A weak force sensor based on water-drop-shaped micro-nano fiber and a preparation method thereof

Abstract

This invention discloses a weak force sensor based on a teardrop-shaped micro / nano optical fiber and its fabrication method. The sensor comprises a biconical micro / nano optical fiber, a single-conical optical fiber, a quartz capillary tube, and a PDMS microsphere. The biconical micro / nano optical fiber is symmetrically bent, with the central waist region fiber forming a teardrop-shaped ring. The two ends of the teardrop-shaped micro / nano optical fiber ring are sequentially connected to portions of the waist region fiber, a transition region fiber, and a standard fiber. This invention utilizes the sensitive self-sensing capability of micro / nano optical fibers to achieve integrated signal input and output, greatly simplifying the measurement feedback mechanism of the sensing system and reducing overall cost. Furthermore, it is simple and intuitive to operate, reducing the technical requirements for operators. In terms of sensor fabrication, the main raw materials are standard optical fiber and quartz capillary tube, both of which are extremely low-cost. Therefore, this micro / nano optical fiber sensor exhibits significant cost-effectiveness.

Description

Technical Field

[0001] This invention relates to the field of biomechanics, utilizing micro / nano optical fibers to monitor changes in output light intensity, and to testing systems and devices for control, particularly to a weak force sensor based on a teardrop-shaped micro / nano optical fiber and its fabrication method. Background Technology

[0002] Precise measurement of microforces plays a crucial role in fields such as micromanipulation, materials science, biomolecular science, and biomechanics. Especially in biomechanics, measuring the biomechanical properties of single cells or living biological tissues (such as tension, pressure, adhesion, and elastic modulus) can provide more multidimensional and critical information for life science, physiology, and medical research, advancing our understanding of life processes, physiological mechanisms, and pathological progressions. Therefore, developing microforce sensing technologies with high measurement sensitivity and excellent biocompatibility is of great significance.

[0003] Currently, weak force sensing technologies used for biomechanical property testing mainly include atomic force microscopy, optical tweezers, magnetic tweezers, and ultrasonic shear waves. These technologies possess mechanical detection resolution on the pN to nN scale. However, these technologies generally suffer from high costs and complex detection equipment and feedback mechanisms, which significantly limit their further development and application. For example, the relatively long scanning time of atomic force microscopy may complicate or even limit the detection of dynamic living biological samples. To address these challenges, researchers have conducted numerous studies on fiber-optic optical sensors. Among them, a micro-force sensor with a Fabry-Pérot (FP) cavity, constructed by building a microstructure on the end face of a standard optical fiber, has achieved pN-level mechanical detection. However, as an interferometric sensor that detects peak wavelength drift, this type of sensor still requires a precise light source and spectrometer for signal demodulation. Furthermore, the non-integral materials and geometry of the sensor may lead to insufficient robustness and the risk of device damage.

[0004] Micro- and nano-fibers are special optical waveguides that are directly close to or smaller than the wavelength. Optically, they possess characteristics such as low transmission optical loss, strong optical field confinement, and extreme sensitivity to changes in the refractive index of the surface evanescent field. Mechanically, their bending radius can be as low as the micrometer scale, and their tensile strength reaches the GPa level, stronger than spider silk. Currently, U-shaped cantilever beam mechanical sensors based on micro- and nano-fibers exist, but directly applying them to the detection of biomechanical properties of single cells or biological samples encounters the following problems:

[0005] (1) The device is relatively large and has no clear position when in contact with the sample, making it difficult to accurately locate a single cell. At the same time, it is impossible to apply standard mechanical models to quantitatively evaluate biomechanical properties.

[0006] (2) During the detection, direct contact between micro-nano optical fibers and biological samples will cause surface contamination of the optical fibers and introduce unnecessary scattering signal noise.

