Methods of manufacturing and devices

Abstract

The present invention provides methods of manufacturing devices with a multilayer structure in which a layer of the multilayer structure is treated with a laser and devices in which one of the layers has been treated with a laser. The invention also relates to devices manufactured by the methods. In some embodiments the invention provides a method of manufacturing a device, the method including the steps of: forming a multilayer structure having a first layer in contact with a second layer, wherein the first layer is formed of a conductive or semiconductive material and has a thickness of no more than 1 pm, and the second layer is formed of an insulating material; and selectively removing, by laser ablation, one or more portions of the second layer to form a pattern in said second layer without damaging the first layer. In other embodiments the invention provides a method of manufacturing a device, the method including the steps of: forming a first layer of a first material; forming a second layer of a second material, different to the first material, on the first layer; and selectively treating, using a laser, one or more portions of the second layer to form a pattern in said second layer, wherein: the laser ablation uses a pulsed laser having a pulse length of no more than 1 picosectond; and the laser that has a beam divergence of at least 10 degrees, preferably at least 12 degrees.

Description

[0001] METHODS OF MANUFACTURING AND DEVICES

[0002] The present invention relates to methods of manufacturing. The methods are of particular, but not exclusive, relevance to manufacturing multi-layer devices, for example medical devices including implantable medical devices. The invention also relates to devices manufactured by the methods.

[0003] Flexible electronic devices have been developed for a wide range of applications from sensors to thermoelectric devices to displays to wearable and implantable devices for healthcare. Similar devices have been developed which have fluidic components (with or without accompanying electronic components).

[0004] Such devices normally consist of alternating layers of materials. For example, in electronic devices these may be alternating layers of insulating and conducting materials. The layers are typically patterned to create one or more individually addressable electronic components such as an electrode, photodetector or transistor as well as connectors between those components.

[0005] Manufacturing techniques that enable patterning of the materials in flexible devices include photolithography, ink jet printing, shadow masking, and screen printing among others. However, these processes are generally limited to only patterning on flat, 2D surfaces. Furthermore, lithographic processes require the relevant “masks” to achieve the patterning to be designed and produced in advance of manufacturing, which can significantly increase the time, complexity and cost of the design process for new devices. The cost and time to design and produce masks also makes it difficult to prototype and iteratively adapt designs.

[0006] Lithographic processes also typically involve toxic chemicals, both for the deposition of photoresist layers and the solvents used to remove such layers. This can cause problems where the final device is intended to be biocompatible, as well as requiring safety protocols to be adopted during manufacture and for safe disposal of such chemicals.

[0007] Ultrafast lasers have also been used to cut, mill, and ablate insulating and conducting materials. Such laser-based processes are in many cases faster than other techniques while also offering potential to account for changes in the z-axis (i.e. out of plane) while patterning, for example on a surface with curvature or significant roughness. Typical laser machining processes use nanosecond or longer laser pulses. These are easily achievable but cause large amounts of local heating effects which can damage surrounding materials.

[0008] Typical laser machining processes also have relatively large spot sizes (typically >20|im). This means that a high laser power is needed to achieve significant energy density at the sample. This is not an issue for cutting most materials; the laser power can simply be increased until the material is cut through. Moreover metals absorb nearly 100% of the input laser energy due to their electronic band structure and so they can be readily cut with such processes. However, the spot size places a limit on the minimum dimensions of features in the pattern.

[0009] The optics in standard machining processes are generally designed to produce a relatively low beam divergence. This is good for cutting through materials as the beam stays almost in focus throughout the depth of the material. However, it makes it very difficult to precisely machine multi-layer components as it is hard to control the depth of the beam effects and therefore very difficult to cut a pattern in one layer of a multilayer device whilst leaving the layer beneath undamaged.

[0010] Most standard laser processing is carried out with laser wavelengths >500nm. However, many polymers, particularly transparent polymers, barely interact with light at these wavelengths. Again, this is generally not an issue for pure cutting applications; the laser power can be increased until the polymer absorbs enough to be cut, and there is no need to worry about any sacrificial carrier layer(s) underneath. However, it is not practical if the intent is to pattern the top layer whilst leaving the lower layers undamaged.

[0011] In some processes, UV lasers are used which do interact with many of these polymers, but an underlying metal layer will absorb significantly more strongly than the polymer layer and so end up being damaged if sufficient power is provided to ablate the pattern in the polymer layer.

[0012] Long-term stability is another critical requirement for such devices, particularly where they are intended for implantation in a human or animal body, as device degradation can compromise signal fidelity, biocompatibility, and overall functionality in clinical applications. For example, flexible thin-film polyimide / platinum (Pl / Pt) electrodes are widely used due to their mechanical compliance and conductivity, but they can suffer from adhesion issues, delamination, and electrochemical instability over time. Addressing these challenges is important to ensure reliable, long-lasting neural implants.

[0013] One area of application for implantable devices is in the provision of neural interfaces. A neural interface refers to a system that facilitates communication between the nervous system and external electronic circuits. This integration has become an important area of research aimed at enabling seamless interaction between biological systems and electronic devices. The development of neural interfaces began in the 1960s with the creation of the cochlear implant, the first successful device to restore hearing through electrical stimulation of the auditory nerve [1],

[0019] . In the 1980s, brain-computer interfaces (BCIs) were developed, allowing direct communication between the brain and non-biological signal circuits. These early breakthroughs paved the way for further advancements, including the use of neural interfaces in the treatment of Parkinson's disease in the late 1980s, where deep brain stimulation (DBS) has been employed to alleviate motor symptoms

[0019] ,

[0023] . Throughout the 2000s, advances in soft biocompatible materials and conductive polymers led to the adoption of flexible polymers in neural interfaces. These soft neural interfaces offer several advantages over conventional rigid devices, including flexibility, better conformability to, and enhanced stability [2],

[0014] Today, cutting-edge applications focus on the unique features enabled by these soft materials, such as shape actuation and origami / kirigami designs, which allow for complex, dynamic and even personalised performances [2], [7]. Additionally, the integration of closed-loop control systems has become a significant advancement, enabling real-time feedback and adaptation between the neural interface and the biological system for more precise and effective interventions [4], [8]-[9]. These innovations enable new possibilities for neural interfaces in the treatment of neural disorders and spinal cord injuries.

[0015] However, current neural interface devices still face challenges of long-term stability, particularly during continuous recording and stimulation. Over the time, the materials used in these devices will delaminate and degrade, which reducing the performance and ultimately leading to unfunctional devices. Therefore, how to ensure these thin-film devices are still functional after years of recording or stimulation has become a key area of research

[0011] ,

[0021] , The present invention aims to solve one or more of the above problems.

[0016] Aspects of the present invention aim to provide methods of manufacturing devices that can pattern multilayer devices without damaging the underlying layer(s). In particular, aspects of the present invention aim to provide methods of manufacturing devices that can pattern an insulating layer formed on top of a conductive or semiconductive layer without damaging the underlying layer.

[0017] Aspects of the present invention aim to provide methods of manufacturing devices that are rapidly adaptable and thus allow easy and inexpensive prototyping and iterative design processes.

[0018] Aspects of the present invention aim to provide methods of manufacturing devices that provide for strong adhesion between layers of the device, thus increasing the functional life of the devices.

[0019] At their most general, aspects of the present invention provide methods of manufacturing devices with a multilayer structure in which a layer of the multilayer structure is treated with a laser and devices in which one of the layers has been treated with a laser.

[0020] At their most general, other aspects of the present invention provide methods of manufacturing devices with a multilayer structure in which a surface of one layer in the device is treated with a laser so as to improve the adhesion of a second layer to that surface and devices in which this treatment has been performed.

[0021] A first aspect of the present invention provides a method of manufacturing a device, the method including the steps of: forming a multilayer structure having a first layer in contact with a second layer, wherein the first layer is formed of a conductive or semiconductive material and has a thickness of no more than 1 pm, and the second layer is formed of an insulating material; and selectively removing, by laser ablation, one or more portions of the second layer to form a pattern in said second layer without damaging the first layer.

[0022] In the present application, “without damaging” preferably means that the first, conduct! ve / semiconductive layer remains functionally intact, for example that it is able to conduct in a similar fashion (i.e. as effectively) as it would have done prior to the laser treatment.

[0023] Preferably the laser will not remove any of the first layer when removing the second layer. However, in practical terms it is likely that an ablation process will, in certain situations, remove small amounts of the first layer. Such ablation which removes small amounts of the first layer is explicitly included in the above aspect and other aspects of the invention provided that the functionality of the first layer is retained.

[0024] For example, and without limitation, removal of no more than 100 nm, preferably no more than 50 nm, more preferably 10 nm and more preferably no more than 5 nm of a 1 pm thick first layer is likely in most situations to mean that the functionality of the first layer is retained.

[0025] Alternatively, for layers of other thicknesses, removal of no more than 10%, preferably no more than 5%, more preferably no more than 1% and more preferably no more than 0.5% of the first layer is unlikely to affect the functionality of the first layer.

[0026] The method may also include the step of removing, by laser ablation, one or more portions of the first layer. This removal is typically performed prior to the forming of the second layer, but may be done afterwards (for example, if the layers are formed on a substrate and subsequently removed before the laser ablation is performed on the first layer).

[0027] In certain embodiments, the first layer is formed on a substrate. The method may further include the step of removing the device from the substrate after the patterning is completed.

[0028] Preferably the final device (for example when removed from the substrate, if any) is flexible. This may mean that the final device has a bend radius of no more than about 1 cm, preferably no more than 100 mm. In certain embodiments the final device has a bend radius of no more than about 10 mm, no more than about 5 mm, or no more than about 1 mm. In embodiments of particularly flexible devices, the final device may have a bend radius of not more than 100 pm. The indicated bend radii of the final device are in at least one direction of the device (and, in the case of substantially planar devices, in a direction lying in the plane of the device). Preferably the bend radii are in more than one direction, for example, in the case of substantially planar devices, in all directions lying in the plane of the device. Bend radius, which is measured to the inside curvature, is the minimum radius that a component (in this case the device) can be bent in at least one direction without damaging it. The bend radius as defined here refers to elastic deformation as opposed to plastic deformation such that a device bent under an applied force to a radius greater than the minimum bend radius would return at least part way to its original shape with the removal of the applied force. In other words, in these embodiments, the device of this aspect can be bent to an inside curvature of 10 mm, for example by rolling when a device is being arranged for deployment, for example, in the case of an implantable device, for insertion into a patient, and subsequently deployed (e.g. unrolled) to an expanded, less bent configuration (e.g. a substantially planar configuration) and still function exactly as it did prior to bending.

[0029] In certain embodiments, the thickness of the first layer is no more than 500 nm.

[0030] In certain embodiments the method may further include the step of cutting through all layers to define the external dimensions of the device. This cutting may be performed by the same laser that performs the ablation step(s) or by a different laser. In such embodiments, a plurality of devices may be prepared with one or more of: a common substrate, common layers and / or common patterning, and then separated from each other after patterning. This can significantly improve the throughput and efficiency of the production.

[0031] In certain embodiments the pattern formed by the laser ablation includes at least one feature with a minimum dimension of no more than 5 pm, preferably no more than 2 pm. Features in the pattern may be, for example, elements of a electrical circuit (tracks, connectors, contact, electrodes, etc.) or holes, vias or other openings in a layer.

