2, University of Cambridge, Cambridge, , United Kingdom
Large, bulky, power-hungry traditional mechanical actuators are poorly suited for small, biological applications such as medical devices. Shape changing polymers are an emerging class of actuators which can utilize environmental conditions to undergo large, complex shape changes. Liquid crystalline self-assembly is one promising strategy to program structural orientation and resulting actuation in polymeric materials. This molecular ordering can be spatially patterned, resulting in monolithic materials that undergo complex shape change. However, liquid crystal polymer networks are typically hydrophobic and only respond to stimuli that would be incompatible with biological environments, such as high temperatures and organic solvents. We have used two strategies to overcome these limitations: 1) engineering liquid crystal elastomers chemistry to respond near body temperature and 2) building gels from water-soluble, chromonic liquid crystals to respond to aqueous stimuli.
Thermotropic, hydrophobic liquid crystal elastomers have significant advantages of facile patterning, a variety of available monomers, and well-studied elastomer chemistries. These patternable elastomers can undergo high amounts of actuation strain, but these strains are typically not realized below 100-200 C, as determined by the liquid crystalline phase transition. To lower this actuation temperature to near body temperature, we use a 2-click thiol-acrylate/thiol-ene chemistry to copolymerize mesogens with isotropic monomers, which can shift the transition temperature and resulting actuation temperature from 105 C to 28 C. This control of phase behavior enables liquid crystal elastomers which can morph in response to contact with skin or warm tap water. Additionally, these low-temperature elastomers can be 3D printed, allowing structural freedom which is unfeasible in many other liquid crystal elastomer chemistries.
Alternatively, we can synthesize gels which respond to aqueous stimuli, such as pH, by building hydrogels from chromonic liquid crystals. We have synthesized the first hydrogel from chromonic liquid crystals which can be ordered through surface alignment and copolymerized with responsive monomers. By controlling the crosslink density of the gel with non-polymerizable chromonic precursors, we can polymerize gels ranging from ~10 kPa to ~300 kPa. The modulus of the gel is ~2x stiffer along the axis of molecular alignment, which guides anisotropic actuation in response to temperature and pH. This control over crosslink density not only allows for control over the magnitude of shape change but also allows us to synthesize gels similar to soft tissues with anisotropic mechanical properties.
With these strategies, we have taken steps towards the use of liquid crystalline polymers in biological applications. Both liquid crystal elastomers and liquid crystal gels have been used as substrates for mammalian cell culture to confirm the biocompatibility of these materials. By controlling the molecular orientation of these ordered gels and elastomers, we can potentially affect cellular behavior both statically by anisotropic modulus and dynamically by temperature-based or pH-based actuation.