Processing advances in liquid crystal elastomers provide a path to biomedical applications

Abstract

Liquid crystal elastomers (LCEs) are a class of stimuli-responsive polymers that undergo reversible shape-change in response to environmental changes. The shape change of LCEs can be programmed during processing by orienting the liquid crystal phase prior to crosslinking. The suite of processing techniques that has been developed has resulted in a myriad of LCEs with different shape-changing behavior and mechanical properties. Aligning LCEs via mechanical straining yields large uniaxial actuators capable of a moderate force output. Magnetic fields are utilized to control the alignment within LCE microstructures. The generation of out-of-plane deformations such as bending, twisting, and coning is enabled by surface alignment techniques within thin films. 4D printing processes have emerged that enable the fabrication of centimeter-scale, 3D LCE structures with a complex alignment. The processing technique also determines, to a large extent, the potential applications of the LCE. For example, 4D printing enables the fabrication of LCE actuators capable of replicating the forces generated by human muscles. Employing surface alignment techniques, LCE films can be designed for use as coatings or as substrates for stretchable electronics. The growth of new processes and strategies opens and strengthens the path for LCEs to be applicable within biomedical device designs.

Light-driven continuous rotating Möbius strip actuators

“…To prepare the desired Möbius strip actuators, we chose a liquid crystal elastomer (LCE) as the fundamental stimulus-responsive polymeric material because LCEs, as classical and predominant two-way shape-memory materials, can exhibit reversible, complex and huge amplitude shape deformations along with the external-stimuli-triggered order–disorder phase transition of the internal mesogenic polymer network and have prosperous application prospects in smart actuators, robotic technology and biomedical engineering 23 , 26 – 37 . Moreover, we embedded a NIR absorbing dye into the LCE matrix so that the prepared ribbon actuators could be stimulus triggered by remote light control, relying on the photothermal conversion effect of the incorporated NIR dye 38 .…”

4D Printing of Liquid Crystals: What's Right for Me?

“…[98] Having the LCE thermal response occur at physiological temperatures, ≈37 °C, is imperative for biotechnological applications, a field where LCEs hold great promise. [199,200] The T NI of an LCE can be lowered by destabilizing the mesophase of the LC. Introducing "disorder" by using multiple reactive mesogens, a flexible dithiol (1,1′-(1,2-ethanediyl) bis(3-mercaptopropanoate), Figure 14 and Table 3) and adding isotropic components (triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione) can lower the T NI after crosslinking to 28 °C.…”

“…[125] Other methods of creating photo-responsivity besides employing molecular photoswitches is through the incorporation of absorbing species to obtain a photothermal response instead (Section 2.2.3). This type of response to NIR light has been achieved in LCEs by adding carbon black or poly (3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT :PSS) [199,205,206] or to visible light with Au NPs [207] (see Figure 14). Furthermore, the photothermal effect may also be initiated by dyes with narrower absorption bands, such as croconaine dyes which release heat in response to excitation at NIR wavelengths around λ max = 796 nm or λ max = 1002 nm, depending on the dye used.…”

Liquid Crystal Elastomer Metamaterials with Giant Biaxial Thermal Shrinkage for Enhancing Skin Regeneration

“…The ability to shrink substantially upon heating (from T room ) to a relatively low temperature (e.g., <50 °C) that does not cause any evident discomfort or pain of the carrier represents an important concern for biointegrated applications. The temperature T NI is controlled mainly by the crosslinking density of LCEs, which is associated with many factors in the first thiol-acrylate Michael addition reaction, such as the type and ratios of compositions (liquid crystal molecule, [40] crosslinker, [41] spacer [42] ), applied stress for alignment, [43] the curing temperature, [44] and the presence of solvent. [45] Figure 4a-i elucidates the synergistic effect of the curing temperature as well as mass ratios of crosslinker (PETMP) and liquid crystal (RM257) on the nematic-isotropic transition temperature (T NI ) and the magnitude of uniaxial actuation strain/stress.…”

“…[5][6][7] Because of the performances (e.g., biaxial strain > 30% and biaxial stress > 50 kPa) remains a challenge. Additionally, the use of LCEs in biointegrated applications requires a low actuation temperature (e.g., < 50 °C for skin regeneration), [30,31] but the strategy for the low-temperature actuation of LCEs has not been well developed.…”