[0032] The method may include further steps of forming one or more further layers sequentially on the second layer and selectively removing, by laser ablation, material from one or more of the further layers. In this way a multi-layer device can be built up and patterned as each layer is added to create a composite device which may have significant embedded functionality.

[0033] In certain embodiments, the further layers alternate layers of insulating and conductive material. In this way a multi-layer electronic device can be assembled with each conductive layer insulated from other conductive layers, except where interconnection is desired through vias in the insulating layer(s). In certain embodiments the laser ablation is performed so that it changes the surface properties of either the first or the second layer. For example, the laser ablation may be used to modify the surface roughness of a layer prior to the application of a subsequent layer.

[0034] In certain embodiments, the change in surface properties causes changes in the hydrophobic or hydrophilic nature of one or more of the layers. This can be advantageous if, for example, fluid channels are being formed in the device, for example to form a microfluidic device.

[0035] In certain embodiments the change in surface properties causes a change in the adhesion properties between one or more of the layers. For example, this can provide for improved adhesion between layers and thus reduce or avoid the need for separate adhesive. Alternatively, this can provide for reduced adhesion between layers in certain portions of the device, thus allowing for the creation of cavities in a flexible device, which may be advantageous for example for shape-actuation.

[0036] With the high-precision laser treatment of this method, structures and devices which have a three-dimensional footprint (i.e. more than the thickness dimension of a typical laminate structure) can be patterned and treated. Thus, in certain embodiments, the device or the multilayer structure of the device is larger in all directions than the combined thickness of the first and second layer in all directions. For example the device may be cylindrical, whether rounded or polygonal, a prism, cuboid or other regular or irregular three-dimensional structure (or formed on an appropriately-shaped substrate).

[0037] Alternatively or additionally, the method may be used to pattern and / or treat layers which do not have a perfectly laminar structure. For example the first layer and / or the second layer may have one or more projecting features which extend in the thickness direction of the layers. In such cases the step of removing may include selectively removing portions of one or more of the layers from locations on the projecting features and / or from locations not on the projecting features.

[0038] In certain embodiments the device manufactured by the method of this aspect is implantable. This can be achieved by appropriate selection of the materials from which the layers of the device are formed. The characteristics of the laser used for the ablation steps are preferably specifically chosen so as to permit ablation of the insulating layer without damaging the thin conducting layer underneath.

[0039] In certain embodiments the ablation is performed using a pulsed laser having a pulse length of no more than 1 picosecond. Such lasers are sometime referred to as “femtosecond” lasers as their pulse lengths are measured in femtoseconds.

[0040] In certain embodiments the laser has a beam divergence angle of at least 10 degrees, preferably at least 12 degrees. A high beam divergence angle allows for a highly focused beam to be formed, particularly in relation to the depth of the focal zone, and so is useful for reducing the effects of the laser on material adjacent to the material which is desired to be removed by the ablation.

[0041] In certain embodiments the ablation is performed using a laser that delivers a power density of at least ImW / pm2, preferably between I mW / pm2and lOOmW / pm2.

[0042] A second aspect of the present invention provides a method of manufacturing a device, the method including the steps of: forming a first layer of a first material; forming a second layer of a second material, different to the first material, on the first layer; and selectively treating, using a laser, one or more portions of the second layer to form a pattern in said second layer, wherein: the laser ablation uses a pulsed laser having a pulse length of no more than 1 picosecond; and the laser that has a beam divergence of at least 10 degrees, preferably at least 12 degrees.

[0043] The characteristics of the laser used for the ablation have been discovered to permit precise treatment of the second layer. In particular, even where the layers involved are thin (for example 1 pm or less), and / or the second layer is not particularly absorbent of laser radiation (and / or is relatively less absorbent of the laser radiation than the first layer), treatment of the second layer without damaging the first layer underneath can be achieved.

[0044] In certain embodiments the ablation is performed using a laser that delivers a power density of at least ImW / pm2, preferably between ImW / pm2and lOOmW / pm2.

[0045] In certain embodiments, the treating may be used to form channels in the second layer. In certain embodiments the treating may be used to cause a change in one or more properties of the first and or second layers.

[0046] For example, the treating may change the solubility of the treated portions of the second layer and the method may then further include the step of removing the treated portions by dissolution.

[0047] In other examples, the treating may change the adhesion properties between the first and second layers and / or change the hydrophobic or hydrophilic properties of the first and / or second layers.

[0048] A third aspect of the present invention provides a method of manufacturing a device, the method including the step of selectively removing, by laser ablation, one or more portions of material from a layer of material in the device to form channels in said layer, such that the channels are contained within the layer.

[0049] By using a laser (for example, a laser having one or more of the characteristics set out in relation to the first and second aspects above in relation to pulse length, beam angle and / or power density), it has been found that channels can be created within layers of material in a device, without affecting the material in that layer above and below the formed channel. This can be achieved even with relatively thin layers (e.g. no more than 20 pm thick, 15 pm thick or 10 pm thick).

[0050] A fourth aspect of the present invention provides a device formed by the method of any of the above aspects, including some, all or none of the optional and preferred features of those aspects.

[0051] A fifth aspect of the present invention provides a device having: a first layer formed of a conductive or semiconductive material and having a thickness of no more than 1 pm and a second layer formed of an insulating material formed on the first layer, wherein one or more portions of the second layer have been selectively removed, by laser ablation, to form a pattern in said second layer without damaging the first layer. Preferably the device is flexible. For example the device may have a bend radius in one or more directions which is as set out in relation to the first aspect above.

[0052] In certain embodiments the pattern formed by the laser ablation includes at least one feature with a minimum dimension of no more than 5 pm, preferably no more than 2 pm. Features in the pattern may be, for example, elements of a electrical circuit (tracks, connectors, contact, electrodes, etc.) or holes, vias or other openings in a layer.

[0053] In certain embodiments the device includes at least one further layer formed on the second layer, wherein a pattern is formed in at least one further layer by selectively removing, by laser ablation, material from the further layer. In this way a multi-layer device can be built up and patterned as each layer is added to create a composite device which may have significant embedded functionality. The same laser may be used for patterning each layer.

[0054] In certain embodiments, the further layers alternate layers of insulating and conductive material. Thus the device can be multi-layer electronic device with each conductive layer insulated from other conductive layers, except where interconnection is desired through vias in the insulating layer(s).

[0055] A sixth aspect of the present invention provides a method of manufacturing a device having a multilayer structure, the method including the steps of: providing a first layer; treating a first surface of the first layer by irradiating the first surface with a pulsed laser have a pulse length of no more than 1 picosecond to increase the adhesion properties of the first surface; and forming a second layer on the treated first surface.

[0056] By increasing the adhesion properties of the first surface, a better adhesion between the first and second layers may be obtained. This can result in the production of multilayer devices which have an improved functional life as they may be less prone to delaminate and / or degrade.

[0057] In certain embodiments the treating of the first surface changes the chemical properties of the first surface and / or increases the surface roughness of the first surface and the adhesion between the first layer and the second layer is increased by the changed chemical properties and / or the increased surface roughness. A seventh aspect of the present invention provides a method of manufacturing a device having a multilayer structure, the method including the steps of: providing a first layer; treating a first surface of the first layer by irradiating the first surface with a laser, wherein the laser treatment changes the chemical properties of the first surface and increases the surface roughness of the first surface, forming a second layer on the first layer, wherein the adhesion between the first layer and the second layer is increased by the changed chemical properties and the increased surface roughness.

[0058] By increasing the adhesion properties of the first surface, a better adhesion between the first and second layers may be obtained. This can result in the production of multilayer devices which have an improved functional life as they may be less prone to delaminate and / or degrade.

[0059] In certain embodiments the laser is a pulsed laser having a pulse length of no more than 1 microsecond, preferably no more than 10 nanoseconds, preferably no more than 100 picoseconds, more preferably no more than 10 picoseconds and, in some embodiments, no more than 1 picosecond.

[0060] Short pulse length laser surface processing (LSP), with pulse lengths ranging down to the femtosecond range (i.e. less than 1 picosecond), has emerged as a promising technique to enhance the durability of these interfaces by modifying surface morphology without thermal damage. By increasing surface roughness and promoting Pt-0 bonding, LSP can significantly improves metal-polymer adhesion, reducing the risk of delamination. Additionally, the present inventors have shown, through electrochemical characterization, that LSP-treated electrodes exhibit reduced impedance and enhanced charge transfer, critical for efficient neural signal transmission.

[0061] Accelerated aging tests further confirm that LSP-treated electrodes maintain stable performance for over 700 equivalent days, demonstrating their long-term reliability. Beyond electrodes, LSP can also enhance bonding in polyimide-based microfluidic channels with parylene-C (PaC) and PDMS, which are important for integrated drug delivery and biofluid monitoring in neural implants. These findings establish LSP as a transformative fabrication approach, enabling the development of fully laser-processed neural interfaces with improved mechanical and electrochemical stability, paving the way for more durable and clinically viable neurotechnolo gies . The following preferred and optional features are applicable to the method of either of the sixth or seventh aspect.

[0062] In certain embodiments the first layer is formed of a first material and the second layer is formed of a second material, different to the first material. For example, the first layer may be formed of a polymer material. The second layer may be formed of a different polymer material, or a metal or semi-conductive material.

[0063] In other examples, the first layer may be formed of an insulator material (which may be a polymer) and the second layer may be formed of a conductive or semi-conductive material, for example a metal or conductive polymer.

[0064] However, in certain embodiments the first and second layers may be formed of the same material, for example of the same polymer material.

[0065] The methods of the above sixth and seventh aspects have particular application where the first layer is a thin layer. Using femtosecond laser processing, or laser processing which changes the chemical properties of the first surface as well as the surface roughness of that surface can allow good adherence of subsequent layers to a thin layer as it can treat only the portion of the first layer close to the first surface.

[0066] In certain embodiments the first layer has a thickness of no more than 10 pm, optionally no more than 5 pm.

[0067] One particular potential advantage of the methods of the above sixth and seventh aspects is that they permit strong adhesion of the second layer to the first layer without damaging or affecting the properties of the first layer, even when the first layer is very thin. Preferably the method is such that the laser treatment of the first surface does not damage the first layer.

[0068] In the present application, “not damaging” preferably means that the first layer remains functionally intact, for example, if it is a conductor or semi-conductor, that it is able to conduct in a similar fashion (i.e. as effectively) as it would have done prior to the laser treatment. In practical terms the laser treatment of the first surface removes small amounts of the first layer. However, preferably the amount removed is small.

[0069] For example, and without limitation, removal of no more than 250 nm, preferably no more than 200 nm, and in some cases no more than 100 nm of a 1 pm thick first layer is likely in most situations to mean that the functionality of the first layer is retained.

[0070] Alternatively, for layers of other thicknesses, removal of no more than 10%, preferably no more than 5%, more preferably no more than 1% of the first layer is unlikely to affect the functionality of the first layer.

[0071] In certain embodiments the laser treatment of the first surface forms a regular pattern of peaks and troughs on the first surface. This pattern can increase the surface roughness of the first surface. A regular pattern made be created by a repeated variation in the laser fluence.

[0072] In certain embodiments the irradiation of the first surface is performed in a predetermined pattern so as to cause the second layer to preferentially adhere to the portions of the first surface which have been treated relative to those portions which have not been treated. In such embodiments the method may further include the step of removing portions of the second layer which have not adhered to the first layer, thereby creating a patterned second layer. Thus the methods can provide an approach to producing a patterned layer on top of a first or substrate layer. For example, where the second layer is formed from a conductive or semiconductive material, the patterned layer could be the tracks and connections forming an electronic circuit.