[0007] To address the problems and challenges of applying the aforementioned weak force sensing technologies in the field of biomechanics, and to achieve in-situ, rapid, flexible, and reliable detection of living biological samples, a novel weak force sensor based on teardrop-shaped micro / nano optical fibers is proposed. This sensor possesses high sensitivity characteristics sufficient for biomechanical detection, while also exhibiting significant advantages in response time, ease of use, and robustness. It successfully achieves accurate Young's modulus measurement of single-cell and living biological samples, providing a practical and reliable new method. Summary of the Invention

[0008] This invention addresses the shortcomings of existing technologies by providing a weak force sensor based on teardrop-shaped micro / nano optical fibers and its fabrication method, thus enabling the application of weak force sensors in the field of biomechanical property detection. This invention is achieved through the following technical solutions:

[0009] This invention discloses a weak force sensor based on a teardrop-shaped micro / nano fiber, comprising a biconical micro / nano fiber, a single-conical fiber, a quartz capillary tube, and a PDMS microsphere. The biconical micro / nano fiber is symmetrically bent, with the central waist region fiber forming a teardrop-shaped ring structure. The two ends of the teardrop-shaped micro / nano fiber ring are sequentially connected to a portion of the waist region fiber, a transition region fiber, and a standard fiber. The waist region fibers, transition region fibers, and standard fibers at both ends are tightly bonded together. Two standard fibers pass through the quartz capillary tube and are constrained within it. The remaining portions of the waist region fiber, transition region fiber, and standard fiber outside the capillary tube are attached to the single-conical fiber and supported and fixed by it. The tail end of the single-conical fiber is fixed to the outer wall of the quartz capillary tube, with the remaining portion extending out of the quartz capillary tube. The PDMS microsphere is located at the center of the top of the teardrop-shaped micro / nano fiber ring.

[0010] This invention discloses a method for fabricating a weak force sensor based on a teardrop-shaped micro / nano optical fiber, comprising the following steps:

[0011] 1) The pre-drawn biconical micro / nano fiber is folded at the center of symmetry and the waist fiber is transformed into a teardrop-shaped ring structure by twisting, resulting in a teardrop-shaped micro / nano fiber ring. The remaining fibers on both sides, including part of the waist fiber, the transition fiber and the standard fiber, are tightly bonded together.

[0012] 2) Draw another standard optical fiber into a single-cone optical fiber, fix the tail end of the single-cone optical fiber to the outer wall of the quartz capillary tube with UV glue, and let the remaining part extend out of the quartz capillary tube.

[0013] 3) Pass the two standard fibers at the tail end of the biconical micro-nano fiber of the prepared teardrop-shaped micro-nano fiber ring through a quartz capillary tube, leaving one end of the teardrop-shaped micro-nano fiber ring extending out of the quartz capillary tube. Then, fix the waist region fiber, transition region fiber, and standard fiber of the biconical micro-nano fiber outside the quartz capillary tube onto the single-conical fiber with low-refractive-index UV adhesive, leaving only the teardrop-shaped micro-nano fiber ring unsupported.

[0014] 4) Using a micro-nano manipulation platform, uncured PDMS droplets are transferred to the top of the symmetrical center of the teardrop-shaped micro-nano fiber ring through another single-cone fiber, forming a PDMS microsphere structure. The PDMS is then cured by heating to obtain a complete weak force sensor based on a teardrop-shaped micro-nano fiber.

[0015] As a further improvement, the biconical micro / nano fiber described in this invention is pre-drawn using flame brush tapering technology, and the fiber diameter in the central waist region of the biconical micro / nano fiber is in the range of 1-3 μm.

[0016] As a further improvement, the width of the teardrop-shaped micro / nano fiber ring described in this invention is adjustable in the range of 60-130 μm.

[0017] As a further improvement, the refractive index of the low refractive index UV adhesive described in this invention is in the range of 1.4-1.45.

[0018] As a further improvement, the diameter of the PDMS microspheres described in this invention can be adjusted according to the number of PDMS droplets transferred, and the diameter is adjustable in the range of 10-30 μm.

