Thermo-responsive electrospun fibers

A thermos-responsive material is able to cause a reaction in response to the changes in temperature. A common example is shape memory materials which are often triggered by changes in the ambient temperature. Another response for thermos-responsive materials is the change in surface contact angle of electrospun fibrous mat. Thermo-responsive gel which undergoes volume phase transition from swollen to shrunken state has also been fabricated.

Electrospinning has been used to construct nanofiber membrane with shape memory material and shape memory composite with the fibers as filler. A composite made out of electrospun poly-e-caprolactone embedded within a silicone rubber (Sylard 184) matrix has demonstrated strain fixing/recovery of more than 97% for 3 cycles [Luo et al 2009]. Zhang et al (2013) constructed a nanofibrous membrane out of polyethylene oxide and Nafion blend. The resultant membrane was able to memorize five shapes (four temporary and one permanent shape) with shape recovery of more than 70% [Zhang et al 2013]. Recovery of electrospun shape memory fibrous membrane may be faster than other bulk form due to its higher specific surface area. Biodegradable and shape memory polymer, poly(D,L-lactide-co-trimethylene carbonate) (PDLLA-co-TMC) has been electrospun and its shape memory effect demonstrated with recovery of more than 98%. Shape recovery of PDLLA-co-TMC nanofibrous membrane and tube has been shown to take less than 1 min [Bao et al 2014].

Changes in the water contact angle of electrospun fibers may be induced by changes in temperature. Oh et al (2014) constructed an electrospun nanofiber-based material where its water contact angle reduces significantly when the solution temperature was reduced from 37 °C to 20 °C. This is achieved by graft polymerization of thermoresponsive poly(N-isopropylacrylamide)(PIPAAm) on polystyrene (PS) nanofibrous mats. Compared to grafted polystyrene dish, the change in contact angle on electrospun mat is much greater. As the water contact angle of both electrospun mat and dish is similar at the lower temperature, the difference in the magnitude of change in the water contact angle comes from the larger water contact angle of the electrospun grafted PS mat which may be attributed to its larger surface roughness. Such changes in water contact angle in response to temperature have been demonstrated by Oh et al (2014) in cultured cell recovery. For the purpose of easy detachment of cells cultured on a substrate, Young et al (2019) used cross-linked electrospun high molecular weight poly(N-isopropyl acrylamide) (PNIPAM) fibers. The transformation of PNIPAM fibers from hydrophobic to hydrophilic at temperatures below 32° causes rapid hydration of the polymer network and this causes detachment of cells from the substrate. Cross-linking of PNIPAM fibers is necessary to render the substrate insoluble in water. Without this, the fiber would lose its morphology during cell culture. Cell detachment with fibroblast showed that about 50% of the cells were released when the scaffold was washed with culture media at 22°C for 5 minutes. This result is comparable to PNIPAM UpCell, a commercial thermoresponsive cell release substrate.

A thermo-sensitive hydrogel such as poly(N-isopropylacrylamide)(PNIPAAm) typically undergo a two-step shrinking process at the phase transition temperature which rendered it too slow for practical utilization. However, by forming a nanofibrous mesh of PNIPAAm, the high porosity and surface area of the structure is able to respond at a much faster rate than typical gel. Maeda et al (2015) showed that the collective diffusion constant of the nanofibrous PNIPAAm gel to be 1.8x10^-5 cm/s which is over 100 times faster than its bulk gel form. The nanofibrous PNIPAAm gel also did not show a discontinuous volume phase transition. This has been attributed to its porous structure which prevented the formation of a skin layer on the gel surface and its nano scale dimension which made relaxation time short.

Electrospun thermo-responsive polymers nanofibers may be incorporated with other materials to give a different output. Deng et al (2018) electrospun P(NIPAm-co-NMA) composite nanofibers which shows changes in electrical resistance with temperature. The electrical resistance property come from reduction of AgNO₃ salt that was added to P(NIPAm-co-NMA) solution prior to electrospinning into Ag nanoparticles. As temperature of the composite nanofibers increases, the electrical resistance decreases due to shrinkage of the polymer. The composite was able to maintain the same thermal responsive performance with a thousand heating-cooling cycle between 30 - 60 °C. Okutani et al (2022) constructed an ultrathin fiber-mesh polymer positive temperature coefficient (PTC) thermistors via electrospinning. The matrix of the electrospun fibers was 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 and transparent layer, gas permeable and high flexibility. This electrospun thermo-responsive membrane has the potential for use as wearable temperature sensors.

Electrospun fibers have been constructed with polymers that changes its optical properties with temperatures. Fatayati et al (2018) selected liquid crystal (LC) molecules, N-(4-methoxybenzylidene)-4'-butylaniline (MBBA) as the optical thermal responsive material. Poly(vinyl pyrrolidone) (PVP) was selected as the carrier material as it is a transparent linear polymer, so it does not have birefringence. The mixture was electrospun using a single nozzle needle and collected with parallel alignment across two parallel copper collectors. The aligned fiber mesh displays positive birefringence when the slow axis of the retardation plate (RP) coincided with that of LCs and otherwise, it displayed a negative birefringence. Under a polarized optical microscope (POM), when the mesh was heated, the intensity of transmitted light slowly decreased until completely dark at 32.5 °C, this indicates the LC molecules have reached the isotropic phase. During cooling, the intensity of transmitted light became maximum when the LC reached the nematic phase. Cooling to nematic phase was found to be 25.2 °C while the transition from nematic phase upon heating was found to be 26°C.

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