Conducting polymer hydrogels with electrically-tuneable mechanical properties as dynamic cell culture substrates.

Hydrogels are a popular substrate for cell culture due to their mechanical properties closely resembling natural tissue. Stimuli-responsive hydrogels are a good platform for studying cell response to dynamic stimuli. Poly(N-isopropylacrylamide) (pNIPAM) is a thermo-responsive polymer that undergoes a volume-phase transition when heated to 32 °C. Conducting polymers can be incorporated into hydrogels to introduce electrically responsive properties. The conducting polymer, polypyrrole (PPy), has been widely studied as electrochemical actuators due to its electrochemical stability, fast actuation and high strains. We determine the volume-phase transition temperature of pNIPAM hydrogels with PPy electropolymerised with different salts as a film within the hydrogel network. We also investigate the electro-mechanical properties at the transition temperature (32 °C) and physiological temperature (37 °C). We show statistically significant differences in the Young's modulus of the hybrid hydrogel at elevated temperatures upon electrochemical stimulation, with a 5 kPa difference at the transition temperature. Furthermore, we show a three-fold increase in actuation at transition temperature compared to room temperature and physiological temperature, attributed to the movement of ions in/out of the PPy film that induce the volume-phase transition of the pNIPAM hydrogel. Furthermore, cell adhesion to the hybrid hydrogel was demonstrated with mouse articular chondrocytes.

Modulation of hydrogel stiffness by external stimuli: soft materials for mechanotransduction studies.

Mechanotransduction is an important process in determining cell survival, proliferation, migration and differentiation. The extracellular matrix (ECM) is the component of natural tissue that provides structural support and biochemical signals to adhering cells. The ECM is dynamic and undergoes physical and biochemical changes in response to various stimuli and there is an interest in understanding the effect of dynamic changes in stiffness on cell behaviour and fate. Therefore, stimuli-responsive hydrogels have been developed to mimic the cells' microenvironment in a controlled fashion. Herein, we review strategies for dynamic modulation of stiffness using various stimuli, such as light, temperature and pH. Special emphasis is placed on conducting polymer (CP) hydrogels and their fabrication procedures. We believe that the redox properties of CPs and hydrogels' biological properties make CPs hydrogels a promising substrate to investigate the effect of dynamic stiffness changes and mechanical actuation on cell fate in future studies.

Mechanical Characterization for Cellular Mechanobiology: Current Trends and Future Prospects.

Accurate mechanical characterization of adherent cells and their substrates is important for understanding the influence of mechanical properties on cells themselves. Recent mechanobiology studies outline the importance of mechanical parameters, such as stress relaxation and strain stiffening on the behavior of cells. Numerous techniques exist for probing mechanical properties and it is vital to understand the benefits of each technique and how they relate to each other. This mini review aims to guide the reader through the toolbox of mechanical characterization techniques by presenting well-established and emerging methods currently used to assess mechanical properties of substrates and cells.