[0019] As a further improvement, the micro-nano manipulation platform of the present invention includes a high-magnification optical microscope and a three-dimensional adjustment frame. The optical microscope is used for observation, and the three-dimensional adjustment frame uses a single-cone optical fiber for precise control of movement and transfer.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] (1) Compared with large-scale precision weak force sensing technology, this invention realizes the integration of signal input and output through the sensitive self-sensing capability of micro-nano optical fibers. It does not require expensive external detection equipment, which greatly simplifies the measurement feedback mechanism of the sensing system and reduces the overall cost. At the same time, it is simple and intuitive to operate, reducing the technical requirements for operators. In terms of the fabrication of the sensor itself, the raw materials it relies on are mainly standard optical fibers and quartz capillaries, both of which are extremely low in cost. Therefore, the micro-nano optical fiber sensor has significant and excellent cost-effectiveness.

[0022] (2) As an optical sensor that relies on optical transmission, the present invention has an ultra-fast response speed compared with electrical testing systems, which is beneficial for the rapid detection of dynamic living biological samples.

[0023] (3) Compared with fiber optic sensors based on the FP cavity principle, the present invention has higher stability and robustness. The main component of the micro-nano fiber optic sensor is a biconical micro-nano fiber, with a high degree of integration between the material and the geometry. At the same time, the high flexibility of the micro-nano fiber itself ensures that it can withstand relatively large deformations during detection, thus exhibiting high durability. In addition, the force measurement range of the present invention can be adjusted by adjusting the width of the teardrop-shaped ring and the diameter of the micro-nano fiber, providing high flexibility.

[0024] (4) The teardrop-shaped structure of this invention is itself a linear elastic structure, which conforms to Hooke's Law. The stress on the sensor and the strain of the sensor have a strict linear relationship, and the change in light intensity and the strain of the sensor also have a strict linear relationship. Therefore, the quantitative relationship between stress, strain, and change in light intensity can be obtained through the two linear relationships. The linear relationship greatly simplifies the calculation model.

[0025] (5) Compared with the U-shaped micro / nano fiber cantilever beam sensor, the most distinctive feature of this invention is the addition of a PDMS microsphere on the micro / nano fiber. This PDMS microsphere, serving as a direct contact medium, avoids surface contamination of the micro / nano fiber caused by direct contact between the micro / nano fiber and the sample during testing, further preventing the introduction of scattering noise. Furthermore, the PDMS microsphere's structure approximates the geometric characteristics of a standard sphere, facilitating the quantification of the pressing depth during contact testing, and allowing for the accurate calculation of the Young's modulus value by substituting it into the Hertzian contact model under the spherical probe.

[0026] (6) PDMS microsphere raw material: PDMS, short for polydimethylsiloxane, is an organosilicon polymer and a key material in fields such as biomedical engineering and bionics. PDMS has low cytotoxicity; fully cured PDMS itself has extremely low toxicity to most cell types, so it is highly safe to use it as a material that comes into contact with biological samples to be tested. At the same time, PDMS has high elasticity and flexibility, with a Young's modulus value typically between 0 and 5 MPa, similar to many soft tissues. Furthermore, the mechanical properties of PDMS can be adjusted by changing the ratio of prepolymer to curing agent.

[0027] (7) Since micro-nano optical fibers are lightweight, a single-cone optical fiber is specially designed and added to the sensor structure as a support unit to ensure that it can maintain horizontal self-support in its natural state when no external force is applied, thereby improving measurement accuracy.

[0028] (8) To better protect the overall optical fiber, quartz capillary tubes are used to fix and encapsulate the standard optical fiber, which makes it convenient for operators to clamp the sensor and perform other related operations. Attached Figure Description

[0029] Figure 1 This is a top view schematic diagram of a weak force sensor based on a teardrop-shaped micro / nano optical fiber provided in an embodiment of the present invention;

[0030] Figure 2 This is a side view schematic diagram of a weak force sensor based on a teardrop-shaped micro / nano optical fiber according to an embodiment of the present invention.

[0031] Figure 3 This is a schematic flowchart of a method for fabricating a weak force sensor based on a teardrop-shaped micro / nano optical fiber according to an embodiment of the present invention.

[0032] Figure 4 This is a schematic diagram of a test system for a weak force sensor based on a teardrop-shaped micro / nano optical fiber, provided in an embodiment of the present invention.

