatible.

[0181] It should be noted here that, through sensible structuring, it is not necessary to provide all areas of the auxetic SMA structure 11, 12, 13 with corresponding layers and / or components. Instead, there can be empty island structure areas and connections that serve a supporting function but do not support electrical components or conduct electrical current. This differs from the methods described in US 2020 / 0144431 A1, where a semiconductor device had to be structured with serpentine connections on every island structure area and every serpentine of an auxetic lattice structure.

[0182] Not shown in detail, but also possible, is to position the first electrode above the electrical components or electronic parts, so that an electrically insulating layer is first built up on the auxetic SMA structure, followed by the electronic components or parts in general, and then another electrically insulating layer, followed by the electrode. Additional conductors can also be built on top of each other, each separated by electrically insulating layers. Design-related deviations from this are, of course, possible.

[0183] Once all MEMS components have been fabricated, the SMA is heated above its martensitic phase transition temperature (typically body temperature ~37°C) to restore the shape previously established during annealing. If the component is intended as a medical implant, it can now be crimped for insertion into a microcatheter. If a flat 2D shape was chosen, the functionalized stretchable / flexible component is ready to conform to the underlying substrate (e.g., the skin of a wearable e-tattoo or biosensor). At this stage, the functionalized devices can optionally be embedded (or coated) in a biocompatible polymer if required for the application.

[0184] Various thin-film processes known in the prior art can be used to apply the different functional layers. Sputtering, especially magnetron sputtering, is particularly suitable. However, other thin-film processes are also viable, such as sol-gel or vapor deposition. Reference is also made to US patent 8,758,636 B2, which discloses a process for manufacturing a medical functional element with a self-supporting lattice structure. This patent specifically describes the production of special components made of a shape-memory alloy with a thin film, intended for use in stent devices.

[0185] Fig. 22 shows the from Fig. 21Section d) known structure, supplemented by at least one additional electrode 6. This electrode 6 can additionally perform further functions of the electronic components, for example communication or additional energy transmission, or the like. Further electrodes can also be provided, which can then connect different electronic components to each other.

[0186] Complex network structures can be built.

[0187] In Fig. 23 A first exemplary embodiment of an implantable structure according to the invention in the form of a tube, which is inserted into an artery, is described using a modified reentrant auxetic geometry, as in Fig. 20 already introduced, shown.

[0188] Fig. 23aFigure 1 shows a reentrant auxetic arrow island structure in a 3D cylindrical geometry with low-stress island structure surfaces (in the shape of arrows) that are equipped with MEMS components according to the diagram in Figure 2. Fig. 20 The presented process flow has been functionalized.

[0189] This embodiment can be used as a smart stent, with the 2D cutout in Fig. 23b three different electronic components (represented by a square, a circle and a triangle) are shown mounted on the arrow-shaped island structure surfaces of the modified low-voltage auxetic structure. Fig. 23cFigure 1 shows the component that is inserted into the artery. The auxetic framework of the smart stent enables conformal contact with the non-linearly curved surface of the artery. Next-generation medical implants must be equipped with a range of electronic devices that measure vital patient data and transmit it to the physician in real time. A functional cardiovascular stent, for example, could monitor a patient's blood pressure and many other vital parameters from home and then wirelessly transmit the data to the hospital or a physician.

[0190] Auxetic shape-memory alloys would be particularly attractive for use in medical implants and stents in arteries of the cardiovascular and neurological systems, as the implants could be delivered to the patient via non-invasive endovascular procedures. The negative Poisson's ratio allows auxetic structures to change their shape and adapt to the surrounding surface, the complex arteries and veins of the body, as demonstrated in [reference to specific example]. Fig. 23c) depicted. After a stent has been placed in a patient's body for several years, it can become trapped by large and small blood clots. Over time, this can lead to a complete blockage of the arteries and cause serious and life-threatening medical complications such as a pulmonary embolism, a heart attack, or a stroke. The accumulation of the body's own material (e.g., tissue, cells) on the stent could be monitored with a force sensor in a smart auxetic stent. An antenna could transmit a signal to notify the physician when a critical threshold is reached. The functional stent could also be self-powered if a piezoelectric energy collector is integrated into the island or backbone of the auxetic structure.

