Kristofer K. Westbrook

Actuator Designs Using Environmentally Responsive Hydrogels

Environmentally responsive (ER) hydrogels are hydrogels that can experience an abrupt volume change and hence hold or release a large amount of water under the change of certain environmental conditions, such as temperature, pH, or an electric field. Because of their unique capability to achieve a large yet reversible volume change, hydrogels have been widely used in microfluidics and biomedical applications, such as hydrogel sensors and actuators for microfluidic channels, novel drug delivery systems, and scaffolding materials for tissue engineering. In most applications, ER hydrogels are used to create an isotropic volume change or one-dimensional motion by constraining the hydrogel in the other two directions. In this study, using finite element based simulations, actuator designs are demonstrated that can generate a variety of shape changes by carefully patterning ER hydrogels on a non-ER hydrogel matrix. In these designs, as the environment changes, the ER hydrogels undergo volume change under the constraints imposed by the non-ER matrix, causing an eigenstrain (mismatch strain) and hence the deformation of the non-ER matrix. By properly controlling the locations of the ER hydrogel sections with respect to the non-ER hydrogel matrix, one can achieve a desired deformation of the composite structure. As examples, designs of a single material actuator, one linear spring composite actuator, and two coil composite actuators are demonstrated. The feasibility to produce these actuators and the possible applications are discussed at the end of the article.

Thermomechanical behavior of a two-way shape memory composite actuator

Shape memory polymers (SMPs) are a class of smart materials that can fix a temporary shape and recover to their permanent (original) shape in response to an environmental stimulus such as heat, electricity, or irradiation, among others. Most SMPs developed in the past can only demonstrate the so-called one-way shape memory effect; i.e., one programming step can only yield one shape memory cycle. Recently, one of the authors (Mather) developed a SMP that exhibits both one-way shape memory (1W-SM) and two-way shape memory (2W-SM) effects (with the assistance of an external load). This SMP was further used to develop a free-standing composite actuator with a nonlinear reversible actuation under thermal cycling. In this paper, a theoretical model for the PCO SMP based composite actuator was developed to investigate its thermomechanical behavior and the mechanisms for the observed phenomena during the actuation cycles, and to provide insight into how to improve the design.

Time and Temperature Dependent Recovery of Epoxy-Based Shape Memory Polymers

Shape memory polymers (SMPs) are a group of adaptive polymers that can recover the permanent shape from a temporary shape by external stimuli on demand. Among a variety of external stimuli for polymer actuation, temperature is the most extensively used. In SMP applications, one of the major design considerations is the time necessary to recover the shape without external deformation constraints, or free recovery, and the amount of the recoverable strain. This paper investigates the amount of the recoverable strain and the recovery rate of an epoxy-based SMP (Veriflex ® E, VFEI-62 (CRG, Dayton, OH)) under different thermal conditions. In particular, the free recovery behaviors of the SMPs under two experimental protocols, isothermal and shape memory (SM) cycle, are studied. It is found that free recovery in isothermal experiments is much faster than that in a SM cycle at the same recovering temperature and the material is fully recoverable at the temperature above differential scanning calorimetry Tg. Furthermore, for the recovery in SM cycle experiments, reshaping the sample at a low temperature and recovering from the deformation at a high temperature yield the fastest recovery rate, while reshaping at a high temperature and recovering at a low temperature cannot recover the original shape within this works experimental time frame. The possible mechanism for these observations is discussed.

Mechanical loading regulates human MSC differentiation in a multi-layer hydrogel for osteochondral tissue engineering

A bioinspired multi-layer hydrogel was developed for the encapsulation of human mesenchymal stem cells (hMSCs) as a platform for osteochondral tissue engineering. The spatial presentation of biochemical cues, via incorporation of extracellular matrix analogs, and mechanical cues, via both hydrogel crosslink density and externally applied mechanical loads, were characterized in each layer. A simple sequential photopolymerization method was employed to form stable poly(ethylene glycol)-based hydrogels with a soft cartilage-like layer of chondroitin sulfate and low RGD concentrations, a stiff bone-like layer with high RGD concentrations, and an intermediate interfacial layer. Under a compressive load, the variation in hydrogel stiffness within each layer produced high strains in the soft cartilage-like layer, low strains in the stiff bone-like layer, and moderate strains in the interfacial layer. When hMSC-laden hydrogels were cultured statically in osteochondral differentiation media, the local biochemical and matrix stiffness cues were not sufficient to spatially guide hMSC differentiation after 21 days. However dynamic mechanical stimulation led to differentially high expression of collagens with collagen II in the cartilage-like layer, collagen X in the interfacial layer and collagen I in the bone-like layer and mineral deposits localized to the bone layer. Overall, these findings point to external mechanical stimulation as a potent regulator of hMSC differentiation toward osteochondral cellular phenotypes.

Design considerations for shape memory polymer composites with magnetic particles

Recent experimental investigations demonstrated that by incorporating magnetic particles into shape memory polymer matrices, a fast and remote heating of materials and shape recovery could be achieved by exposing the shape memory polymer composite to an electromagnetic field. The particles served as internal mini-antennas to transform the electromagnetic energy to inductive Joule heat, and subsequently to initiate the recovery of shape memory polymers (SMPs). In this paper, a three-dimensional computational study was carried out to study the coupling between heat transfer from spherically shaped particles and heat-induced shape recovery of SMP composites. The influence of particle size, particle volume fraction, particle heating temperature and rate to the magnetically induced shape recovery behavior was studied. The results in this paper provided a meaningful guidance for further designs and applications of the magnetic particles reinforced shape memory polymer composites.

Time Dependent Recovery of Shape Memory Polymers

Shape memory polymers (SMPs) are polymers that can recover the permanent shape from a temporary shape by external stimuli. In SMP applications, one of the major design considerations is the time necessary to recover the shape without external constraints, or free recovery rate. This paper investigates the free recovery behaviors of an epoxy-based SMP under shape memory (SM) cycles. We found that reshaping the sample at a low temperature and recovering from the deformation at a high temperature yields the fastest recovery rate whilst reshaping at a high temperature and recovering at a low temperature cannot recover the original shape within this work’s experimental time frame. The possible mechanism for these observations is discussed.