[0033] Figure 5 This is a schematic diagram of the mechanical sensing principle of a weak force sensor based on a teardrop-shaped micro / nano optical fiber provided in an embodiment of the present invention.

[0034] In the figure, 1-double-cone micro / nano fiber, 2-PDMS microsphere, 3-single-cone fiber, 4-quartz capillary, 5-sensing and testing equipment, 101-teardrop-shaped micro / nano fiber ring, 102-partial waist region fiber, 103-transition region fiber, 104-standard fiber, 501-computer, 502-tungsten lamp, 503-spectrometer, 504-controller, 505-three-dimensional electrically controlled displacement stage, 6-soft material under test. Detailed Implementation

[0035] This invention discloses a weak force sensor based on a teardrop-shaped micro / nano optical fiber and its fabrication method. The technical solution of this invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] Example 1

[0037] like Figure 1 , 2The figures shown are top and side views of a weak force sensor based on a teardrop-shaped micro / nano fiber. The sensor structure includes a biconical micro / nano fiber 1, a PDMS microsphere 2, a single-conical fiber 3, and a quartz capillary 4. The biconical micro / nano fiber 1 consists of three fiber parts arranged sequentially: a waist region fiber with the smallest and most uniform diameter; a transition region fiber 103 with a gradually increasing diameter; and a standard fiber 104. The waist region fiber located at the symmetry center of the biconical fiber is fabricated into a teardrop-shaped micro / nano fiber ring 101, and the remaining waist region fiber is a partial waist region fiber 102. The biconical micro / nano fiber 1 is symmetrically bent, with the central waist fiber forming a teardrop-shaped micro / nano fiber ring 101. The two ends of the teardrop-shaped micro / nano fiber ring 101 are sequentially connected to a portion of the waist fiber 102, a transition fiber 103, and a standard fiber 104. The portions of the waist fiber 102, the transition fiber 103, and the standard fiber 104 at both ends are tightly fitted together. Two standard fibers 104 pass through a quartz capillary tube 4 and are constrained within it. The remaining standard fibers 104, portions of the waist fiber 102, and the transition fiber 103 outside the quartz capillary tube 4 are all attached to a single-conical fiber 3 and supported and fixed by it. The tail end of the single-conical fiber 3 is fixed to the outer wall of the quartz capillary tube 4, with the remaining portion extending out of the quartz capillary tube 4. The PDMS microsphere 2 is located at the top center of the teardrop-shaped micro / nano fiber ring 101. PDMS, or polydimethylsiloxane, is a polymer material. It is a transparent silicone rubber material with excellent optical and deformation properties, as well as excellent biocompatibility, and is widely used in the field of biomechanics.

[0038] like Figure 3 As shown, the fabrication process of this weak force sensor based on teardrop-shaped micro / nano optical fiber is presented.

[0039] S1: The pre-drawn biconical micro / nano fiber 1 is folded at its center of symmetry and twisted to transform the waist region fiber into a teardrop-shaped ring structure, resulting in a teardrop-shaped micro / nano fiber ring 101. The remaining fibers on both sides, including a portion of the waist region fiber 102, the transition region fiber 103, and the standard fiber 104, are tightly bonded together. The specific operation process includes: stripping a small section of the coating layer from the middle of the standard multimode fiber, fixing it on a three-dimensional electrically controlled displacement stage 505, heating the fiber with an oxyhydrogen flame to a molten state, and simultaneously operating the three-dimensional electrically controlled displacement stage 505 to stretch the fiber at both ends. During the heating and stretching process, the diameter of the fiber gradually decreases to the micrometer level, resulting in the biconical micro / nano fiber 1. The drawn biconical micro / nano fiber 1 is gently folded at its narrowest point and twisted once to obtain a teardrop-shaped ring with a relatively large diameter. Then, a pigtail of the biconical micro / nano fiber 1 is twisted. During the twisting process, the waist region fiber 102 and the transition region fiber 103 at both ends of the folded fiber are wrapped together. At the same time, the diameter of the teardrop-shaped micro / nano fiber ring 101 decreases until the ring width reaches the target value.

