Wearable Sensor and Smart Technology using electrospinning

With high adoption of smartphones, there is an increasing interest in development in wearable sensors and smart technologies for monitoring human health and performance. Flexible sensors may be attached to the skin to detect movement or monitor health. Body movement may be used to generate electricity to power sensors. Tiny and flexible electronics may be worn and used to interact with nearby processors. These are just some possibilities as more research and development are carried out in this field.

With highly sensitive pressure sensors from electrospun fibers, it is possible to detect body movements. During electrospinning, the electric field encourages dipolar alignment in the direction of the electric field. Such alignment can be further enhanced with a rotating collector that stretches the nanofiber and physically aligns the fibers. Persano et al (2013) demonstrated the advantage of electrospun P(VDF-TrFE) fibers alignment using a rotating collector in enhancing the performance of pressure sensors. The aligned electrospun fibers showed greater fraction of polar β phase with overall crystallinity of 48% while random mat has a crystallinity of 40%. At a low pressure range between 0.4 and 2 kPa, the sensitivity of the device was 1.1 VkPa^-1. The voltage output was also found to be unaffected by the length of the fiber array in the range between 2 cm and 8 cm. To further demonstrate the sensitivity of highly aligned P(VDF-TrFE) fiber bundles to strain, Hsu et al (2017) attached it perpendicularly to the brachioradialis of the forearm and the biceps of the upper arm. The piezoelectric bundle was able to detect direction of forearm rotation by showing positive or negative voltage output. For the piezoelectric bundle attached to the biceps, level of isometric contraction was detected by increasing or decreasing voltage output. This study demonstrated the feasibility of using electrospun bundles in wearable textile sensors.

(a) Photograph of a simple, P(VDF-TrFe) nanofiber-based accelerometer. Scale bar, 1cm. (b) Output voltage collected from this device exposed to 70 dB sound intensity. (c,d) Photographs of a flexible device mounted on the skin of the arm and wrapped around a finger. Scale bar, 2 cm. (e) Characterization of an orientation sensor, based on a pressure sensor with an attached test mass. The output voltage changes with orientation angle, θ, in an expected manner. The inset provides a sketch of the device (yellow: PI; grey: fibres; blue: test mass) and the measurement geometry [Persano et al 2013].

Abolhasani et al (2022) demonstrated the feasibility of a wearable pressure sensor using electrospun porous PVDF fibers. Electrospun PVDF fibers with inner pores showed a high β-phase of up to 92%. A mechanical impact (0.2 MPa) on the electrospun nanofibers gave a voltage response of 22.6 V. A pressure sensor was assembled with a 6x5 cm² electrospun PVDF membrane sandwiched between two electrodes composed of Kapton tapes coated with aluminum. The pressure sensor was able to give a voltage output of 1.5 V for wrist movements, 2 V for elbow movements, 4.96 V for walking and 0.47 V for finger tapping. With a cyclic pressure caused by finger tapping, the voltage output was constant at 0.47 V which demonstrated the stability and reproducibility of the sensor response.

(a) Schematic of the pressure sensor. The output voltages of the pressure sensor under (b) wrist bending and (c) elbow bending [Abolhasani et al 2022].

Li et al (2023) developed a low intensity sensitive, wearable heart sound sensor using electrospun PVDF layer as the tribo-negative and electrospun polyurethane (PU) layer as a tribo-positive layer. With this triboelectric pair, the output voltage can be enhanced and the generated charge can be trapped to prevent charge recombination at the electrode. The PU fiber layer was created using near field electrospinning to form ordered grids and the microcavities between the fibers help to reduce air damping and these create resonance and improve sensitivity while the PVDF fibers were randomly oriented from conventional electrospinning setup. The assembled sensor consists of the PVDF tribo-negative layer sandwiched between two PU tribo-positive layers using double sided tapes so that there is an air gap between the layers. These electrospun layers were in turn sandwiched between two electrode layers. When there is an acoustically induced vibration the center tribo-negative layer contacts the upper positive tribo-positive layer thus creating electrification. Separation of the layers result in electrostatic induction and electron flow from the lower electrode to the upper electrode. The bottom tribo-positive layer functions as a barrier to prevent the recombination of the surface charge on the center tribo-negative layer from the induced positive charge on the lower electrode. When the negative tribo-negative layer touches the lower positive tribo-electic material, the upper positive tribo-electric layer is the barrier to prevent recombination at the upper negative tribo-electric layer. The PU fibers grid spacing was also found to affect the sensitivity of the sensor with 0.4 mm grid spacing being the optimum. Lower grid spacing reduces sensitivity due to increased air damping while higher grid spacing reduces the ability of the PU layer to prevent charge recombination.

