Electrospun pressure sensors

Electrospinning is a highly versatile nanofiber production method with numerous materials being electrospun into nanofibrous structures. Electrospun products typically take the form of a flat membrane and this makes it suitable as a pressure sensor. Materials with piezoelectric properties may be electrospun into membranes and used as pressure sensors. Conductive nanomaterials may be loaded into otherwise non-conductive materials and electrospun into membranes for use as pressure sensors. Other creative methods have also been utilized to incorporate electrospun nanofibers in pressure sensor applications.

Piezoelectric polymers have been routinely electrospun to form nanofibrous membrane. Due to significant electrical response to mechanical stimulus, electrospun piezoelectric polymers have the potential to be used in highly sensitive pressure sensors. A study by Chang (2009) found that the average energy conversion efficiency for a single poly(vinylidene fluoride) (PVDF) nanofiber was 12.5%, going as high as 21.8%, which is much higher than the energy conversion efficiency of PVDF thin film which average about 1.3%. This may be due to the presence of β-phase in electrospun PVDF fibers [Costa et al 2010] while normal PVDF exists in α-phase.

Capacitive pressure sensors can be constructed with electrospun membrane as the dielectric layer. The sensor works by measuring the changes in the capacitance value between two electrodes which is influenced by the gap between them, overlapping surface area and electrical properties of the dielectric layer. With electrospun poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) as the dielectric layer, Kim et al (2017) was able to increase the sensitivity of the pressure sensor. Unlike spin coated polydimethylsiloxane (PDMS), electrospun P(VDF-TrFE) has much lower stiffness (6 times lower) which makes it a better candidate for use in wearable technology. In their device, the electrospun dielectric layer was sandwiched between two aluminum sheets (electrodes). Thickness of the electrospun membrane was found to have a significant influence in the sensitivity towards high or low pressure. With thicker membrane, the pressure sensor was very sensitive to low pressure (less than or equal to 0.12kPa) application with initial sensitivity of 2.81 kPa^-1. However, the sensitivity reduces dramatically at higher pressure. This has been attributed to lower membrane stiffness for thicker membrane. Response time of the sensor was good at 42 ms with low hysteresis.

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].

The sensitivity of the pressure sensor may be increased by adding suitable additives into the base piezoelectric material. Liu et al (2021) demonstrated this by blending ZnO nanoparticles into poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) solution and electrospinning into nanofibers. ZnO is a dielectric material and it facilitates the nucleation of β-phase crystals in PVDF-TrFE. Annealing is also carried out on the electrospun fibers to increase the crystal size. With the addition of ZnO and annealing, Liu et al (2021) was able to achieve an optimal peak-to-peak voltage response of 1.788 V which was a 75% increase compared to that of the pristine PVDF-TrFE sensor. This electrospun PVDF-TrFE/ZnO nanofiber membrane pressure sensor is sensitive enough to pick up a human pulse with a frequency of around 1 Hz.

While P(VDF-TrFE) exhibits good dielectric property, the copolymer has a higher cost compared to poly(vinylidenefluoride) (PVDF). To improve the dielectric property of PVDF, Li et al (2014) doped PVDF with silver nanowires (AgNW) through blending into the PVDF solution for electrospinning. Interaction between the polymer and AgNW significantly increases the crystalline polar β phase in PVDF and this increases its dielectric property. Assembled into a pressure sensor by sandwiching the membrane between two aluminum sheets, a sensitivity of 30 pC/N was recorded for the nanofiber membrane containing 1.5 wt% AgNW. Another method of improving the voltage output from electrospun PVDF membrane is to construct porous fibers. Abolhasani et al (2022) was able to use phase separation to electrospin fibers with inner pores with smooth outer surface. The electrospun non-porous PVDF fibers exhibited a β-phase of about 77% while porous PVDF fibers exhibited a β-phase of up to 92% for the most porous fiber. They attributed this to trapped molecular chains in the β-phase due to confined space between the pores. To obtain porous fibers, water which is a non-solvent was added to the PVDF solution to induce phase separation during electrospinning. The higher β-phase translates into higher voltage response when the membrane experiences a mechanical vibration. For the non-porous PVDF fibers, a mechanical impact (0.2 MPa) on the electrospun nanofibers gave a voltage response of 4.4 V but with highly porous fibers, a voltage response of 22.6 V was recorded. To increase the β-phase in non-porous PVDF fibers, 0.1wt% of graphene was able to increase the β-phase to 84%. However, further increase in graphene leads to reduction in the β-phase. The increased β-phase with the graphene addition was able to significantly increase the voltage response of non-porous fibers from 4.4V to 14.8 V although this is still lower than the voltage obtained from porous PVDF fibers. When assembled as a pressure sensor, the porous fiber PVDF membrane was able to give an output voltage of 4.96 V in a walking test which can be detected in a wearable electronic circuit. With finger tapping, a consistent voltage output of 0.47 V was recorded demonstrating stability and reproducibility of the sensor response.