[0073] The methods of the above sixth and seventh aspects are particularly suited to creating a pattern which includes small dimensional features (i.e. features which are less than a certain dimension in a particular direction, for example the width of a conductive track or similar). For example, in certain embodiments the pattern has at least one feature having a dimension in the plane of the first surface of no more than 1 mm, and may have at least one feature having a dimension that plane of no more than 100 pm or no more than 50 pm.

[0074] One way in which the methods of the above sixth and seventh aspects can improve adhesion between the first and second layers is by increasing the surface roughness of the first surface and thus the total overall contact area between the materials of the first and second layers for a given planar contact area. The surface roughness may be characterised by a representation referred to as RMS (root mean square) surface roughness. This metric quantifies the average height of peaks and valleys on a surface and is calculated by taking the square root of the mean of the squared differences between the surface profile heights and the mean line. RMS surface roughness is typically measured by a profilometer, or by optical measurement.

[0075] The methods of the above sixth and seventh aspects may be such that the irradiation of the first surface results in the first surface having an RMS surface roughness of greater than 10 nm, preferably greater than 50 nm.

[0076] Advantageously, the methods of the above sixth and seventh aspects may involve applying the second layer in a manner that some or all of the surface roughness of the first surface is retained by the second layer. This can mean that a second layer having a desired surface roughness can be obtained without needing to treat the second layer. That may be advantageous for example when the second layer is formed of a material that is less easily treated to obtain the desired surface roughness. This may be particularly applicable where the second layer is a thin layer.

[0077] In other words, if the second layer has a second surface which is in contact with the first surface and a third surface opposite the second surface then the irradiation of the first surface and forming of the second layer can result in the third surface having an RMS surface roughness which is at least 75%, potentially at least 80% or at least 90% of the RMS surface roughness of the first surface.

[0078] Alternatively or additionally, in absolute terms, the third surface may have an RMS surface roughness greater than 10 nm, preferably greater than 50 nm.

[0079] This obtained surface roughness of the third surface may be particularly advantageous where the second layer is a metal. In this situation the roughness of the third surface may cause the second layer to have a capacitance and / or surface charge density which is greater than that of a smooth surface of the same metal.

[0080] A particularly advantageous effect of the surface roughness of the first layer being retained by the applied second layer is that the increased surface roughness on the second layer may allow a third layer to be applied to the second layer, on the opposite side to the interface with the first layer, and improved adhesion between the second and third layers obtained. In certain embodiments, the methods may include the further step of forming a third layer on the second layer, wherein the treatment of the first surface and the thickness of the second layer are such that the adhesion properties between the second layer and third layer are increased.

[0081] In certain embodiments, when the second layer is a metal, this layer may be deposited by sputtering. Forming the second layer by sputtering can enable complete coverage of the roughened surface, whilst other deposition techniques (for example e-beam and thermal deposition) are more unidirectional and may therefore result in uneven or patchy coverage of the roughened surface as only the area of the surface with a direct line of site from the source would be covered.

[0082] The methods of the above sixth and seventh aspects may be performed on the interface between more than one layer in a device having three or more layers.

[0083] The methods of the above aspects may include any combination of the above-described optional and preferred features.

[0084] A further aspect of the present invention provides a device formed by the method of any of the above aspects, including some, all or none of the optional and preferred features of those aspects.

[0085] A further aspect of the present invention provides a multilayer device having: a first layer formed of a first material and a second layer of a second material formed on the first layer, wherein a first surface of the first layer adjacent the second layer has been surface treated with a laser to increase the surface roughness of the first surface and improve the adherence of the second layer to the first layer.

[0086] By increasing the adhesion properties of the first surface, a better adhesion between the first and second layers may be obtained. This can result in multilayer devices which have an improved functional life as they may be less prone to delaminate and / or degrade.

[0087] In certain embodiments the first layer is a thin layer, for example having a thickness of no more than 100 pm, and in some embodiments no more than 50 pm or no more than 10 pm. The laser treatment of the first layer can be capable of increasing the surface roughness even on such thin layers.

[0088] Preferably the laser treatment does not damage the first layer. The meaning of damage in this context has been discussed above.

[0089] Increased surface roughness can increase the total overall contact area between the materials of the first and second layers for a given planar contact area. In certain embodiments the first surface has an RMS surface roughness of greater than 10 nm, preferably greater than 50 nm.

[0090] Advantageously, the device of this aspect may be constructed such that some or all of the surface roughness of the first surface is retained by the second layer. This can mean that a second layer having a desired surface roughness can be obtained without needing to treat the second layer. That may be advantageous for example when the second layer is formed of a material that is less easily treated to obtain the desired surface roughness. This may be particularly applicable where the second layer is a thin layer.

[0091] In other words, if the second layer has a second surface which is in contact with the first surface and a third surface opposite the second surface then the irradiation of the first surface and forming of the second layer can result in the third surface having an RMS surface roughness which is at least 75%, potentially at least 80% or at least 90% of the RMS surface roughness of the first surface.

[0092] Alternatively or additionally, in absolute terms, if the second layer has a second surface which is in contact with the first surface and a third surface opposite the second surface then the third surface may have an RMS surface roughness of greater than 10 nm, preferably greater than 50 nm.

[0093] This obtained surface roughness of the third surface may be particularly advantageous where the second layer is a metal. In this situation the roughness of the third surface may cause the second layer to have a capacitance and / or surface charge density which is greater than that of a smooth surface of the same metal. In certain embodiments the irradiation of the first surface is performed in a predetermined pattern so as to cause the second layer to preferentially adhere to the portions of the first surface which have been treated relative to those portions which have not been treated. Thus the second layer can be formed in a predetermined pattern on the first layer and the areas of the first surface to which the second layer is not adhered have not been surface treated. For example, where the second layer is formed from a conductive or semiconductive material, the patterned layer could be the tracks and connections forming an electronic circuit.

[0094] The devices of this aspect are particularly suited to arrangements in which a pattern having small features is formed on the second layer (i.e. features which are less than a certain dimension in a particular direction, for example the width of a conductive track or similar). For example, in certain embodiments the pattern has at least one feature having a dimension in the plane of the first surface of no more than 1 mm, and may have at least one feature having a dimension that plane of no more than 100 pm or no more than 50 pm.

[0095] In certain embodiments the first material is a polymer, which may provide a substrate for subsequent layers. The second material may be a conductive or semi-conductive material and / or may be a polymer.

[0096] In certain embodiments the first surface may have a regular pattern of peaks and troughs formed by the surface treatment.

[0097] The devices of the above aspect may have any combination of the above-described optional and preferred features.

[0098] Unless indicated otherwise, any of the features (including the optional or preferred features) described in relation to one of the above aspects are equally applicable in combination with the devices and methods of any of the other above-described aspects.

[0099] The invention is described below, by way of example, with reference to the accompanying figures in which:

[0100] Figure 1 shows, schematically, the steps in a method according to an embodiment of the present invention; Figure 2 shows, schematically, steps in a method according to an embodiment of the present invention in cross-section;

[0101] Figure 3 shows the details of the patterns that can be formed in a device according to an embodiment of the present invention;

[0102] Figure 4 shows, schematically, the steps in a method according to a further embodiment of the present invention;

[0103] Figure 5 shows, schematically, the steps in a method according to a further embodiment of the present invention;

[0104] Figure 6 shows examples of alternative patterning formed in devices according to embodiments of the present invention;

[0105] Figure 7 shows the formation of a device according to a further embodiment of the present invention;

[0106] Figure 8 shows cross-sections of a device according to a further embodiment of the present invention;

[0107] Figure 9 shows, schematically, a laser patterning method;

[0108] Figure 10 illustrates the laser surface processing pattern as used in embodiments of the present invention;

[0109] Figure 11 shows the CAD images used to form electrodes in embodiments of the present invention and comparative examples;

[0110] Figure 12 shows, schematically, the steps in the fabrication of laser surface processed devices according to embodiments of the present invention; Figure 13 shows images of fabricated electrodes in embodiments of the invention and comparative examples;

[0111] Figure 14 plots the diameter of laser-milled electrode openings of embodiments of the invention and comparative examples;

[0112] Figure 15 shows the effects of sonication on a sample having areas treated by LSP and untreated areas;

[0113] Figure 16 shows the steps in an image processing algorithm for analysing the regions in an adhesion test;

[0114] Figure 17 shows the result of an adhesion test performed on embodiments of the invention and comparative examples;

[0115] Figures 18 and 19 show the ratio of remaining metal films formed on an PI layer for different surface processing techniques;

[0116] Figure 20 shows, schematically, different samples whose surface characteristics were evaluated;

[0117] Figures 21 and 22 are SEM images of a platinum surface formed on a PI substrate that has been treated with laser surface processing and the boundary between the LSP and non-LSP regions respectively;

[0118] Figure 23 shows FIB images of a platinum surface formed on a PI substrate that has been treated with laser surface processing (top images) and at the boundary between the LSP and non-LSP regions (bottom images);

[0119] Figure 24 shows AFM images of, from left to right, pristine PI, O2 plasma treated PI and LSP treated PI without a platinum coating (top images) and with a lOOnm platinum coating (bottom images); Figure 25 plots RMS roughness of pristine PI, O2 plasma treated PI and LSP treated PI without a platinum coating as determined from the images in Figure 24;

[0120] Figure 26 plots RMS roughness of pristine PI, O2 plasma treated PI and LSP treated PI with a platinum coating as determined from the images in Figure 24;

[0121] Figure 27 shows the results of a general XPS survey for a PI film with different surface processing techniques applied;

[0122] Figure 28 shows, from left to right, the Ols, Cis and Nls peak of the PI surface with different surface modification techniques, and (right hand plot) the Pt4f peak of the interface between Pt thin film and a PI surface;

[0123] Figure 29 shows the results of an EIS scan of electrodes formed according to embodiments of the invention and in comparative examples;

[0124] Figure 30 shows the results of electrode impedance tests performed on electrodes formed according to embodiments of the invention and in comparative examples;

[0125] Figure 31 shows the CV curve of electrodes formed according to embodiments of the invention and in comparative examples;

[0126] Figure 32 shows the surface charge density on electrodes formed according to embodiments of the invention and in comparative examples;

[0127] Figure 33 shows, schematically, an experimental setup used for testing accelerated aging of embodiments and comparative examples;

[0128] Figure 34 shows the results of impedance increase during the aging tests carried out on embodiments of the invention and comparative examples;

[0129] Figure 35 shows the maximum current density during DC acute stimulation tests on embodiments of the invention and comparative examples; Figure 36 shows the VI curve of the stimulation during acute stimulation tests on embodiments of the invention and comparative examples;

[0130] Figure 37 shows a design of two channels formed using laser surface processing and the LSP patterns used;

[0131] Figure 38 shows images of PFPaC micro fluid channels fabricated using methods according to embodiments of the invention and the progression of liquid flow over time; and

[0132] Figure 39 is an image of a PI / PDMS channel fabricated using methods according to embodiments of the invention.

[0133] Embodiments of the present invention provide a methodology to create flexible electronic devices in a layer by layer process in which patterning steps are done with an ultrafast laser system.

