The objective of this paper is to investigate the role played by the structural water on the intermolecular sliding between collagen-like 1QSU peptides in a microfibril under deformation. Three modes of deformation are used to generate intermolecular sliding: forced axial stretching (case I) or sliding (case II) of a central peptide monomer (while other surrounding monomers are fixed); and cantilever bending (case III) under a terminal lateral load. The force-displacement curve of each deformation mode is derived using a module called Steered Molecular Dynamics (SMD) in a molecular dynamics package NAMD under the CHARMM22 force field. Each calculation is carried out twice, one in the presence of structural water, one without. It is found that the structural water is a weak "lubricant" in forced axial stretching (case I), but it functions as a "glue" in forced axial sliding (case II) and cantilever bending (case III). A change in the pulling speed does not significantly alter the force-displacement behavior in axial stretching (case I) and sliding (case II), but it does in cantilever bending (case III). The additional resistance contributed by the structural water is attributed to the additional energy cost in breaking the water-mediated hydrogen bonds (water bridges).

Besides biological and chemical cues, cellular behavior has been found to be affected by mechanical cues such as traction forces, surface topology, and in particular the mechanical properties of the substrate. The present study focuses on completely characterizing the bulk linear mechanical properties of such soft substrates, a good example of which are hydrogels. The complete characterization involves the measurement of Young's modulus, shear modulus, and Poisson's ratio of these hydrogels, which is achieved by manipulating nonspherical magnetic microneedles embedded inside them. Translating and rotating these microneedles under the influence of a known force or torque, respectively, allows us to determine the local mechanical properties of the hydrogels. Two specific hydrogels, namely bis-cross-linked polyacrylamide gels and DNA cross-linked polyacrylamide gels were used, and their properties were measured as a function of gel concentration. The bis-cross-linked gels were found to have a Poisson's ratio that varied between 0.38 and 0.49, while for the DNA-cross-linked gels, Poisson's ratio varied between 0.36 and 0.49. The local shear moduli, measured on the 10 μm scale, of these gels were in good agreement with the global shear modulus obtained from a rheology study. Also the local Young's modulus of the hydrogels was compared with the global modulus obtained using bead experiments, and it was observed that the inhomogeneities in the hydrogel increases with increasing cross-linker concentration. This study helps us fully characterize the properties of the substrate, which helps us to better understand the behavior of cells on these substrates.

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Despite cellular environments having dynamic characteristics, many laboratories utilized static polyacrylamide hydrogels to study the ECM-cell relationship. To attain a more in vivo like environment, we have developed a dynamic, DNA-crosslinked hydrogel (DNA gel). Through the controlled delivery of DNA, we can temporally decrease or increase gel stiffness while expanding or contracting the gel, respectively. These dual mechanical changes make DNA gels a cell-ECM model for studying dynamic mechanoregulated processes, such as wound healing. Here, we characterized DNA gels on a mechanical and cellular level. In contrast to our previous publication, in which we examined the increasing stiffness effects on fibroblast morphology, we examined the effects of decreased matrix stiffness on fibroblast morphology. In addition, we quantified the bulk and/or local stress and strain properties of dynamic gels. Gels generated about 0.5 Pa stress and about 6-11% strain upon softening to generate larger and more circular fibroblasts. These results complemented our previous study, where dynamic gels contracted upon stiffening to generate smaller and longer fibroblasts. In conclusion, we developed a biomaterial that increases and decreases in stiffness while contracting and expanding, respectively. We found that the dynamic deformation directionality of the matrix determined the fibroblast morphology and possibly influences function.

Pluripotent embryonic stem cells represent a promising renewable cell source to generate a variety of differentiated cell types including hepatocyte lineage cells, and may ultimately be incorporated into extracorporeal bioartificial liver devices and cell replacement therapies. Recently, we and others have utilized sodium butyrate to directly differentiate hepatocyte-like cells from murine embryonic stem cells cultured in a monolayer configuration. However, to incorporate stem cell technology into clinical and pharmaceutical applications, and hopefully increase the therapeutic potential of these differentiated cells for liver disease treatment, a major challenge remains in sustaining differentiated functions for an extended period of time in their secondary culture environment. In the present work, we have investigated the use of polyacrylamide hydrogels with defined mechanical compliances as a cell culture platform for improving and/or stabilizing functions of these hepatocyte-like cells. Several functional assays, e.g., urea secretion, intracellular albumin content, and albumin secretion, were performed to characterize hepatic functions of cells on polyacrylamide gels with stiffnesses of 5, 46.6, and 230 kPa. In conjunction with the mechanical and cell morphological characterization, we showed that hepatic functions of sodium butyrate differentiated cells were sustained and further enhanced on compliant substrates. This study promises to offer insights into regulating stem cell differentiation via mechanical stimuli, and assist us with designing a variety of dynamic culture systems for applications in tissue and cellular engineering.