[0191] Furthermore, it is possible to directly manufacture MEMS and NEMS components to equip the island structure surfaces of the auxetic structure with integrated sensors and readout components.

[0192] As another example, the invention disclosed here could represent a platform for integrating components such as cameras, lights (LEDs), and sensors for guiding, positioning, and tracking the stent position during placement. With further innovations of this invention, this could be a method that, in the future, eliminates the need for MRI or X-ray imaging for stent placement. In addition to adapting to the shape of the body, the auxetic structure can also be designed to achieve a compromise between the areal density of the unit cell and the maximum volume expansion of the entire structure. For example, changes in blood flow (pressure) in some arteries can be controlled by designing the areal density of the auxetic structure (i.e., large vs. small island structure areas or large vs. small openings between the island structure areas) in its deployed state.Changing the device's surface density to restrict blood flow to an aneurysm is desirable, for example, in the case of a flow diverter stent. Furthermore, non-invasive measurements of medical data, such as blood pressure, could be wirelessly transmitted to the physician if the corresponding sensor elements are functionalized on the low-stress island structure surfaces. These island structure surfaces can also be equipped with radiopaque markers that detect rotation of the stent or device, allowing the devices to be inserted at a specific position and angle, thus enabling easy placement of the functional islands precisely where needed.

[0193] The high tensile stresses associated with the superelastic effect enable auxetic shape memory materials to offer extremely high strength, high work performance, and high actuation density. Auxetic stent structures have already been shown to exhibit radial forces at the stent circumference that are 10 times higher than current stent designs. This means that thinner SMA devices can be manufactured that are easier to compress, maneuver, and deploy within the microcatheter.

[0194] The combination of flexibility and strength offered by auxetic stents made of shape-memory alloys could also be useful for other medical applications requiring the temporary or permanent opening of a narrow passage. These include stents that require greater radial forces than those needed for the brain, for example, for use in the esophagus, urethra, and prostate.

[0195] Due to the aforementioned properties of auxetic shape-memory materials, this invention is suitable for functionalizing the exterior of surgical instruments or for other temporary medical implants. For example, a flexible / stretchable 3D implant with integrated CO₂ / O₂ sensors, cameras, LEDs, and pressure sensors could be a useful aid for surgeries / procedures involving the insertion of tubes into the patient's airway, as described by Ullah, Ramzan, et al. in "Real-Time Optical Monitoring of Endotracheal Tube Displacement." Biosensors 10.11 (2020): 174. If such a device were wrapped around the exterior of an endotracheal tube, for instance, it could assist physicians / respiratory therapists in inserting the tube more quickly and accurately. The cameras / LEDs help locate the correct position in the trachea for placement (2 cm - 5 cm above the carina).The CO₂ / O₂ sensors integrated into the outside of the endotracheal tube (i.e., directly on or above the cuff) could help detect air leakage (indicating misplacement of the device in either the lungs or esophagus). The current method for verifying correct tube placement relies on X-ray imaging. Integrated pressure sensors in the cuff could help the clinician monitor the pressure applied to the patient while inflating the cuff (balloon) to secure the endotracheal tube. This is just one example of how an auxetic medical SMA implant could be used in conjunction with existing surgical instruments.

[0196] Like the previously mentioned problems with stent placement, confirming the placement of intubation tubes often requires an X-ray, which is both time-consuming and expensive. With the right arrangement (co-integration) of several functional devices, the medical implant disclosed in this invention could eliminate this imaging step in the future.

[0197] The treatment of brain aneurysms also requires a miniaturized medical device that can be used non-invasively with current endovascular treatment methods, e.g. with a catheter.