[0134] An example of an ultrafast laser system such as used in the embodiments below is the CARBIDE series of lasers produced by Light Conversion of Vilnius, Lithuania (https: / / lightcon.com / product / carbide-femtosecond-lasers / ), such as the Carbide CB5, which has the following properties:

[0135] Power: 6W

[0136] Pulse length: 290fs

[0137] Repetition rate: Single shot up to 1 MHz

[0138] Wavelength: 1030nm

[0139] The output from this laser was then sent through a frequency doubler (such as the 2H model produced by Light Conversion) to halve the wavelength to 515 nm and halve the power to 3W.

[0140] A microscope objective lens, such as the PAL-10-HR, produced by OptoSigma (https: / / sea.optosigma.com / en_sg / pal-10-hr.html) is then used to focus the beam, resulting in a beam divergence on the output beam of around 220 mrad (12.7 degrees).

[0141] In a simple embodiment of the present invention, illustrated in Figure 1 , a substrate (not shown) is first coated with an insulating material (not shown) followed by coating of a conducting material 10, as shown in Figure la. An ultrafast laser system 100, such as that described above, is then used to selectively remove (ablate) the conducting material without damaging the underlying insulating material in order to create one or more independent tracks 12 of conducting material and optionally other features in the conductive material such as contacts or electrodes, in accordance with a desired design of an electronic circuit and / or connections (Figure lb). This continues until the complete desired pattern of tracks and other features has been formed in the conducting material, as shown in Figure 1c. In a further step a second insulating layer 20 is coated on top of the patterned conducting material (Figure Id). Subsequently, an ultrafast laser system 100 (which is preferably the same system as used to pattern the conductive layer 10) is used to selectively remove one or more portions of the second insulating layer on top of one or more locations of a track of conducting material without damaging to the conducting material. This creates a device in which certain portions 14 of the conductive material 10 are exposed through the insulating layer 20 (for example as contacts or electrodes, e.g. sensor electrodes), whilst other portions of the conductive material 10 remain coated by the insulating layer 20 as seen in the resultant device of Figure If.

[0142] In this context it will be appreciated that without damaging preferably means that the conductive layer 10 remains functionally intact and able to conduct in a similar fashion (i.e. as effectively) as it would have done prior to the laser treatment. Ideally the laser will not remove any of the underlying conductive layer 10 when removing the insulating layer 20, but in practical terms it is likely that any ablation process will remove extremely small amounts of the conductive layer, but will not change the functionality of that layer, even if the conductive layer 10 is very thin (e.g. 1 pm or less).

[0143] Figure 2 shows the step of removing the upper insulating layer, shown in Figures le and If, in cross-section. By appropriate choice of the laser 100, as discussed further below, a narrow and highly-focused beam can be produced which is able to remove the insulating layer 20 above the conducting layer 10 and expose contacts 14 without damaging the conducting layer. Figure 2 also shows the mounting of the conducting layer 10 and the insulating layer 20 on a substrate 1.

[0144] A further step in the process may be to use an ultrafast laser system or other approach to cut through all of the deposited layers thereby defining the final outline of the device. It will be appreciated that multiple devices may be formed on one substrate using the same layers and patterned sequentially (or simultaneously) on the substrate. The cutting step may be used to define and separate these into individual devices.

[0145] In a final step the device(s) can be removed from the substrate.

[0146] Similarly, in an alternative embodiment, the method could start with patterning the first insulating layer to create openings in desired locations in order to have a portion of subsequent (conductive) layers exposed on the bottom side of the device when the device is removed from the substrate.

[0147] It will be appreciated that the layering / patteming steps can be repeated multiple times in order to build up a multilayer device. This can be used to provide a range of functionality in the different layers of the device. Connections between conducting layers (e.g. vias) may be created in known fashion. This is shown, schematically, in Figure 4 in which a first layer of insulating material 2 is formed on the substrate 1, before the conducting layer 10 is formed on top and then patterned by laser 100 (Figure 4a). A further insulating layer 20 is formed over and around the patterned conducting layer 10 and then selectively ablated to expose portions 14 of the conducting layer 10. A second conductive layer 30 is then formed which can extend through the insulating layer 20 to the exposed portions 14 of the first conductive layer 10, thus forming vias between the two conductive layers and is itself patterned by the laser 100. A third insulating layer 40 is then formed over and around the patterned second conductive layer 30 and then selectively ablated to expose portions 34 of the second conductive layer 30.

[0148] Additional processes may then be used to electrically connect the manufactured device to other technologies. For example, wire bonding, soldering, or laser welding may be used to attach microwires or ACF bonding used to attach flat flexible cables. Alternatively, the device stack may incorporate patterned conductive materials which are configured to enable wireless connectivity (e.g. an embedded antenna structure).

[0149] Intermediate steps before the deposition of any layer may include standard techniques to clean surfaces and / or improve adhesion such as oxygen plasma treatment, reactive ion etching, deposition of an adhesion promoter or sonication in a suitable solvent or cleaning solution. An anti-adhesion promoter may also be employed to better facilitate removal of the device from the substrate. For example, a thin soap layer or a water-soluble polymer such as polyvinyl alcohol may be used.

[0150] The ultrafast laser system may be tuned to selectively cut, mill and / or ablate material layers with micrometer precision. For example, this could allow electrode interconnects to be defined with track widths of 10 pm and down 1 pm with no upper limit on the maximum dimensions aside from the working area of the laser system (which is typically measured in centimetres). Similar separations between Figure 3 shows typical patterns that can be formed and the potential size and separations between tracks 12.

[0151] The substrate on which the device is fabricated can take many forms. For example, it could be a relatively smooth / flat two-dimensional surface such as a silicon or glass wafer, as in the embodiments shown in Figures 1-4.

[0152] However, the substrate could also have areas of significant curvature or roughness or measurable changes in height relative to a baseline value. This could be achieved for example by 3D printing a surface with the desired dimensions or by other known means of manufacturing.

[0153] For example, the substrate could be cylindrical, as shown in Figure 5. Figure 5 shows the process of manufacturing a device according to an embodiment of the present invention in a similar sequence of steps to those shown in Figure 1, but using a cylindrical substrate (polymer tube 1 ’).

[0154] First, a metal conductive layer 10’ is deposited on the substrate 1 ’ (Figure 5b). This metal layer 10’ is patterned (Figure 5c) using laser 100 to form the desired circuitry pattern on the surface of the substrate 1 ’ (Figure 5d). The structure is then coated in an insulating polymer layer 20’ (Figure 5e). The insulating polymer layer 20’ is then ablated using the laser 100 to open holes in the insulating layer 20’ and expose contacts / electrodes 14’ of the metal layer 10’ to produce the final device shown in Figure 5f.

[0155] In other embodiments the substrate may be flat but significant topology may be introduced into subsequent layers by introducing a patterned feature of significantly different thickness. For example, the method may include milling a 100 pm deep channel of 50 pm width into an insulating layer. Alternatively, the method may include selectively placing a strip of relatively thick material (e.g. a microtube or an x-ray imaging marker) onto a portion of the device layout and adhering it to the device.

[0156] However, the configuration is obtained and whatever configuration is being processed, the ultrafast laser system may be programmed to accommodate the variations in the arrangement of the layer(s) that is / are being patterned to achieve the desired patterning of the layer(s).

[0157] Example insulating materials include polymers such as parylene, polyimide, PET, and silicone. These may be deposited by techniques such as physical vapor deposition, spin coating, blade coating, spray coating and / or dip coating.

[0158] Example conducting materials include gold, platinum, copper, titanium, chromium, magnesium as well as conjugated polymers such as PEDOT:PSS. Example semiconducting materials include silicon, germanium, zinc oxide as well as conjugated molecules and polymers. Thin layers of metallic materials from nanometer thickness up to 1 micrometer are often deposited by techniques such as sputter coating, thermal evaporation, electron or ion beam deposition. Some conjugated molecules as well as other inorganics may also be deposited by physical vapor deposition processes. Conjugated polymer layers may be deposited from solution (inkjet, spin coating, blade coating, etc.) or electrochemical deposition with thickness ranging from nanometer scale to hundreds of microns.

[0159] In embodiments of the invention, the method includes steps of patterning layers which are subsequently removed to create channels which may in turn be used to transport liquids and / or gases in the device. For example, the method may include a step of patterning an intermediate layer of a water-soluble polymer or metal such as calcium followed by a later step to dissolve the soluble material to create a void space / channel. Figure 6 illustrates an example of this method and the resulting devices.

[0160] In Figure 6a the laser 100 is used with highly focused, depth-controlled ablation to create a channel 120 in a substrate 110. As illustrated in the left-hand part of Figure 6a, the high beam angle of the laser means that the power of the laser is focused on a very small volume 105 of the substrate 110 close to the surface, allowing a microchannel 120 with width and depth dimensions in the order of micrometres to be formed.

[0161] As shown in Figure 6b, the same process can be used to create a surface pattern 122 on the substrate 110 by varying the depth of the material of the substrate which is ablated at different positions, whether in a linear fashion (as shown in Figure 6b), or in a more complex pattern which may include, for example, interconnected, joining or dividing channels.

[0162] In a variation on this approach, the focus of the laser 100 may be adjusted so that it is inside the substrate 110. This can allow cavities or channels 120’ to be formed which are embedded in the material without any damage to the surface of the substrate.

[0163] Devices of embodiments produced in this manner may be used for example as devices for drug delivery, shape-actuation or local cooling. In variations of these methods, the method can incorporate patterned materials that prevent adhesion to the subsequent layer to create regions or channels that can later be separated. Figure 7 illustrates an example approach to achieve this.

[0164] A first material layer 210 is formed on a substrate 200. A selected area 212 of the first layer 210 is treated by the laser 100 so as to reduce the adhesive properties of the first layer for example by patterning the surface of the first layer at a micron scale.

[0165] A second material layer 220 (which may be the same material as first layer 210) is then coated on the first layer. At the portions not treated by the laser, the material layers 210, 220 adhere. However, at the portion 212 treated by the laser, the layers do not adhere, allowing a chamber 222 to be formed when the layers are removed from the substrate 200.

[0166] In variations of the above approach, the laser may be used to treat the areas of the first layer 210 where adhesion is desired, with the application of the second layer 220 being such as to only cause the second layer to adhere to the treated areas of the first layer.

[0167] Such methods may be used, for example, to create an inflatable cavity in a flexible device for shape-actuation. In other embodiments, the methods can also be used to pattern of materials that do not interact with the laser system or for which re-deposition of ablated particles is a concern. For example, a metal layer could be patterned in a multistep “lift-off’ process in which a polymer layer is first deposited and patterned via laser such that polymer is removed only where it is desirable to have the metal layer. The metal layer could then be deposited (for example by e-beam deposition) across the surface and the underlying polymer layer then dissolved which in turn carries with it the metal layer, except where the polymer layer was first removed.

[0168] In some embodiments the methods may include using the laser treatment to selectively change the surface properties of one or more layers in a device in one or more locations. For example, this approach could be used to either increase or decrease the hydrophobicity of the layer. Tailoring hydrophobicity is of particular interest to applications involving micro fluidics as well as drug delivery.

[0169] In another example, one or more layers in the device may be processed with laser pulses to increase the surface roughness on nanometer to micrometer length scales. One example application for this approach would be to increase or decrease the adhesion for a subsequently deposited layer. For example, as shown in Figure 8, a layer 310 (such as a polyimide insulating layer) formed on a substrate 300 is laser processed to increase surface roughness 315, as shown in the exploded view, such that a subsequent layer 320 (for example a sputter-coated layer of platinum) exhibits stronger adhesion. In certain embodiments, the surface roughness 315 can be increased by creating hierarchical structures (structures upon structures). The increase in adhesion is particularly increased when the subsequent layer 320 is deposited from a gaseous form.