[0040] S2: Another standard optical fiber is drawn into a single-cone optical fiber 3. The tail end of the single-cone optical fiber 3 is fixed to the outer wall of the quartz capillary tube 4 with UV adhesive, and the remaining part extends out of the quartz capillary tube 4. The specific operation process includes: first drawing a double-cone micro / nano optical fiber 1 with a waist region fiber diameter of about 20μm using the above-mentioned flame brush technique, then turning off the oxyhydrogen flame, stopping the heating, and further operating the three-dimensional electrically controlled displacement stage 505 to continue stretching the optical fiber to both ends until the optical fiber breaks at the narrowest point in the center, thus obtaining two single-cone optical fibers 3. The tail end of the single-cone optical fiber 3 is still a standard optical fiber. The standard optical fiber end is fixed to the outer wall of the quartz capillary tube 4 with UV adhesive, leaving a length of about 5mm beyond the quartz capillary tube 4. This reserved part of the single-cone optical fiber 3 will serve as a support later.

[0041] S3: Pass the two standard fibers 104 at the tail end of the biconical micro / nanofiber 1, which has already been fabricated into a teardrop-shaped micro / nanofiber ring 101, through a quartz capillary tube 4, leaving one end of the teardrop-shaped micro / nanofiber ring 101 protruding outside the quartz capillary tube 4. Secure the portion of the biconical micro / nanofiber 1 outside the quartz capillary tube 4—the waist region fiber 102, the transition region fiber 103, and the standard fibers 104—to the single-conical fiber 3 using low-refractive-index UV adhesive, leaving only the teardrop-shaped micro / nanofiber ring 101 unsupported. The specific operation includes: combining the two standard fibers 104 of the biconical micro / nanofiber 1, which has been bent into a teardrop shape in S1, and passing them through the quartz capillary tube 4, confining them within the quartz capillary tube 4, leaving the teardrop-shaped micro / nanofiber ring 101, the portion of the waist region fiber 102, and the transition region fiber 103 outside the quartz capillary tube 4. A separate optical fiber is dipped in a small amount of uncured low-refractive-index UV adhesive. Using a scraping technique, a portion of the waist region fiber 102 and the transition region fiber 103 are adhered to the supporting single-cone optical fiber 3, leaving only the teardrop-shaped micro / nano fiber ring 101 in a naturally suspended, unattached state. UV light is used to cure the UV adhesive, completing the fixation of the double-cone micro / nano fiber 1. This allows the double-cone micro / nano fiber 1 to maintain self-support even under natural, stress-free conditions. Furthermore, during testing, the sensor can be manipulated by clamping the quartz capillary tube 4, significantly reducing the damage rate of the double-cone micro / nano fiber 1 and improving the sensor's durability.

[0042] S4: Using a micro-nano manipulation platform, uncured PDMS droplets are transferred through another single-cone optical fiber to the apex of the teardrop-shaped micro-nano fiber ring 101, forming a PDMS microsphere 2 structure. The PDMS is then cured by heating to obtain a complete weak force sensor based on the teardrop-shaped micro-nano fiber. The specific operation includes: preparing a thinner single-cone optical fiber, dipping it in uncured PDMS, and forming PDMS droplets on the surface of the single-cone optical fiber under the action of liquid surface tension. Under the micro-nano manipulation platform, the single-cone optical fiber is controlled to be tilted and tangent to the teardrop-shaped micro-nano fiber ring 101, with the tangency position at the apex of the teardrop-shaped micro-nano fiber ring 101. The single-cone optical fiber is slowly removed, and the PDMS droplets on its surface are transferred to the teardrop-shaped micro-nano fiber ring 101. This transfer operation is repeated multiple times, controlling the number of transferred PDMS droplets, until the formed PDMS microsphere 2 reaches the target diameter. After obtaining PDMS microspheres 2 of the target diameter, the entire sensor was placed on a heating stage to solidify the PDMS at 80°C for approximately 20 minutes. Thus, a weak force sensor based on teardrop-shaped micro / nano optical fibers was successfully fabricated.