[0198] Flow diverter stents, coiling, and woven endobridges (WEBs) are common methods for treating aneurysms. While most types of cerebral aneurysms can be treated with flow diverter stents, there are currently no suitable stent-based intrasaccular devices for treating geodesic dome aneurysms. Stents also cannot be used for bifurcation aneurysms because they can obstruct blood flow in an artery, potentially leading to a stroke. The most common method for treating this type of aneurysm is to physically occupy the space (e.g., with potassium phosphate coils) to restrict further blood flow into the aneurysm. However, a significant side effect of this treatment is that the potassium phosphate coils introduce noise, preventing accurate measurement of blood flow within the aneurysm using conventional MRI techniques.The auxetic design according to the invention is suitable for offering the same advantages as a web-based device, as well as additional advantages, since the auxetic design allows more freedom in determining the size of the component by photolithography than a conventional mesh design. Furthermore, the low-stress island structure surfaces of the auxetic structure according to the invention are the only type of intrasaccular medical device that is functionalized and allows for the direct reading of information relating to the healing process, such as blood flow in the aneurysm, device failure, or the reduction of the body's response, such as inflammation, to the device.

[0199] The disclosed invention is a unique type of device that overcomes the limitations of modern treatment for intrasacular aneurysms. The modified stretchable auxetic structure according to the invention exhibits synclastic (conformal) bending around dome-shaped curves (as in the embodiment shown in Figure 1). Fig. 7 (shown). This type of bending behavior is extremely difficult or impossible to achieve with current stent technologies. The use of the structure according to the invention enables conformal bending around a dome shape, which is required for intravascular devices.

[0200] The synclastic bending properties of auxetic SMA implants could also be beneficial for improving surgical instruments. Because the auxetic substrate can adapt to the shape of its surroundings, it could wrap around the exterior of existing surgical instruments. This could improve surgical outcomes, as surgeons could equip their instruments with new features that were previously unavailable (e.g., lights, cameras, sensors). Integrating sensors onto the island structure surfaces would make it possible to non-invasively monitor blood pressure or blood flow inside an aneurysm using this device. Non-invasive blood flow monitoring without MRI imaging could be revolutionary for the treatment of cerebral aneurysms.Furthermore, the use of a dome-shaped SMA device inside the aneurysm would be safer for the patient, as it would shorten the operating time and increase patient safety through reduced anesthesia time, radiation and contrast agent exposure.

[0201] Fig. 24 shows a second embodiment of an "S-shaped island structure surface" auxetic geometry, modified to provide island structure surfaces for the integration of devices from conventional "S-shaped" auxetic structures, represented as a 2D surface ( Fig. 24A ), as an excerpt ( Fig. 24B ) and as a 3D intelligent stent ( Fig. 24CFigure 22B also shows that three different types of functional components are integrated onto the substrate. Conventional "S-shaped" auxetic structures are known from Meena, Kusum, and Sarat Singamneni. "A new auxetic structure with significantly reduced stress concentration effects." Materials & Design 173 (2019): 107779.

[0202] In Fig. 25 are the results of finite element simulations of the exemplary embodiment of an "S-shaped island structure surface" auxetic structure made of Fig. 24 with the structure in equilibrium ( Fig. 25A ) and under uniaxial tensile force (y-direction) ( Fig. 25B ) shown. It is demonstrated that the modified auxetic structure retains its auxetic behavior.

[0203] Fig. 26The "S-shaped island structure surfaces" exhibit auxetic geometry, which shows synclastic bending when formed into a 3D sphere. This suggests a possible use of the structure as an intrasaccular aneurysm device.

[0204] Applications of the technology, in particular the substrate structure, as disclosed herein, include: Functionalized medical devices and implants (conformal skin sensors, stents, percutaneous heart valve repair, intrasaccular devices, expandable / collapsible endotracheal tube, functional inferior vena cava (IVC) filter); space applications (deformable lightweight space applications, i.e., substrate structures that allow more functionalization on rovers and spacecraft, deployable solar sails); energy harvesters integrated directly into car and bicycle tires (e.g., piezoelectric elements); flexible / stretchable LEDs, displays, and cameras (individual pixels / sensors integrated onto island structure surfaces); general structures for wearable and stretchable electronics; stretchable electrodes for soft actuators; functionalized means of transport (e.g., indoor / outdoor aircraft, spacecraft, boats, hot air balloons); functionalized outdoor equipment (e.g.,Solar-heated sleeping bag and tent; impact-resistant energy absorbers for vehicles; architectural buildings (structures likely able to bend around any shape); auxetic clothing and functionalized garments (e.g., compression socks, shirts, pants, shoes, backpacks, tourniquets); shock-resistant military equipment (e.g., bulletproof vest).