[0170] Another application for this approach would be to deliberately increase the surface area of a subsequently deposited conductive layer in order to improve charge storage capacitance.

[0171] The methods according to embodiments of the invention are not restricted to patterning of insulating, semiconducting and conducting layers; other advanced functional materials may also be included in the method and in the resulting device structures. For example, a shape memory polymer or metal alloy could be selectively patterned and incorporated into a layer. Other functional materials which can be used in methods of embodiments of the invention may include piezoelectric materials and / or drug-load polymers or hydrogels. Other embodiments of the present invention provide a methodology to create multilayer devices in which a first layer is surface treated by laser processing to improve the adhesion of a second layer to that first layer. In various embodiments the surface treatment may provide for preferential adhesion of the second layer to the first layer, which may assist in patterning the second layer. In other embodiments the whole (or substantially the whole) of the interface between the first and second layers may be treated to create strong adherence between the first and second layers.

[0172] Whilst the description of the embodiments below will focus on examples of adhesion between two layers, it will be appreciated that devices according to embodiments of the present invention can have more than two layers and that the techniques set out herein can be used to improve adhesion between any two or more of these layers, but that alternative approaches can also be used in the formation of such multilayer devices.

[0173] Femtosecond (fs) lasers are known for their ultra-short pulse duration and high precision. These lasers enable fine-scale modifications to materials, allowing for the creation of intricate microstructures and precise patterning at the nanoscale.

[0174] The present inventors have designed and manufactured sample devices according to embodiments of the present invention and have tested them in a number of ways, as set out in more detail below.

[0175] AutoCAD was used to design two types of devices for comparative testing, referred to herein and in the figures as “LSP” (Laser Surface Processed - i.e. according to embodiments of the present invention) and “non-LSP” devices (for control / comparison). Both device types featured thin-film polyimide (PI) / Pt structures with similar overall geometries, including electrode arrays and flexible substrate regions. Two specific device designs were created: 8-channel devices with 1 mm electrodes and 16-channel devices with 15 pm electrodes. The non-LSP devices retained the same shape and layout but without any surface modifications, serving as controls to evaluate the impact of LSP on performance. Device Fabrication

[0176] The devices utilized a sandwich structure (Pl / Pt / PI). The fabrication began with the spincoating of PI thin films onto glass substrates, followed by soft baking and curing at specific temperatures to achieve uniformity and mechanical stability. The CAD design was converted into G-Codes to control the precise motion of the fs laser beam, followed by fine-tuning the laser's frequency and power to optimize the results. For LSP devices, tracks of the electrodes were laser processed, while non-LSP devices skipped this step. A 100 nm Pt thin film was then deposited using physical vapor deposition (PVD), followed by laser patterning of the Pt layer using G-Code and optimized laser parameters. Finally, an insulation layer of PI was spin- coated, and electrode openings were created based on electrode size: 15 pm openings were milled with the fs laser, while 1 mm openings were made using tape masking.

[0177] Characterizations

[0178] The comparison between LSP and non-LSP devices involved comprehensive surface and performance characterizations. For surface characterization, techniques such as SEM / FIB, AFM, and XPS were used to analyse morphology, roughness, and chemical bonding. Adhesion strength was tested using ASTM D3359 tape tests and sonication on Pl / Pt samples. Device performance comparison included electrochemical characterizations via EIS, CV, and DCs acute stimulation tests, and in vitro accelerated aging tests exceeding one year

[0025] . These evaluations assessed the impact of LSP on structural integrity and long-term recording and stimulation stability in comparison to non-LSP devices.

[0179] Other Materials

[0180] In addition to PI and Pt, further experiments included adhesion tape tests on PI and Au samples to evaluate the robustness of laser surface modifications on alternative materials. The adhesion performance of PI / Au interfaces was assessed using ASTM D3359 tape tests to determine the impact of LSP on gold's bonding strength. Furthermore, microfluidic channels made of PI combined with parylene-C (PaC) or PDMS were fabricated and analysed to explore the compatibility of LSP with hybrid structures.

[0181] A summary of the equipment and the materials is listed in below Table 1. Table 1. Equipment summary

[0182] * Provided by Oxford Materials Characterisation Service at Begbroke Science Park

[0183] ** Provided by Electron Microscopy Facility at Dunn School of Pathology

[0184] Preparation of PI thin films

[0185] To fabricate PI thin films for use, for example, in neural interfaces, a standard thickness of 5 pm was chosen for its optimal balance between flexibility and stiffness. The process began with substrate preparation, including sequential sonication in acetone, IP A, and deionized (DI) water for 10 minutes each, followed by a 1 -minute O2 plasma treatment (short plasma) at 100 W with a flow rate of 1.5 seem. For achieving the 5 pm thickness of PI-2611 polyimide as shown in Figure 2, a spin-coating process was performed at a speed of 2750 RPM with an acceleration of 150 RPM / s for a duration of 1 minute on a 2x1 inch glass wafer. Then the film underwent soft baking of 70°C for 3 minutes followed by 120°C for 3 minutes to partially remove solvents. Then it was cured with a temperature ramp of 2°C / min to 300°C in a nitrogen oven, followed by isothermal curing at 300°C for 6.5 hours.

[0186] Femtosecond Laser surface processing for PI & metal thin films

[0187] Embodiments of the present invention used a green femtosecond laser from Oxford Laser with the wavelength of 1064nm. There are multiple parameters in such an fs laser system including frequency, power, attenuator, speed of laser movement and repletion. The laser energy distributed on a unit area is defined as laser fluence (F, in J / cm2). The laser fluence could be derived as indicated below, based on the characteristics illustrated in Figure 9, and assuming a total laser energy E on the area of A.

[0188] Ppulse' pulse'f / < . d-V ' Where, Ppuise is the laser power of each laser pulse = 6W, tpuise is the duration of each pulse = 190fs, / is the laser frequency, d is the diameter of the laser spot, v is the speed of the laser movement, P% and Atten% are the percentage of the laser power and attenuator.

[0189] The general process for femtosecond (fs) laser surface processing according to embodiments of the invention begins with converting the CAD design into G-code. Then the LSP pattern is designed as a filled shape (electrodes and tracks) consisting of parallel laser paths with 5 pm intervals as shown in Figure 10. The laser should be focused exactly on the sample surface with the parameters set out in Table 2 below. An F-theta lens is used, providing a spot size of 5 pm for high-resolution processing.

[0190] Table 2. Laser parameters in the experiment of finding the best parameter for LSP with F-theta

[0191] Finally, the processed samples with different laser fluences were analysed with a profilometer to find the RMS roughness to assess the damage to the PI thin films, and then the samples were analysed under an optical microscope with an image processing algorithm as described below with respect to Figure 12 to analyse the carbonization the laser has introduced. The optimum LSP parameters should have a homogeneous surface roughness with least laser-induced carbon on the surface.

[0192] For femtosecond laser surface processing (Fs-LSP) on polyimide (PI), it was identified that at low fluence (<0.5 kJ / mm2), roughness remains insufficient for effective surface modification. In the next region (0.5-1 kJ / mm2), roughness increases while grey value decreases moderately, indicating enhanced structuring with minimal carbonization — the optimal fluence range. Beyond 10 kJ / mm2, excessive ablation leads to carbonization, which introduces side products to the devices. Thus, the middle region provides the best balance of roughness enhancement and minimal thermal damage for LSP-treated electrodes formed on PI.

[0193] Fabrication of non-LSP devices

[0194] The fabrication of non-LSP devices for comparative examples begins with the preparation of 5 pm PI thin films on 2x1 inch glass substrates, as described above. A 10-minute O2 plasma treatment (long plasma) at 100 W with a flow rate of 1.5 seem is then applied to the PI with the subsequent 100 nm Pt layer PVD deposition at 1 A / s in vacuum. Then, the Pt thin film is patterned into the desired shape using the laser parameters specified in Table 3 below.

[0195] Table 3. Laser parameters for Pt patterning on PI with F-theta lens

[0196] Two device types were fabricated for both LSP and non-LSP devices: 1 mm electrode devices and 15 pm electrode devices, as shown in Figure 11, differing in the final insulation PI fabrication step. For the 15 pm electrode devices, tape masking is applied to the ACF ribbon cable connection area before spin-coating an additional 5 pm PI layer over the patterned Pt. A 10xobjective lens (spot size: 2.6 pm) is then used to mill the top insulation layer, exposing the electrodes without damaging the underlying Pt, as detailed in Table 4. For the 1 mm electrode devices, low-adhesion tape masking is applied to both the ACF connection area and electrodes before spin-coating the insulation PI layer. The tape is removed after soft baking.

[0197] Table 4. Laser parameters for PI milling on Pt with lOx objective lens

[0198] Finally, the devices were bonded with the 16CH or 8CH FFC cables with the ACF bonder following a 100°C, 3s pre-attach of the ACF adhesive tape to the cable with 0.2MPa and then a 175°C, 10s final attach for the FFC cables to the device with 0.3MPa pressure. All the connections were sealed carefully with DOWSIL 732 sealant.

[0199] Fabrication of LSP devices

[0200] Fabrication of LSP devices is similar to those non-LSP devices as in the previous section with an additional step of surface processing on PI with the best laser parameter found, as discussed in the preceding section, before metal deposition as illustrated in Figure 12. Note that only the track region is surface processed (i.e. the region with metal conductors) to reduce the overall processing time. Figures 13a, 13b and 13c show images of devices with 15pm or 1mm electrodes with or without the LSP. Figure 13a shows the 15pm devices with LSP (left image) and without (right image). The electrode opening of the 15pm devices as shown in Figure 13a shows a good uniformity across all devices with or without LSP. Figures 13b and 13c show, respectively, the devices with 1mm electrodes with and without LSP.

[0201] Figure 14 shows that the diameter of the 15 pm electrodes was kept constant and almost identical for LSP and non-LSP devices. Testing of these devices (e.g. testing of the EIS and CV performance) will therefore directly reflect the impact of LSP rather than variations in electrode sizes.

[0202] Sonication Test

[0203] The sonication test was conducted to evaluate the adhesion and durability between of LSP and non-LSP PI to the thin- film Pt. The LSP region was designed as the Oxford logo with the laser parameter being as indicated in Table 2 above. Prior to laser surface processing, the substrate underwent O2 plasma treatment for 10 minutes at 300W with a 1.5sccm O2 flow rate. The LSP pattern was then applied immediately, followed by the deposition of a lOOnm Pt layer using PVD sputtering. Sonication was performed in DI water to assess the adhesion strength of the Pt layer on both LSP and non-LSP surfaces, providing insights into the impact of LSP on mechanical stability and long-term device performance.

[0204] Figure 15 demonstrates the effect of sonication on LSP-treated and non-LSP regions of a test device. The Oxford logo in the device is formed on the LSP area and the surrounding surface is formed on a non-LSP region of the PI substrate. It can be seen that the non-LSP regions exhibit significant delamination, with large portions of the metal layer peeling off, indicating poor adhesion between the Pt layer and PI substrate. In contrast, the LSP-treated Oxford logo remains largely intact after the two-hour sonication, demonstrating the increase of adhesion between Pt and PI after Fs-LSP treatments.