[0043] The pre-drawn biconical micro / nanofiber is specifically produced using flame-brush tapering technology. Standard optical fibers are uniformly stretched under oxyhydrogen flame heating, and the diameter of the fiber's central waist region is controlled according to the stretching length. Therefore, the diameter of the biconical micro / nanofiber 1 required for the weak force sensor is within the range of 1-3 μm. The width of the teardrop-shaped micro / nanofiber ring 101 is adjustable within the range of 60-130 μm, and its size is controlled during the twisting process to prepare the teardrop-shaped micro / nanofiber ring 101. The larger the diameter of the biconical micro / nanofiber 1 or the smaller the width of the teardrop-shaped micro / nanofiber ring 101, the greater the stiffness of the weak force sensor based on the teardrop-shaped micro / nanofiber, i.e., the larger the corresponding spring constant k value. Therefore, in practice, the spring constant k of the sensor can be adjusted by regulating the diameter of the biconical micro / nanofiber 1 and the width of the teardrop-shaped micro / nanofiber ring 101.

[0044] When bonding part of the waist region fiber 102 and the transition region fiber 103 to the support single-cone fiber 3, the selected UV adhesive is a low-refractive-index UV adhesive, with a selectable refractive index in the range of 1.4-1.45, which is less than the refractive index of the fiber core, to meet the conditions for total internal reflection light transmission and ensure higher transmission transmittance.

[0045] The PDMS microspheres 2 are obtained through a micro-nano manipulation platform. Their diameter can be adjusted based on the number of PDMS droplets transferred. Each time a PDMS droplet is added and merges with the already formed PDMS microspheres 2, the diameter of the existing PDMS microspheres 2 is measured using a microscope. The PDMS microspheres 2 are gradually increased in size until the target diameter is reached. The target diameter is determined based on actual measurement needs; for example, smaller diameter PDMS microspheres 2 are used to test smaller cells, or smaller diameter PDMS microspheres 2 are used if a greater pressing depth is required. The diameter of the PDMS microspheres 2 is adjustable within the range of 10-30 μm. The micro-nano manipulation platform consists of a high-magnification optical microscope and a three-dimensional adjustment frame. The high-magnification optical microscope is used to observe the relative positional relationship between the single-cone fiber used for transfer and the teardrop-shaped micro-nano fiber ring 101, and to measure the diameter of the PDMS microspheres 2. The three-dimensional adjustment frame is used to precisely control the movement of the single-cone fiber used for transfer.

[0046] like Figure 4 The diagram shows a test system for applying this teardrop-shaped micro / nano fiber-based weak force sensor to perform mechanical sensing. In addition to the sensor, the sensing test system includes a computer 501 for receiving and processing data, a tungsten lamp 502 as an input source of optical signals, a spectrometer 503 for receiving and outputting spectral signals, a three-dimensional electrically controlled displacement stage 505 for high-precision adjustment of the sensor position, and a controller 504 for controlling the displacement stage.

[0047] During the test, under the monitoring of an optical microscope, the interaction between the sensor and the sample is precisely controlled by a three-dimensional electrically controlled displacement stage 505. The signal light is provided by a tungsten lamp 502 and transmitted into the sensor via a standard optical fiber 104 at one end. The output signal is output from the other end of the standard optical fiber 104, received by the spectrometer 503, and transmitted to the computer 501 for signal processing.

[0048] Figure 5 This paper demonstrates the mechanical sensing principle of a weak force sensor based on a teardrop-shaped micro / nano fiber. The left side of the figure shows the teardrop-shaped micro / nano fiber ring 101 in its initial unstressed state, while the right side shows the teardrop-shaped micro / nano fiber ring 101 after interacting with and deforming with the soft material 6 being measured. Comparing the two states, it is clear that the teardrop-shaped micro / nano fiber ring 101 expands outwards after being subjected to force, tending towards a standard circle. During this process, the radius of curvature of the micro / nano fiber at the central waist region increases, meaning the degree of bending decreases, i.e., bending loss decreases. In actual testing, this is visually manifested as an increase in output light intensity. Therefore, this invention can achieve self-sensing of the sensor's own deformation by monitoring changes in output light intensity. The magnitude of the force corresponding to the sensor's deformation can be obtained by testing its interaction with a cantilever beam of known stiffness, thus achieving calibration of the sensor's mechanical properties.