[0205] The structures revealed here can be used in the same applications as conventional auxetic structures. In particular, the areas of application include: Biomedicine: Arterial dilator / stent, drug-eluting stent, stent-supported coiling, smart wound dressing, tissue engineering (i.e., artificial skin), artificial blood vessels, wound pressure pads, surgical devices; Aerospace: Blades for gas turbine engines, thermal insulation, aircraft noses, wing fairings, acoustic vibration rivets; Automotive: Bumpers, cushions, thermal protection, vibration dampers; Sensors / Actuators: Wave propagation, vibration / damping structures, smart strain sensors, hydrophone, piezoelectric components; Military / Defense / Sports: Impact / energy-absorbing military equipment with integrated sensor components (e.g., bulletproof vest, helmet, knee pads, etc.).

[0206] When the auxetic bridge or field structure according to the invention is used for functionalized medical devices and implants, it must be able to withstand extreme demands. Biosensors, also called wearable devices, for example, which can be integrated directly onto the skin or a substrate worn close to the skin, must be able to adapt to extreme movements and the resulting changes in the body's radius of curvature. Stretchable devices can be fabricated using serpentine structures, mesh structures, and island bridge structures. When the auxetic structures shown here are made of shape memory alloys, they offer many advantages over the stretchable designs for stretchable electronics known in the prior art. A platform for an extremely compliant and deformable substrate is presented here to enable the development of novel stretchable thin-film devices.Auxetic substrates made from a shape memory alloy offer superior mechanical properties compared to the state of the art for flexible / extensible designs based on copper serpentines in island-bridge configurations. Shape memory alloys have superior mechanical properties compared to copper, enabling them to elastically recover from extremely large applied strains (theoretically up to > 8% elasticity).

[0207] There are many applications for shape memory / piezoelectric composites in various energy harvester and microactuator applications. For example, sensitive magnetoelectric sensors based on piezoelectric and magnetostrictive materials can be integrated onto shape memory alloys. Miniaturized magnetoelectric composites are needed for applications such as biomagnetic sensors, antennas, energy harvesters, and surface acoustic wave (SAW) sensors. In principle, all auxetic sensor concepts can be powered by body movement or blood flow itself when paired with a piezoelectric (or magnetoelectric) energy harvester. The selective fabrication of AIN components on low-voltage islands can enable the development of sensitive magnetoelectric sensors for the biomagnetic detection of delicate signals originating from the heart and brain.Such magnetoelectric sensors would also be useful for applications such as deep brain stimulation.

[0208] The unique properties of shape memory alloys can increase the mechanical strength / robustness of a stretchable circuit while simultaneously extending the device's lifetime. Since SMAs exhibit significantly greater intrinsic strain compared to traditional metals like copper, enormous global strains can be achieved through SMA Archimedean spiral connections and / or self-similar fractal design connections. This enables structures with higher areal density, allowing for the integration of more active circuit components and thus enabling more efficient stretchable devices. Furthermore, the higher areal density structure allows for the realization of more complex integrated circuits.By developing the composition and applying the right heat treatment, nickel-titanium-based shape memory alloys, such as TiNiCu or TiNiCuCo, can be designed so that the phase transformation is reversible for up to 10 million cycles. Therefore, the implementation of auxetic substrates made from shape memory alloys would be extremely useful for enabling new wearable and deformable electronic applications.