a) Fabrication of the heart sound sensor. b) Structure of the sensor, including electrodes, PU grid spacer, and a PVDF layer. c) Schematic of near-field/far-field hybrid electrospinning. d) Image of a PU grid spacer. e) Scanning electron microscopy image of a PVDF film [Li et al 2023]. The sensor showed good output at heart sound frequency range of 50-150 Hz with a sensitivity of 7027 mV Pa-1 and a low detection limit of 47 dB. The sensor demonstrated excellent stability with no degradation during 7 h continuous operation [Li et al 2023].

Another method of detecting movement is to use a strain sensor instead of pressure sensor. Wu et al (2023) used electrospun thermoplastic polyurethane (TPU) membrane and coated it with a uniform and dense conductive layer of carbon nanotubes (CNT) on the surface of the TPU membrane. When the sensor was stretched, the conductive layer fractured which increased the resistance of the sensor but recovered to its interlocking position when relaxed. This allowed the strain sensor to be reused and reproduced the same linear change in relative resistance. The optimized strain gauge exhibited a gauge factor of 420 within the 0 - 200% strain range. When tested as a wearable flexible electronic skin, it has been shown to detect movements through the deformation of the strain sensor. Deformation of the sensor leading to stretching or overlapping layers caused changes to the relative resistance of the sensor. Accurate responses were recorded when the fingers were bent at 30°, 60° and 90°.

In a step towards the development of smart textiles, self-powered flexible humidity sensors based on electrospun fibers have been fabricated. Wang et al (2021) used electrospun poly(vinyl alcohol)/Ti₃C₂T_x (PVA/MXene) as the humidity sensing material. This electrospun membrane was deposited on an interdigital electrode. To power the humidity sensor, a monolayer molybdenum diselenide (MoSe₂) piezoelectric nanogenerator (PENG) was fabricated on a flexible terephthalate (PET) substrate. The MoSe₂ PENG converts mechanical energy to electric energy and the electrical current runs through the PVA/MXene sensor material. The PVA/MXene sensor material showed large response, fast response/recovery time, low hysteresis and excellent repeatability through the detection of output voltage with changes in humidity. At higher humidity, the resistance of PVA/MXene membrane increases. This is probably due to greater layer spacing between the highly conductive MXene layers as water molecules enter the hydrophilic membrane.

Triboelectric power generation is the harvesting of charges induced from triboelectric processes to convert mechanical energy into electric power. The relative ease of generating power using this setup has encouraged its incorporation into devices. Peng et al (2020) constructed a self-powered electronic skin using electrospun triboelectric nanogenerators. This triboelectric nanogenerator is made out of electrospun polylactic-co-glycolic acid (PLGA) fibers layer and an electrospun polyvinyl alcohol (PVA) fibers layer with silver nanowires (AgNW) sandwiched between the two layers. To have this AgNW layer, a vacuum filtration method was used to pass a suspension of AgNW solution through the electrospun PVA membrane layer where the nanofibers membrane traps the AgNW. Electrospinning of PLGA was carried out with the electrospun PVA membrane loaded with AgNW as the collector. A copper foil was attached to the surface of the membrane as the lead out. Electricity generation and transmission is realized by the periodic contact and separation between the constructed electronic skin (e-skin) and a contact object. The induced potential difference between the inner AgNW inner electrode and the external ground object will cause a charge transfer between them and generate a current. A maximum areal power density of 130 mW m^-2 at a matched resistance of ~500 MΩ was recorded for the PLGA/Ag NWs/PVA e-skin. The electrical output from triboelectric nanogenerator (TENG) constructed using electrospun fibers can be easily used for detecting and measuring movements. Zhou (2024) constructed a TENG with thermoplastic polyurethane (TPU)/PVDF as the negative triboelectric material and nylon as the positive triboelectric materials (TP-TENG). A maximum output voltage of 699 µW was recorded for the TP-TENG. When installed in a running shoe, the output voltage signal was able to accurately reflect the step rate of 120 step min^-1 and at a faster step rate of 180 step min^-1.