[0049] When the sensor is subjected to the same load and interacts with soft materials 6 of varying hardness, the deformation at the sensor end differs, resulting in different responses to changes in output light intensity. When interacting with the relatively harder soft material 6, the deformation at the sensor end is greater, meaning the change in light intensity is larger. Therefore, this characteristic can be used to distinguish the hardness of objects. In quantitative calculations, from... Figure 5 The schematic diagram clearly shows the geometric positional relationship between the teardrop-shaped micro / nano fiber optic ring 101 and the tested soft material 6 during the test. We can obtain l0 – l1 + sample deformation = compression displacement. Here, l0 and l1 are the initial length and the deformed length of the teardrop-shaped micro / nano fiber optic ring 101, respectively, and the difference between them is defined as the sensor deformation. Therefore, the sample deformation can be obtained from the compression displacement and the sensor deformation. The sensor deformation can be obtained from the change in light intensity. Finally, the magnitude of the external force acting on the tested soft material 6 and the corresponding deformation can be obtained. Substituting these values ​​into the Hertzian contact model under the spherical probe, we can achieve the quantitative calculation of the Young's modulus of the tested soft material 6.

[0050] Example 2

[0051] In this invention, the addition of PDMS microspheres 2 to the teardrop-shaped micro / nano fiber ring 101 serves not only as a pressing point with an approximately standard geometric structure, but more importantly, as a sensor-enhancing effect. PDMS has a refractive index of 1.41, higher than that of air (n=1). Compared to the absence of PDMS microspheres, this reduces the refractive index difference between the fiber and the external environment, weakening the fiber's ability to constrain the transmitted light. Consequently, when the teardrop-shaped micro / nano fiber ring 101 undergoes bending deformation, more light that was originally transmitted in waveguide mode is converted to radiation mode and dissipated into the environment, resulting in greater light intensity loss. Furthermore, with the addition of PDMS microspheres 2, light passing through the waist region of the fiber needs to cross an "air-PDMS-air" interface. This crossing of the "air-PDMS" interface generates significant interface reflection and scattering losses, which further amplify the light intensity loss caused by bending deformation. Therefore, the final output light intensity response is the result of the superposition of radiation loss, interface scattering loss and interface reflection loss. In the absence of PDMS microspheres 2, the loss caused by fiber bending deformation is only micro-bending loss. Moreover, in the case of an external environment of air, the fiber has a strong ability to constrain the transmitted light, so the micro-bending loss is small.

[0052] Sensitivity tests were conducted on a comparative experiment of a weak force sensor with and without PDMS microspheres 2. In the experiment, the weak force sensor without PDMS microspheres 2 was first subjected to a pressing operation, with the glass slide pressed in 1.5 μm increments. At this point, the teardrop-shaped micro / nano fiber ring 101 made direct contact with the glass slide, resulting in significant scattering and a momentary drop in light intensity. However, the overall light intensity change was not significant with increasing pressing depth. The same pressing operation was then performed on the sensor after adding PDMS microspheres 2. In this case, the direct contact medium was the PDMS microspheres 2, which facilitated mechanical conduction. The teardrop-shaped micro / nano fiber ring 101 did not make direct contact with the glass slide. The light intensity drop caused by scattering at the moment of contact was negligible. Furthermore, with increasing pressing depth, the light intensity increased significantly, forming a distinct incremental step signal. Calculations showed that the mechanical sensitivity of the weak force sensor increased by approximately three times after adding PDMS microspheres 2 compared to when there were no PDMS microspheres, confirming the sensitizing effect of PDMS microspheres 2. Simultaneously, the change in light intensity caused by scattering upon contact with the glass slide also indicates that PDMS microspheres 2 have an isolation and protective function, protecting the light field and preventing direct interaction between the light field and the measured object to avoid introducing unnecessary scattering losses.