[0205] This result qualitatively confirms that LSP treatment improves the durability of metal films formed on a polymer substrate against mechanical stress, making it a promising technique for long-term stable neural interfaces and flexible electronics.

[0206] ASTM Adhesion Test To further quantitatively evaluate the adhesion between Pt and PI under different surface treatment methods, a modified ASTM D4541 peel test was conducted. The process began with laser ablation of the Pt layer into an 8><8 mesh pattern on the sample surface, ensuring controlled testing regions. A high-adhesion Kapton tape (12.5N / mm) was then applied over the mesh, using a 41b weighted roller to ensure uniform contact. The sample was left undisturbed for 10 minutes to allow the adhesive to stabilize before the tape was peeled off as quickly as possible.

[0207] Then, the peeled regions were examined with an image processing algorithm, illustrated in Figure 16, under an optical microscope to calculate the percentage of the metal removed, which provided a quantitative comparison of adhesion strength across different surface treatments.

[0208] For sample fabrication, three groups of PI substrates were used: pristine PI, O2 plasma-treated PI, and LSP-treated PI. Two metal films, Pt and Au, were tested to compare adhesion performance. Additionally, an extra set of pristine PI with Ti / Au and LSP-treated PI with Au was fabricated to further evaluate the effect of adhesion layers on Pl / metal bonding. The O2 plasma treatment was conducted at 300W for 10 minutes with a 1.5sccm flow rate, and LSP processing followed the same parameters detailed above. All PI substrates were 5 pm thick, and the deposited Pt or Au layers were lOOnm, with the Ti adhesion layer being 10 nm where applicable.

[0209] Figure 17 shows the peeled results in which an 8x8 grid was clearly marked by the laser and the peeled parts are the white regions showing inside the grids. The left hand image in Figure 20 shows pristine PI, the centre image O2 plasma-treated PI and the right image LSP treated PL From Figure 17 it can be qualitatively observed that treatment of the PI improves adhesion, with the LSP treatment performing better than the O2 plasma-treated PL

[0210] The images in Figure 17 were processed by computer vision analysis and the results plotted in Figures 18 and 19 for the Au / PI devices (Figure 18) and the Pt / PI devices (Figure 19). The LSP-treated samples (Au LSP and Pt LSP) exhibit the highest adhesion in both cases, indicating a strong metal-polymer bond induced by femtosecond laser surface processing (Fs-LSP). The Ti / Au and O2 plasma-treated Pt samples also show improved adhesion compared to untreated surfaces, highlighting the benefits of surface activation and adhesion layers. These results validate the potential of LSP as an alternative to traditional adhesion layers, ensuring long-term stability in flexible electronic applications.

[0211] Surface characteristics

[0212] To further evaluate the surface characteristics, SEM / FIB and AFM were performed to visualize the surface morphology and precisely quantify roughness for different PI surface treatment techniques. SEM / FIB imaging was conducted at the Electron Microscopy Facility at Dunn School of Pathology, providing high-resolution visualization of surface modifications induced by LSP. Additionally, AFM measurements were carried out by the Oxford Materials Characterisation Service, allowing for nanoscale roughness analysis across pristine PI, O2 plasma-treated PI, and LSP-processed PI surfaces. To investigate possible chemical changes induced by LSP, XPS analysis was also performed at the Oxford Materials Characterisation Service on these three PI conditions.

[0213] This comprehensive characterization provided insights into both the physical and chemical transformations resulting from different surface treatment methods. For the SEM / FIB, a sample of 5 pm PI with half O2 plasma treated and half LSP and then deposited lOOnm Pt was used. For the AFM, three types of samples of pristine PI, O2 plasma and LSP treated samples were used with and without lOOnm Pt deposition. For the XPS samples, the pristine PI, O2 plasma and LSP treated samples were utilized and then a 1.5nm Pt was deposited onto them to investigate the interface chemistry after the Pt deposition. The samples are illustrated as in Figure 20.

[0214] Surface characterisation - SEM / FIB & AFM

[0215] Figure 21 is an SEM image of the LSP treated surfaces, whilst Figure 22 is an SEM image showing the boundary between the LSP and non-LSP regions.

[0216] Figure 23 shows FIB image of the interface cross-section between PI surface and Pt thin films, with the LSP treated PI substrate shown in the upper images and the boundary between the LSP and non-LSP regions shown in the lower images.

[0217] An AFM was used to precisely quantify the increase of roughness between the different surface modification techniques. Figure 24 shows AFM images of the pristine PI (left), O2 Plasma treated PI (centre) and LSP PI (right) without a Pt thin film (top images) or with a lOOnm Pt thin film (botom images). A clear increase of roughness can be observed from the images in Figure 24 between O2 plasma treatment and LSP. Quantitative data is provided in Figure 25, which shows the surface roughness of the surfaces without the Pt film, and Figure 26 which shows the surface roughness of the surfaces after the Pt film has been added.

[0218] These figures demonstrate that LSP significantly increases surface roughness compared to pristine PI and O2 plasma-treated surfaces. In both cases, LSP-treated samples (red) exhibit significantly higher RMS roughness, confirming that femtosecond laser processing effectively modifies the surface physical morphology. However, after Pt coating (second image), the roughness is reduced, similar to how snow covers footprints, smoothing out the finer details of the laser-structured surface. Despite this reduction, the LSP-treated surface remains significantly rougher than O2 plasma-treated and pristine PI in both cases.

[0219] Surface characterisation - XPS

[0220] To investigate the surface chemical change before and after the LSP, the general survey of the XPS result for the PI thin film with different surface modification techniques and without the Pt coating is shown in Figure 27. The most interested element peaks: Ols, Cis and Nls are shown in Figure 28.

[0221] From Figure 27, since the PI molecule only contain carbon, oxygen, nitrogen and hydrogen (cannot be detected in XPS), therefore the general survey indicates that the scanning was reasonable. The two silicon peaks are most likely from the contamination from the glass substrate under the PI sample. From the Ols and Cis peak, both carbon-oxygen single bond (C-0) increased after O2 plasma and LSP, but the C-0 bond increased the most after LSP. Also, the carbon-oxygen double bond (C=0) decreased significantly after LSP, which potentially indicated significantly amount of C=0 break into C-0 after the laser treatment on the PI surface.

[0222] Both O2 plasma treated PI and LSP PI surface showed a similar amino group (-NH2) increasement, which could indicate similar breakdown of PI molecules (PI to PMDA and ODS) after O2 plasma and LSP, which is well known in the literatures

[0026] . It could be clearly seen that not only the physical roughness but also several chemical reactions happened on the PI surface after the LSP. Therefore, several hypotheses on the chemical reaction on the PI surface during the LSP could be purposed below.

[0223] 31

[0224] The reaction A, where the PI molecules break down into PMDA and ODS could both happen similarly during the O2 plasma and LSP treatment as supported by the Nls XPS information. There were two amino groups in the PMDA which could form a weak interaction bond to the Pt atoms (deposited Pt thin film). However, the high- intensity fs laser energy could trigger reaction B on the PI surface, which not only knocks off the PMDA from the chain but also directly free one pair of the electron in the C=O double bond and forms a covalent bond with the deposited Pt atoms. This will result a PtO bond and a more secure bond between the PI chain and the Pt thin film. From the left hand image in Figure 28, there was a significant increase in the PtO and PtO2 bond after LSP, which supports this hypothesis.

[0225] Electrochemistry tests

[0226] Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were conducted to evaluate the electrochemical properties of electrodes with and without LSP. EIS measurements were performed using the devices fabricated as set out above, assessing impedance across a frequency sweep from 1 MHz to 1 Hz with 10 samples per decade. The experiment PlamSens4 was used as the three-probe measurement equipment, employing a standard Ag / AgCl electrode as the reference and a 5><5 cm Pt mesh as the counter electrode. Additionally, CV was performed at each electrode to assess the charge storage capacity at the electrode interface, using the same test setup with a voltage sweep from -0.6 V to 0.8 V with 25 cycles. Both the test electrode, reference electrode and Pt mesh were submerged in IxPBS, where the distance between the test electrode to the reference and counter electrode was approximately 4 cm. The CV loop was then integrated to determine the surface charge and then divided by the electrode area to calculate the surface charge density in pC / cm2. An additional converter board was designed and manufactured to adapt the FFC cables from the devices to the PlamSense4 (male jumper wires) to ensure secure electrical connection during the measurements.

[0227] Figure 29 shows the EIS scan of the 15pm electrodes from IHz-lMHz. Figure 30 shows the comparison of the electrode impedance at 1kHz between electrodes with and without LSP treatment.

[0228] The figures show that both electrodes were capacitive whereas LSP-treated electrodes exhibit a much lower impedance compared to non-LSP electrodes, with significantly less variation, indicating enhanced consistency and reliability. The LSP-treated electrodes (red, lower line) consistently show lower impedance across all frequencies compared to non-LSP electrodes (black, upper line).

[0229] This reduction in impedance can be attributed to increased surface roughness arising from LSP, which enhances the electrode-electrolyte interface and improves charge transfer efficiency, making LSP an effective strategy for optimizing neural electrodes.

[0230] The CV result was shown in Figure 31 to examine the change of surface charge density after LSP with both 15pm and 1mm electrodes.

[0231] The CV curve shows that LSP-treated electrodes (red) exhibit a significantly larger current response compared to non-LSP electrodes (black), indicating improved charge transfer capabilities. The surface charge density plot in Figure 32 further confirms this, showing a substantial increase in charge storage for LSP-treated electrodes. The improved performance can be attributed to the increased surface roughness from LSP, which enhances the effective electrode area, leading to greater charge injection capacity.

[0232] Accelerated ageing tests

[0233] To evaluate the long-term stability of LSP devices compared to non-LSP devices, accelerated ageing tests were conducted using the setup configuration shown in Figure 33. This test simulated extended implantation conditions to assess the degradation of electrode performance over time. The ageing environment was maintained at 70°C, with an additional 30 mM H2O2 to simulate the oxidative functional groups present in implanted environments. A closed-loop control system, based on pH sensors, ensured a constant concentration of H2O2 throughout the test, stabilizing the oxidative conditions for a more accurate evaluation of device longevity. Therefore, according to the Arrhenius equation as in the ASTM F1980 standard for the accelerated ageing tests, the ageing factor in this case could be calculated as, 23 3= 9.85 (2)

[0234] Where the constant Q10 is a constant that determined by experiments. Here, a value of 2 is common practice for these test protocols and is considered conservative. TAA is the temperature for the accelerated ageing and TRT is the real-time temperature we would like to study, which is 70°C and 37°C respectively.

[0235] The 16CH devices, both with and without LSP, fabricated as set out above, were placed inside the reaction chamber for continuous monitoring with only their open electrode regions were dipped into the solution in the reaction chamber. Their impedance at 1kHz was recorded periodically using the same experiment electrochemistry setup discussed in the previous section to track any changes in electrochemical stability.

[0236] From the impedance increase figure in Figure 34, the electrodes with LSP stayed stable during the 400 days-equivalent period whereas the impedance of non-LSP electrodes increased from around day 50 and increased rapidly from day 100 to around 9 times their initial impedance. This means that the non-LSP electrodes were not functional by this point. This comparison indicates that using LSP could improve the long-term electrochemistry stability of devices compared to those without LSP.