[0209] The auxetic structure improves the adaptability of the electronic device to the skin or wearable substrate, thus preventing premature failure due to delamination issues. Improved skin contact enables more effective sensors that measure sensitive biomagnetic signals. Suitable wearable sensors for magnetoencephalography (MEG), magnetocardiography (MCG), electroencephalography (EEG), and electrocardiogram (ECG) are possible by fabricating active sensor components on the island structure surfaces of the auxetic structure. Ultrasound sensors integrated into the structure would also allow the wearable device to "see" beneath the skin. These types of designs are also suitable and preferred for electronic skin, soft electronics, exoskeletons, and other soft robotics applications.If the application requires it, it is also possible to electrically isolate island structure areas from each other. Similarly, island structure areas could, in particular, preferably share a common ground electrode through the SMA.

[0210] Not every auxetic island structure surface requires an active electronic component. Furthermore, the electronic component need not be a semiconductor device. For example, an implantable structure could be designed with a sensor on one island structure surface, an actuator on another, and a transistor (or any other type of electronic component) on yet another. A significant advantage is that the low stress on each island structure surface allows for virtually any type of active or passive device, and these can be electrically interconnected across multiple island structure surfaces, thus providing a large surface area for such components. Moreover, many other types of materials and devices could now become "stretchable" (i.e., flexible).The material does not have to be a semiconductor (as is the case, for example, in US Publication 2020 / 144431 A1). Essentially, any type of component can remain in its original, traditional, rigid structure.

[0211] The following are the reference symbols used in the figures: 1 Island structure area of ​​an SMA auxetic structure 11 First island structure area of ​​an SMA auxetic structure 12 Second island structure area of ​​an SMA auxetic structure 13 Connection of an SMA auxetic structure 2 Electrically insulating material 3 First electrode 41 First electronic component 42 Second electronic component 43 Third electronic component 5 Biocompatible inclusion 6 Supplementary electrode 7 Substrate

Claims

1. Auxetic web structure or field structure comprising: - island structure areas and - interconnections between the individual island structure areas, wherein - there are open spaces between the individual island structure areas, and - the connections with the island structure areas form a web structure or field structure; wherein - the connections are extensible and are configured as Archimedean spiral connections and / or self-similar fractal design connections; - the connections intersect the island structure areas at the same location and at the same angle as the previous rigid connectors intersect the auxetic framework; - the island structure areas are not bendable or are only slightly bendable when subjected to forces, - wherein the stresses on the island structure areas are at least one order of magnitude below those of the extensible connection and / or - are not or only insignificantly variable in their magnitude and - the Poisson's ratio of the structure under uniaxial deformation is negative.

2. Auxetic web structure or field structure according to claim 1, wherein the individual island structure areas and / or the individual connections are designed to vary in size and shape.

3. Auxetic web structure or field structure according to claim 1 or 2, wherein at least in sections over the auxetic web structure or field structure, areas are formed identically and / or periodically.

4. Auxetic web structure or field structure according to one of the preceding claims, wherein the self-similar fractal design connections comprise Koch's lines, Peano's lines, Hilbert's lines, Moore's loops, Vicsek's loops and branched meshes.

5. Auxetic web structure or field structure according to any one of the preceding claims, wherein the auxetic web structure or field structure is planar, 2D fabricated and subsequently adapted to 3D surfaces.

6. Auxetic web structure or field structure according to any one of the preceding claims, wherein at least individual island structure areas for receiving electronics or individual island structure areas equipped with electronics are provided.

7. Auxetic web structure or field structure according to the preceding claim, wherein the electronics located on the island structure areas are electrically connected to each other via at least one conductor track electrically insulated from the auxetic web structure or field structure on at least one connection of the auxetic web structure or field structure.

8. Use of the auxetic web structure or field structure according to any one of the preceding claims as an implantable structure made of a biocompatible material, wherein the biocompatible material is a metallic shape memory alloy and the auxetic web structure or field structure is metallic.

9. Use of the auxetic web structure or field structure according to the preceding claim, wherein the implantable structure has a self-expanding implant shape with a cylindrical and / or spherical and / or hemispherical and / or tubular and / or curved tubular structure at least in sections.

10. Use of the auxetic web structure or field structure according to one of the two preceding claims, wherein the auxetic web structure or field structure is provided in the form of a common ground electrode.