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 6?x?5 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].

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 megohms was recorded for the PLGA/Ag NWs/PVA e-skin.

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.