[0237] Acute stimulation tests

[0238] The purpose of the acute stimulation test was to examine the impact of LSP on the stability of thin-film Pl / Pt / PI devices under extreme DC stimulation conditions. This experiment aimed to assess whether LSP improves the electrode’s resilience against high-voltage stress, a critical factor for long-term neural interface applications. The experiment setup involved connecting the testing electrode to the HI port of a Keithley 2612B, while the reference (GND) was connected to the LO port via a 5><5 cm Pt mesh, ensuring a sufficiently large counter electrode surface area. Both the test electrode and Pt mesh were submerged in IxPBS, maintaining an electrode-to-mesh distance of approximately 10 cm. A DC voltage sweep from 0 V to 60 V was applied to the test electrode, and the corresponding current was continuously recorded for the VI curve.

[0239] The acute stimulation test results demonstrate that LSP significantly enhances the currentcarrying capacity of electrodes under high-voltage conditions. Figure 35 shows that LSP- treated electrodes exhibit a much higher maximum current density compared to non-LSP electrodes, indicating improved charge injection efficiency. Furthermore, Figure 36 illustrates that LSP-treated electrodes sustain a more stable current response over increasing DC voltages, whereas non-LSP electrodes exhibit an earlier breakdown.

[0240] Figure 36 further confirms this, as LSP-treated electrodes sustain higher currents at increased DC voltages, whereas non-LSP electrodes exhibit premature failure. This suggests that LSP enhances electrode stability and conductivity, making it a promising technique for improving the durability and performance of neural stimulation devices.

[0241] Bonding other insulators to PI using LSP

[0242] Femtosecond LSP was also used to directly bond between PI and parylene-C (PaC) and polydimethylsiloxane (PDMS) during their deposition without any chemical or thermal processes. To demonstrate the viability of LSP for bonding these materials, a two-layer 500 pm microfluidic channel was designed with Pl / PaC and PI / PDMS configurations. The LSP- treated PI surface served as the bonding interface, with PaC deposition via chemical vapor deposition (CVD) and PDMS attachment via spin-coating.

[0243] The Pl / PaC bonding process was optimized for improved adhesion by modifying the laser pattern and surface preparation. The LSP region of the channel, shown in Figure 37a, was designed differently from the patterns used for metal adhesion. To enhance the mechanical interlocking of PaC molecules, two perpendicular LSP orientations (horizontal and vertical) were applied, creating a rougher surface with more potential anchoring sites. The fabrication of the Pl / PaC microfluidic channel began with an O2 plasma treatment (300 W, 1 minute, 1.5 seem flow rate) on a cleaned 5 pm PI surface to increase its hydrophilicity. Next, a 2% soap solution was gently sprayed onto the plasma-treated surface, followed by N2 drying to reduce Pl / PaC adhesion in the unwanted region. LSP was then applied using the optimized parameters listed in Table 5, followed by the deposition of a 5 pm PaC layer in a parylene coater.

[0244] Table 5. Laser parameters of Pl / PaC microfluid channel in the LSP patern and adhesion mechanism. The shape of the microfluidic channel remained the same, but due to the thicker nature of spin-coated PDMS, a zigzag LSP pattern was designed, as shown in Figure 37b, to create more pronounced surface features for PDMS anchoring. This zigzag structure provided enhanced mechanical interlocking, ensuring a stronger bond between the PI and PDMS layers. While the overall fabrication steps mirrored those of Pl / PaC bonding, the LSP pattern was specifically optimized for PDMS adhesion using the laser parameters in Table 6.

[0245] Table 6. Laser parameters of PI / PDMS microfluid channel

[0246] Finally, a 500 pm diameter PTFE microtube was inserted for the inlet and outlet, and the assembly was sealed using DOWSIL 732 silicone sealant along with two-part epoxies.

[0247] Some qualitative measurements were conducted on the devices with microfluidic channels, with the results shown in Figures 38 and 39.

[0248] Figure 38 shows a Pl / PaC micro fluidic channel fabricated using LSP and the progression of liquid flow over time. The successful and continuous movement of the dye through the channel indicates effective bonding between the PI and PaC layers, ensuring minimal leakage.

[0249] Figure 39 shows a PI / PDMS microfluidic channel fabricated using LSP, withstanding high- pressure liquid flow without visible leakage or delamination. This confirms the strong bonding and structural integrity achieved through LSP, making it a promising technique for robust microfluidic applications requiring high fluidic stability.

[0250] Conclusions

[0251] The experimental results set out and discussed above demonstrate that Fs-LSP significantly improves the adhesion between metal thin films and polymer substrates. Sonication and ASTM adhesion tests confirm that LSP enhances Pt-PI bonding by increasing surface roughness and chemical interactions. The X-ray photoelectron spectroscopy (XPS) analysis further supports this, showing an increase in Pt-0 bonding after LSP treatment, suggesting enhanced chemical interactions between Pt and the underlying PI substrate. These findings highlight the potential of LSP as an alternative to conventional adhesion layers like titanium (Ti), which have biocompatibility and degradation issues over time.

[0252] Additionally, focused ion beam (FIB) and scanning electron microscopy (SEM) imaging reveal significant surface morphology changes post-LSP, confirming increased nano- and micro-scale roughness. These topographical modifications play a crucial role in improving mechanical interlocking between Pt and PI, contributing to the superior adhesion properties.

[0253] Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements further validate the advantages of LSP in neural interface electrodes. EIS results show a substantial reduction in electrode impedance after LSP, directly improving signal transmission and charge transfer efficiency. The CV measurements indicate a significant increase in surface charge density, which can be attributed to the enhanced effective surface area introduced by LSP. These enhancements contribute to better charge injection capacity, making LSP-treated electrodes more efficient for neural stimulation and recording.

[0254] Furthermore, atomic force microscopy (AFM) measurements confirm a notable increase in RMS roughness after LSP, reinforcing the hypothesis that the increased surface area leads to improved electrochemical performance. The rougher electrode surface facilitates stronger electrode-tissue interactions, reducing impedance while increasing charge storage capacity.

[0255] Acute stimulation tests further confirm that LSP-treated electrodes sustain higher current densities under extreme stimulation conditions, demonstrating enhanced stability and durability. The results indicate that LSP improves electrical conductivity while maintaining the structural integrity of the electrodes, making them more suitable for long-term neural stimulation applications.

[0256] A major challenge for neural interfaces is maintaining electrochemical stability over extended periods of implantation. The accelerated ageing tests reveal that LSP-treated electrodes maintain stable impedance over 400 equivalent days, whereas non-LSP electrodes exhibit significant impedance increases due to material degradation and delamination. This confirms that Fs-LSP contributes to long-term stability, making it a promising approach for the next generation of chronic neural interfaces.

[0257] The successful fabrication of Pl / parylene-C (PaC) and Pl / polydimethylsiloxane (PDMS) microfluidic channels demonstrates the versatility of LSP in bonding different materials without the need for additional chemical adhesives. The ability to create strong, leakageresistant bonds suggests that LSP could be utilized in flexible bioelectronic devices, lab-on- chip systems, and other biomedical applications requiring robust material integration.

[0258] The methods according to embodiments of the invention are not restricted to the creation of multi-layered structures of insulating, semiconducting and conducting layers; other advanced functional materials may also be included in the method and in the resulting device structures. For example, a shape memory polymer or metal alloy could be joined to a substrate layer. Other functional materials which can be used in methods of embodiments of the invention may include piezoelectric materials and / or drug-load polymers or hydrogels.

[0259] Methods according to embodiments of the present invention can be used to create a wide range of devices and particularly flexible devices. For example, bioelectronic devices such as a multielectrode array for electrophysiology recordings and / or stimulation can be created using methods according to embodiments of the present invention. Other examples include optoelectronic devices, such as an array of organic LEDs, photodetectors and / or transistors. In certain embodiments, such examples could include a step of laser-patterning a photoactive layer (e.g. an emissive conjugated polymer) or other semiconducting materials. Materials that provide specificity in sensing such as an enzyme or molecular imprinted polymer could also be patterned in order to create biosensors. The forgoing description is exemplary in nature only, and the skilled person will understand that changes and variations on the disclosed embodiments are possible within the scope of the claims. The claims define the invention.

[0260] References:

[0261] [1] S. Xu, J. Yan, Y. J. Heo, W. Lu, and J. A. Rogers, “Soft micro fluidic assemblies of sensors, circuits, and radios for the skin,” Lab Chip, vol. 20, no. 24, pp. 4261-4272, Dec. 2020.

[0262] [2] C. L. Kane, H. Fang, X. Li, C. R. Haney, and J. A. Rogers, “Flexible and Stretchable Electronics in Bio-integrated Systems,” Proc. IEEE, vol. 108, no. 9, pp. 1628-1645, 2020.

[0263] [3] R. J. Rodger, Y. Zhao, E. Song, and J. A. Rogers, “Biomedical Applications of Soft Electronics,” Nat. Rev. Bioeng., vol. 1, no. 5, pp. 245-260, 2023.

[0264] [4] R. Green, S. Baek, C. Proctor, D. T. Simon, and G. G. Malliaras, “Electrophysiology with organic electronics,” Chem. Rev., vol. 120, no. 11, pp. 7348-7377, 2020.

[0265] [5] S. Cea, M. Garcia-Cortadella, S. J. Risse, D. T. Simon, and G. G. Malliaras, “Enhancing the longevity of neural interfaces with organic electronics,” Adv. Healthcare Mater., vol. 10, no. l, p. 2001814, 2021.

[0266] [6] J. Liu, K. Fu, S. Das, T. Li, and G. G. Malliaras, “Integration of Ultrathin Flexible Substrates in Bioelectronics for Chronic Neural Recordings,” ACS Appl. Bio Mater., vol. 6, no. 2, pp. 3237-3249, 2023.

[0267] [7] J. Kim, R. Ghaffari, N. Lu, and D. H. Kim, “Microfabricated neural interfaces for chronic implantations,” Adv. Sci., vol. 8, no. 14, p. 2102131, Jul. 2021.

[0268] [8] X. Dai, W. Hong, and P. S. Lee, “High-performance flexible neural electrodes based on polyimide and metal nanowires,” ACS Nano, vol. 14, no. 7, pp. 7832-7842, Jul. 2020.

[0269] [9] S. T. Lee, Y. Wu, and M. Freedman, “Recent advances in polymer-based neural interfaces,” Biosens. Bioelectron., vol. 202, p. 113980, Mar. 2022.

[0270]

[0010] M. Luo and F. Zhang, “Advanced Materials for Next-Generation Bioelectronic Neural Interfaces,” Adv. Mater. Technol., vol. 8, no. 6, p. 2200413, Jun. 2023.

[0271]

[0011] N. N. Udipi and L. Xie, “Challenges and breakthroughs in neural electrode performance for chronic recordings,” J. Neural Eng., vol. 17, no. 5, p. 051001, Oct. 2020.

[0272]

[0012] R. G. Olberg, J. S. Kim, D. Ha, and J.-U. Chu, “Laser-based surface texturing for enhanced electrode-tissue interaction,” IEEE Trans. Biomed. Eng., vol. 69, no. 2, pp. 609-619, Feb. 2022.

[0273]

[0013] T. M. Roberts and A. R. Wheeler, “Femtosecond laser micromachining of polymer-metal interfaces for flexible bioelectronics,” Appl. Phys. Lett., vol. 116, no. 24, p. 241901, Jun. 2020.

[0014] J. T. Muth, D. A. Dornfeld, and V. Subramanian, “Surface modification of Parylene-C and polyimide by femtosecond lasers for improved adhesion in flexible neural implants,” J. Microelectromech. Syst., vol. 30, no. 3, pp. 381-389, Jun. 2021.