Electrospinning have been used to construct piezoresistive sensor where pressure transduced onto the sensor is converted into resistance signal for detection. Han et al (2019) uses electrospinning to construct a carbon nanofiber networks (CNFNs) with a superior sensitivity of 1.41 kPa^-1. The carbon nanofiber is derived from carbonization of electrospun polyacrylonitrile (PAN) membrane. AlCl₃ was added to the PAN solution prior to electrospinning so that Al₂O₃ nanoparticles are formed on the carbon fibers following heat treatment. Doping with Al₂O₃ increases the fracture toughness of the carbon nanofiber networks (CNFNs). The CNFNs was able to show excellent flexibility during bending and twisting. The constructed sensor also showed outstanding repeatability (5000 times), fast response time (<300 ms), super compressibility (>95%) and ultralight. Preliminary tests demonstrated its feasibility for use as a artificial electronic skin. Chen et al (2021) constructed an array of electrospinning derived Ag nanofibers electrodes as part of a triboelectric nanogenerator (TENG) technology that is flexible with pressure-mapping sensing capability. The Ag nanofibers were produced by electrospinning polyvinyl alcohol (PVA) followed by sputter coating a layer of Ag on the PVA nanofibers. The PVA nanofiber template was removed by dissolving in water and transferred to a flexible polyethylene terephthalate (PET) layer. The resultant Ag nanofibers layer was modified to form a 4 x 4 electrodes array using lithography and wet etching process. The electrodes array was packaged with polydimethylsiloxane (PDMS) to give a pressure sensor array where each square electrode is linked to a pixel on screen to create a TENG capable of detecting mechanical stimulations. The ability of this pressure sensor array to detect localized mechanical stimulation was demonstrated by pressing a "Z"-shaped stamp on the array. The differential voltage output from each electrode was able to give a visual mapping of the stamp on screen.

Left: output voltage of a single pixel as a function of the applied pressure. The inset shows the experimental setup to apply and measure the pressure sensing. Right: the pressure mapping result of the "Z" shaped stamp with output voltage signals, in which pixels contacted with the stamp ridge area show a higher output [Chen et al 2021].

Having incremental electrical conductivity with increasing pressure is another mechanism for pressure sensors. Xue et al (2022) used electrospun membrane as the substrate for a coated layer of Ag paste. A unique feature of their electrospun polyurethane (PU) membrane is that it contains numerous scattered bumps on its surface. The Ag paste was coated on the surface with the bumps. Two pieces of the Ag coated electrospun PU membranes were placed facing each other. When pressure is applied to the piezoresistive sensor, the distance between the two Ag-coated PU membranes is reduced and the contact area increases resulting in a drop in the electrical resistance of the sensor. When the load was removed, the PU membranes recovered to their initial state and the electrical resistance increased. The response and recovery time are about 60 ms to 30 ms respectively, demonstrating rapid detection of the sensor to pressure. The presence of the bumps played an important role as they increased the distance between the layers. These bumps had a diameter of about 200 µm and were made of localised twisted polyurethane fibers while the under surface maintains a smooth and flat profile. Xue et al (2022) hypothesized that these bumps were the result of residual solvents gathering at various spots during evaporation. These semi-wet regions may favor charge dissipation which encourages more fibers to deposit on it. Such a raised profile would further encourage more fibers to deposit on it relative to the neighbouring region and eventually, a raised bump is formed.

SEM images of the electrospun PU nanofibers film: (a) upper surface at low magnification and (b) high magnification [Xue et al 2022].

Bi et al (2023) constructed a high sensitivity and wide working range sensor using a combination of electrospun membrane. A rotating cage collector was used to collect aligned electrospun polyamide acid (PAA) fibers. For randomly oriented electrospun polyamide acid (PAA) fibers, a plate collector was used instead. Imidization and carbonization at high temperature was used to transform the electrospun fibers into aligned carbon nanofibers (p-CNF) and randomly oriented carbon nanofibers (r-CNF). To construct the strain sensor, two layers of the CNFs, one aligned, the other randomly oriented, were placed facing each other. These two layers are then encapsulated within polydimethylsiloxane (PDMS) to form a flexible strain sensor. The resultant strain sensor exhibited high sensitivity and wide working range with a gauge factor (GF) of 1272 for strains under 0.5% and 2266 from 70% to 100%. The working range of the sensor is from 0.005% to 100%. Bi et al (2023) found that stretching would cause parallel cracks on the p-CNF membrane. This leads to a sharp increase in resistance in a small strain range. With the r-CNF membrane, no significant cracks appear even when the strain increases to 100% which enables the sensor to detect changes in strain over a wide range. The sensor was shown to be able to track facial expressions and even speech.

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