[0053] In summary, this invention proposes a weak force sensor based on teardrop-shaped micro / nano optical fibers. This sensor can acquire information on deformation and external force by self-sensing its own deformation. Compared with other sensing technologies, this sensor has advantages such as high testing sensitivity, rapid response, excellent robustness, flexible operation, and strong biocompatibility, providing a practical and reliable new testing method for the field of biomechanics.

[0054] It will be understood by those skilled in the art that the above description is merely a single example of the invention and is not intended to limit the invention. Although the invention has been described in detail with reference to the foregoing examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent substitutions for some of the technical features. All modifications and equivalent substitutions made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A weak force sensor based on teardrop-shaped micro / nano optical fibers, characterized in that, The device comprises a biconical micro / nano fiber, a single-conical fiber, a quartz capillary tube, and a PDMS microsphere. The biconical micro / nano fiber is symmetrically bent, with the central waist fiber forming a teardrop-shaped micro / nano fiber ring with a width ranging from 60 to 130 μm. The two ends of the teardrop-shaped micro / nano fiber ring are sequentially connected to a portion of the waist fiber, a transition fiber, and a standard fiber, with the waist fibers, transition fibers, and standard fibers tightly bonded together. Two standard fibers pass through the quartz capillary tube and are constrained. Inside the quartz capillary, the remaining portion of the waist region fiber, transition region fiber, and standard fiber outside the capillary are all attached to the single-cone fiber and supported and fixed by the single-cone fiber. The tail end of the single-cone fiber is fixed to the outer wall of the quartz capillary, and the remaining part extends out of the quartz capillary. The PDMS microsphere is located at the center of the top of the teardrop-shaped micro / nano fiber ring for direct contact with the biological sample to be tested. The fabrication method of the weak force sensor based on the teardrop-shaped micro / nano fiber includes the following steps: 1) Fold the pre-drawn double-cone micro / nano fiber at the center of symmetry and perform a twisting operation. 1) Transform the waist region fiber into a teardrop-shaped ring structure with a width in the range of 60-130 μm to obtain a teardrop-shaped micro / nano fiber ring. The remaining fibers on both sides, including part of the waist region fiber, the transition region fiber, and the standard fiber, are tightly bonded together; 2) Draw another standard fiber into a single-cone fiber, and fix the tail end of the single-cone fiber to the outer wall of the quartz capillary tube with UV adhesive, leaving the remaining part protruding outside the quartz capillary tube; 3) Pass the two standard fibers at the tail end of the double-cone micro / nano fiber of the already prepared teardrop-shaped micro / nano fiber ring through the quartz capillary tube, leaving one end of the teardrop-shaped micro / nano fiber ring protruding from the quartz capillary tube. Outside the capillary, the waist region fiber, transition region fiber and standard fiber of the biconical micro / nano fiber outside the quartz capillary are fixed to the single-conical fiber with low refractive index UV adhesive, leaving only the teardrop-shaped micro / nano fiber ring unsupported; 4) Using a micro / nano manipulation platform, the uncured PDMS droplet is transferred to the top of the symmetrical center of the teardrop-shaped micro / nano fiber ring through another single-conical fiber, and the PDMS microsphere structure with a diameter of 10-30μm is naturally formed by the surface tension of the liquid. The PDMS is then cured by heating to obtain a complete weak force sensor based on the teardrop-shaped micro / nano fiber.

2. The weak force sensor based on teardrop-shaped micro / nano optical fiber according to claim 1, characterized in that, The refractive index of the low-refractive-index UV adhesive is in the range of 1.4-1.

45.

3. The weak force sensor based on teardrop-shaped micro / nano optical fiber according to claim 1 or 2, characterized in that, The micro-nano manipulation platform includes a high-magnification optical microscope and a three-dimensional adjustment frame. The optical microscope is used for observation, and the three-dimensional adjustment frame uses a single-cone optical fiber for precise control of movement and transfer.

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