[0274]

[0015] X. Tang and H. Wang, “Laser-induced periodic surface structures for biofunctionalization: a route to stable neural electrodes,” ACS Appl. Mater. Interfaces, vol. 14, no. 4, pp. 5676- 5685, Jan. 2022.

[0275]

[0016] J. K. Richardson and M. R. Abidian, “Conducting polymer-based nanocomposites for advanced neural interfaces,” Prog. Polym. Sci., vol. 115, p. 101401, May 2021.

[0276]

[0017] L. R. Hochstein and M. T. Petrossians, “Carbon-based thin- film neural interface electrodes,” Front. Neurosci., vol. 15, p. 773501, Jun. 2021.

[0277]

[0018] B. C. Thompson and G. G. Wallace, “Organic electrodes for tissue engineering: the interface between biology and electronics,” Materials Today Chem., vol. 22, p. 100581, 2021.

[0278]

[0019] A. Benabid, “Deep brain stimulation for Parkinson’s disease,” Curr. Opin. Neurobiol., vol. 72, pp. 1-9, Sep. 2022.

[0279]

[0020] E. Green and S. R. Shin, “Flexible electronics and photonics for biomonitoring,” Adv. Mater., vol. 34, no. 24, p. 2107928, Jun. 2022.

[0280]

[0021] J. R. Capadona and R. D. Brauker, “Progress and challenges in intracortical neural interface technology,” J. Neural Eng., vol. 20, no. 2, p. 026003, Apr. 2023.

[0281]

[0022] G. K. Fedder, “BioMEMS and microsystems in neural engineering,” Proc. IEEE, vol. I l l, no. 2, pp. 309-326, Feb. 2023.

[0282]

[0023] W. M. Grill, “Control of Movement Disorders Through Neuromodulation,” Cold Spring Harb. Perspect. Med., vol. 11, no. 2, a035780, Feb. 2021.

[0283]

[0024] A. Jastrzebska and R. O. Potrzebowski, “Graphene-based neural interfaces: progress and perspectives,” Biosensors, vol. 11, no. 7, p. 260, Jun. 2021.

[0284]

[0025] A. Majidi and E. Song, “Novel strategies for long-term in vivo stability of flexible neural devices,” Adv. Healthcare Mater., vol. 12, no. 4, p. 2201074, Feb. 2023.

[0285]

[0026] J. Q. Chen, M. Li, and K. T. Kim, “Porous Strained Pt Nanostructured Thin-Film Electrocatalysts via Dealloying for PEM Fuel Cells,” Adv. Energy Mater., vol. 12, no. 4, pp. 1234-1240, Feb. 2023, doi: 10.1002 / aenm.202201234.

[0286] All of the above references are hereby incorporated by reference.

Claims

CLAIMS1. A method of manufacturing a device, the method including the steps of: forming a first layer of a first material; forming a second layer of a second material, different to the first material, on the first layer; and selectively treating, using a laser, one or more portions of the second layer to form a pattern in said second layer, wherein: the laser treatment uses a pulsed laser having a pulse length of no more than 1 picosectond; and the laser that has abeam divergence of at least 10 degrees, preferably at least 12 degrees.

2. The method of claim 1, wherein the laser delivers a power density of at least ImW / pm2, preferably between ImW / pm2and 100mW / pm2.

3. The method of claim 1 or claim 2, wherein the treating is used to form channels in the second layer, optionally wherein the treating changes the solubility of the portions of the second layer and the method further includes the step of removing the treated portions by dissolution to form the channels.

4. The method of any one of claims 1 to 3, wherein the treating causes a change in the adhesion properties between the first and second layers.

5. A method of manufacturing a device, the method including the steps of: forming a multilayer structure having a first layer in contact with a second layer, wherein the first layer is formed of a conductive or semiconductive material and has a thickness of no more than 1 pm, and the second layer is formed of an insulating material; and selectively removing, by laser ablation, one or more portions of the second layer to form a pattern in said second layer without damaging the first layer.

6. The method of any one of the preceding claims further including the step of removing, by laser ablation, one or more portions of the first layer.

7. The method of any one of the preceding claims, wherein the first layer is formed on a substrate, and further including the step of removing the device from the substrate after the patterning is completed.

8. The method of claim 7 wherein the removed device is flexible.

9. The method according to any one of the preceding claims further including the step of cutting through all layers to define the external dimensions of the device.

10. The method according to any one of the preceding claims wherein the pattern formed by the laser treatment or ablation includes at least one feature with a minimum dimension of no more than 5 pm, preferably no more than 2 pm.

11. The method according to any one of the preceding claims including the further steps of forming one or more further layers sequentially on the second layer and selectively removing, by laser ablation, material from one of the further layers, optionally wherein the further layers alternate layers of insulating and conductive material.

12. The method according to any one of the preceding claims wherein the laser treatment or ablation changes the surface properties of either the first or the second layer so as to cause a change in the hydrophobic or hydrophilic nature of either the first or the second layer.

13. The method according to any one of the preceding claims wherein the laser treatment or ablation changes the surface properties of either the first or the second layer so as to cause a change in the adhesion properties between the first and second layers.

14. The method according to any one of the preceding claims wherein the multilayer structure is larger in all directions than the combined thickness of the first and second layer in all directions.

15. The method according to any one of the preceding claims wherein the first layer and / or the second layer have one or more projecting features which extend in the thickness direction of the layers and the step of removing includes removing portions of the second layer from locations on the projecting features and from locations not on the projecting features.

16. The method according to claim 5 or any claim dependent on claim 5 , wherein the ablation is performed using a pulsed laser having a pulse length of no more than 1 picosecond.

17. The method according to claim 5 or any claim dependent on claim 5 , wherein the ablation is performed using a laser that delivers a power density of at least ImW / pm2, preferably between ImW / pm2and 100mW / pm2.

18. The method according to claim 5 or any claim dependent on claim 5 , wherein the ablation is performed using a laser that has a beam divergence angle of at least 10 degrees, preferably at least 12 degrees.

19. A method of manufacturing a device, the method including the step of selectively removing, by laser ablation, one or more portions of material from a layer of material in the device to form channels in said layer, such that the channels are contained within the layer.

20. A device formed by the method of any of the above claims.

21. A device having: a first layer formed of a conductive or semiconductive material and having a thickness of no more than 1 pm and a second layer formed of an insulating material formed on the first layer, wherein one or more portions of the second layer have been selectively removed, by laser ablation, to form a pattern in said second layer without damaging the first layer.

22. The device according to claim 21 wherein the device is flexible.

23. The device according to any one of claims 21 to 22 wherein the pattern formed by the laser ablation includes at least one feature with a minimum dimension of no more than 5 pm, preferably no more than 2 pm.

24. The device according to any one of claims 21 to 23 wherein the device includes at least one further layer formed on the second layer, wherein a pattern is formed in at least one further layer by selectively removing, by laser ablation, material from the further layer, optionally wherein the further layers alternate layers of insulating and conductive material.

25. The device according to any one of claims 21 to 24 wherein the adhesion properties between the first and second layers have been adjusted by the laser ablation.

26. A method of manufacturing a device having a multilayer structure, the method including the steps of: providing a first layer; treating a first surface of the first layer by irradiating the first surface with a pulsed laser have a pulse length of no more than 1 picosecond to increase the adhesion properties of the first surface; and forming a second layer on the treated first surface.

27. The method of claim 26 wherein the treating of the first surface changes the chemical properties of the first surface and increases the surface roughness of the first surface and wherein the adhesion between the first layer and the second layer is increased by the changed chemical properties and the increased surface roughness.

28. A method of manufacturing a device having a multilayer structure, the method including the steps of: providing a first layer; treating a first surface of the first layer by irradiating the first surface with a laser, wherein the laser treatment changes the chemical properties of the first surface and increases the surface roughness of the first surface; and forming a second layer on the first layer, wherein the adhesion between the first layer and the second layer is increased by the changed chemical properties and the increased surface roughness.

29. The method of claim 28 wherein the laser is a pulsed laser having a pulse length of no more than 1 picosecond.

30. The method of any of claims 26 to 29 wherein the first layer is formed of a first material and the second layer is formed of a second material, different to the first material.

31. The method of any of claims 26 to 30 wherein the first layer is formed of a polymer.

32. The method of any of claims 26 to 31 wherein the second layer is formed of a conductive or semi-conductive material, for example a metal or conductive polymer.

33. The method of any of claims 26 to 32 wherein the first layer has a thickness of no more than 10 pm, optionally no more than 5 pm.

34. The method of any of claims 26 to 33 wherein the laser treatment of the first surface does not damage the first layer.

35. The method of any of claims 26 to 34 wherein the laser treatment of the first surface forms a regular pattern of peaks and troughs on the first surface.

36. The method of any of claims 26 to 35 wherein the irradiation of the first surface is performed in a predetermined pattern so as to cause the second layer to preferentially adhere to the portions of the first surface which have been treated relative to those portions which have not been treated, the method further including the step of removing portions of the second layer which have not adhered to the first layer, thereby creating a patterned second layer.

37. The method of claim 36 wherein the pattern has at least one feature having a dimension in the plane of the first surface of no more than 2 mm, alternatively no more than 1 mm, alternatively no more than 100 pm, alternatively no more than 50 pm.

38. The method of any of claims 26 to 37 wherein the irradiation of the first surface results in the first surface having an RMS surface roughness of greater than 10 nm, preferably greater than 50 nm.

39. The method of claim 38 wherein the second layer has a second surface which is in contact with the first surface and a third surface opposite the second surface and wherein irradiation of the first surface and forming of the second layer results in the third surface having an RMS surface roughness of greater than 10 nm, preferably greater than 50 nm.

40. The method of claim 39 wherein the second layer is a metal and the roughness of the third surface causes the second layer to have a capacitance and / or surface charge density which is greater than that of a smooth surface of the metal.

41. The method of any of claims 26 to 40 including the further step of forming a third layer on the second layer, wherein the treatment of the first surface and the thickness of the second layer are such that the adhesion properties between the second layer and third layer are increased.

42. A device formed by the method of any of claims 26 to 41.

43. A multilayer device having: a first layer formed of a first material; and a second layer of a second material formed on the first layer, wherein a first surface of the first layer adjacent the second layer has been surface treated with a laser to increase the surface roughness of the first surface and improve the adherence of the second layer to the first layer.

44. The device according to claim 43 wherein the first surface has an RMS surface roughness of greater than 10 nm, preferably greater than 50 nm.

45. The device according to claim 43 or claim 44 wherein the second layer has a second surface which is in contact with the first surface and a third surface opposite the second surface and wherein the third surface has an RMS surface roughness of greater than 10 nm, preferably greater than 50 nm.

46. The device according to any of claims 43 to 45 wherein the second layer is formed in a predetermined pattern on the first layer and the areas of the first surface to which the second layer is not adhered have not been surface treated.

47. The device according to claim 46 wherein the pattern has at least one feature having a dimension in the plane of the first surface of no more than 2 mm, alternatively no more than 1 mm, alternatively no more than 100 pm, alternatively no more than 50 pm.

48. The device according to any of claims 43 to 47 wherein the first material is a polymer.

49. The device according to any of claims 43 to 48 wherein the second material is a conductive or semi-conductive material, or wherein the second material is a polymer.

50. The device according to any of claims 43 to 49 wherein the first surface has a regular pattern of peaks and troughs formed by the surface treatment.

Images