Other smart apparels may also be developed using wearable triboelectric nanogenerators (TENGs). Chen et al (2023) constructed a chemical resistant TENG (CR-TENG) using electrospun polysulfonamide (PSA) with polyurethane (TPU) as the carrier polymer and polytetrafluoroethylene (PTFE) nanoparticles and polyvinyl alcohol (PVA) as the carrier polymer. The electrospun PTFE/PVA fibers subsequently undergo heat treatment to obtain sintered PVA-PTFE (sPTFE/PVA) membrane. After the PSA/TPU and sPTFE/PVA membrane surfaces come into contact, PSA/TPU surface loses electrons and attained a positive potential while the sPTFE/PVA surface attained a negative potential. The changes in potential generated an AC output as the surfaces undergo contact/separation cycles with a short-circuit current of 250 nA and open-circuit voltage of 26 V. The CR-TENG output remained stable after immersing in strong acid and alkali solution for up to 7 days. The CR-TENG embedded into a glove can be used to sense the working state of the protective glove. For an intact glove, the output voltage was 37V but dropped to 27 V when damaged. An alarm may be triggered when the output voltage or current drops below a predetermined value to ensure safety of the user.

Electrospun fibers have been used to construct ultrathin fiber-mesh polymer positive temperature coefficient (PTC) thermistors which has the potential to be used as wearable sensors. Okutani et al (2022) electrospun fibers made of a mixture of octadecyl acrylate, butyl acrylate and poly(3,4-ethylenedioxythiophene)-tetramethacrylate (PEDOT-TMA). Carbon nanofibers were added into the matrix as conductive fillers. A thin layer of parylene was coated using chemical vapor deposition after the fibers have been fabricated. This sheath layer of parylene helps to maintain the integrity of the fibers at temperature of 35° which is the melting temperature of the composite fiber. It is also at this melting temperature that the thermistor showed a change in the resistance by three orders of magnitude. In cyclic temperature test between 25 °C and 37 °C, the electrospun mesh thermistors coated with 1 µm parylene was able to maintain a resistance of over 1.2 GΩ at temperature of 37 °C for up to 400 cycles. At temperature of 25 °C; the resistance varied from 0.45 to 2.8 MΩ. Such variation in the resistance may be attributed to the movement of conductive fillers when the composite material is in its molten state. Advantages of this electrospun thermistor mesh is that it can be made into an ultra-lightweight, breathable and transparent layer with high flexibility. The mesh thermistor, in a wrinkled state, was able to maintain the property of large increase in resistance when the temperature increased. Cyclic testing at 25% stretching for 200 cycles showed an increase of resistance by about 30% but otherwise maintained its large increase in resistance, indicating high mechanical durability.

Having a stretchable and conductive membrane will make integration of electronic parts easier on a fabric. To construct a highly conductive membrane, Codau et al (2024) coated a layer of conductive poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT: PSS) over thermoplastic polyurethane (TPU)/multi wall carbon nanotubes (MWCNTs) fibers using wet electrospinning where fibers were deposited on a coagulation bath. A solution of functionalized multi wall carbon nanotubes was blended with TPU for electrospinning. The electrospinning jet directly deposited fibers into a coagulation bath made of an aqueous solution of PEDOT: PSS. The TPU/MWCNTs fibers were immediately saturated by the PEDOT: PSS conductive polymer solution and formed porous fibers through nonsolvent-induced phase separation. The membrane was maintained in the PEDOT: PSS solution for 180 min before vacuum drying for completion of phase separation. The formation of micropores on the fibers may allow carboxylic groups of the functionalized MWCNT to form hydrogen bonds with the sulfonate groups from PEDOT: PSS and to form conjugate between them which would facilitate movement of charge carriers. An optimal Power Factor value of 4.28 µW m^-1 K^-2 was obtained with an electrical conductivity of 85 Sm^-1 and the TPU base provided a high strain value of 190%. Yang et al (2025) electrospun polyurethane (PU)/sodium dodecyl sulphate (SDS) membrane as a substrate for conductive and stretchable liquid metals (LMs) blended with iron powder. After the LMiron mixture was coated on the PU/SDS membrane, the membrane was stretched through 10 cycles at 200% strain to improve permeability and electrical stability. The resultant conductive composite membrane has a stretchability of 1130?% strain, permeability of 792.71?gm^-2 day^-1), high electrical conductivity of 12105 S cm^-1 and electromechanical stability demonstrated by over 5000 cycles at 100% strain with resistance increased by 1.03?%. The PU/SD/LMiron membrane assembled into an electrode for electrophysiological signals showed lower skin-interface impedance across a broad frequency range compared to commercial gel electrodes.

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