Three-dimensional structures capable of reversible changes in shape, i.e., four-dimensional-printed structures, may enable new generations of soft robotics, implantable medical devices, and consumer products. Here, thermally responsive liquid crystal elastomers (LCEs) are direct-write printed into 3D structures with a controlled molecular order. Molecular order is locally programmed by controlling the print path used to build the 3D object, and this order controls the stimulus response. Each aligned LCE filament undergoes 40% reversible contraction along the print direction on heating. By printing objects with controlled geometry and stimulus response, magnified shape transformations, for example, volumetric contractions or rapid, repetitive snap-through transitions, are realized.

Liquid crystal elastomers (LCEs) are a unique class of materials which combine rubber elasticity with the orientational order of liquid crystals. This combination can lead to materials with unique properties such as thermal actuation, anisotropic swelling, and soft elasticity. As such, LCEs are a promising class of materials for applications requiring stimulus response. These unique features and the recent developments of the LCE chemistry and processing will be discussed in this review. First, we emphasize several different synthetic pathways in conjunction with the alignment techniques utilized to obtain monodomain LCEs. We then identify the synthesis and alignment techniques used to synthesis LCE-based composites. Finally, we discuss how these materials are used as actuators and sensors.

Despite its widespread clinical use in load-bearing orthopaedic implants, polyether-ether-ketone (PEEK) is often associated with poor osseointegration. In this study, a surface porous PEEK material (PEEK-SP) was created using a melt extrusion technique. The porous layer thickness was 399.6±63.3 µm and possessed a mean pore size of 279.9±31.6 µm, strut spacing of 186.8±55.5 µm, porosity of 67.3±3.1%, and interconnectivity of 99.9±0.1%. Monotonic tensile tests showed that PEEK-SP preserved 73.9% of the strength (71.06±2.17 MPa) and 73.4% of the elastic modulus (2.45±0.31 GPa) of as-received, injection molded PEEK. PEEK-SP further demonstrated a fatigue strength of 60.0 MPa at one million cycles, preserving 73.4% of the fatigue resistance of injection molded PEEK. Interfacial shear testing showed the pore layer shear strength to be 23.96±2.26 MPa. An osseointegration model in the rat revealed substantial bone formation within the pore layer at 6 and 12 weeks via µCT and histological evaluation. Ingrown bone was more closely apposed to the pore wall and fibrous tissue growth was reduced in PEEK-SP when compared to non-porous PEEK controls. These results indicate that PEEK-SP could provide improved osseointegration while maintaining the structural integrity necessary for load-bearing orthopaedic applications.

Materials that change shape are attractive candidates to replace traditional actuators for applications with power or size restrictions. In this work, we design a polymeric bilayer that changes shape in response to both heat and water by the incorporation of a water-responsive hydrophilic polymer with a heat-responsive liquid crystal elastomer. The distinct shape changes based on stimulus are controlled by the molecular order, and consequently the anisotropic modulus, of a liquid crystal elastomer. In response to water, the hydrophilic polymer layer expands, bending the bilayer along the path dictated by the anisotropic modulus of the liquid crystal elastomer layer, which is approximately 5 times higher along the molecular orientation than in perpendicular directions. We demonstrate that by varying the direction of this stiffer axis in LCE films, helical pitch of the swollen bilayer can be controlled from 0.1 to 20 mm. By spatially patterning the stiffer axis with a resolution of 900 μm, we demonstrate bilayers that fold and bend based on the pattern within the LCE. In response to heat, the liquid crystal elastomer contracts along the direction of molecular order, and when this actuation is constrained by the hydrophilic polymer, this contraction results in a 3D shape that is distinct from the shape seen in water. Furthermore, by using the vitrification of the dry hydrophilic polymer this 3D shape can be retained in the bilayer after cooling. By utilizing sequential exposure to heat and water, we can drive the initially flat bilayer to reversibly shift between 3D shapes.

Approaches for the synthesis and processing of responsive materials that combine robust mechanical properties and the ability to undergo shape change in response to a stimulus are of intense interest. Here, we report an approach to integrate these properties by synthesizing liquid crystal elastomers (LCEs) that can be aligned and subsequently crystallized. We polymerize LCEs in the isotropic and nematic states and characterize the resulting actuation and mechanical properties. After polymerization, each of these materials can be reversibly crystallized. By crystallizing LCEs, we demonstrate stiffer and tougher shape changing materials. Notably, crystallized samples exhibit moduli 2 orders of magnitude higher and toughness 5 times higher than nematic elastomers. Heating melts the crystallinity and then induces shape change via melting of the liquid crystalline phase. These LCEs are capable of high load bearing during actuation, up to 1.3 MPa, and high work capacity, up to 730 kJ/m 3 . These aligned and crystallized LCEs offer promising benefits as dynamic smart materials with robust mechanical properties.

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The purpose of this study was the synthesis of novel degradable polymer-based devices capable of releasing an encapsulated agent in a controlled manner with specific interest for use as drug delivery materials. Base-catalyzed thiol Michael additions between trithiols and triacrylates containing silyl ether groups were exploited to prepare a series of degradable cross-linked networks. Disodium fluorescein was loaded as a hydrophilic drug surrogate inside the networks, and the degradation of the networks and the release of dye were monitored. The networks were characterized by Fourier transform infrared spectroscopy, and their thermal and mechanical properties were investigated through thermogravimetric analysis and dynamic mechanical analysis. The effects of the monomer structure on degradation, release behavior, and thermal properties were investigated. The networks prepared from more sterically hindered silyl ether monomers exhibited decreased rates of degradation and correspondingly slower release of encapsulated disodium fluorescein dye. The results suggest that the characteristics of the networks can be fine-tuned by manipulation of the group attached to the Si atom in the silyl ether monomers.