Leukocyte-mimicking stem cell delivery via in situ coating of cells with a bioactive hyperbranched polyglycerol.

Since stem cells emerged as a new generation of medicine, there are increasing efforts to deliver stem cells to a target tissue via intravascular injection. However, the therapeutic stem cells lack the capacity to detect and adhere to the target tissue. Therefore, this study presents synthesis of a bioactive hyperbranched polyglycerol (HPG) that can noninvasively associate with stem cells and further guide them to target sites, such as inflamed endothelium. The overall process is analogous to the way in which leukocytes are mobilized to the injured endothelium.

Tailoring the dependency between rigidity and water uptake of a microfabricated hydrogel with the conformational rigidity of a polymer cross-linker.

Many diverse applications utilize hydrogels as carriers, sensors, and actuators, and these applications rely on the refined control of physical properties of the hydrogel, such as elastic modulus and degree of swelling. Often, hydrogel properties are interdependent; for example, when elastic modulus is increased, degree of swelling is decreased. Controlling these inverse dependencies remains a major barrier for broader hydrogel applications. We hypothesized that polymer cross-linkers with varied chain flexibility would allow us to tune the inverse dependency between the elastic modulus and the degree of swelling of the hydrogels. We examined this hypothesis by using alginate and poly(acrylic acid) (PAA) modified with a controlled number of methacrylic groups as model inflexible and flexible cross-linkers, respectively. Interestingly, the polyacrylamide hydrogel cross-linked by the inflexible alginate methacrylates exhibited less dependency between the degree of swelling and the elastic modulus than the hydrogel cross-linked by flexible PAA methacrylates. This critical role of the cross-linker's inflexibility was related to the difference of the degree of hydrophobic association between polymer cross-linkers, as confirmed with pyrene probes added in pregel solutions. Furthermore, hydrogels cross-linked with alginate methacrylates could tune the projection area of adhered cells by solely altering elastic moduli. In contrast, gels cross-linked with PAA methacrylates failed to modulate the cellular adhesion morphology due to a lower, and smaller, elastic modulus range to be controlled. Overall, the results of this study will significantly advance the controllability of hydrogel properties and greatly enhance the performance of hydrogels in various biological applications.

The interplay between cell adhesion cues and curvature of cell adherent alginate microgels in multipotent stem cell culture.

Cell-adherent microcarriers are increasingly used to expand multipotent stem cells on a large scale for therapeutic applications. However, the role of the microcarrier properties and geometry on the phenotypic activities of multipotent cells has not been well studied. This study presents a significant interplay of the number of cell adhesion sites and the curvature of the microcarrier in regulating cell growth and differentiation by culturing mesenchymal stem cells on alginate microgels chemically linked with oligopeptides containing the Arg-Gly-Asp (RGD) sequence. Interestingly, the cell growth rate and osteogenic differentiation level were increased with the RGD peptide density. At a given RGD peptide density, the cell growth rate was inversely related to the microgel diameter, whereas the osteogenic differentiation level was minimally influenced. The dependency of the cell growth rate on the microgel diameter was related to changes in shear stresses acting on cells according to simulation. Overall, this study identifies material variables key to regulating cellular activities on microcarriers, and these findings will be useful to designing a broad array of bioactive microcarriers.

Three-dimensional synthetic niche components to control germ cell proliferation.

Spermatogonial stem cells (SSCs) are increasingly studied for potential use in tissue regeneration due to their ability to dedifferentiate into embryonic stem cell-like cells. For their successful therapeutic use, these cells must first be expanded in vitro using an appropriate culture system. We hypothesized that a hydrogel with proper biochemical and biomechanical properties may mimic the composition and structure of the native basement membrane onto which SSCs reside, thus allowing us to control SSC proliferation. This hypothesis was examined in two-dimensional (2D) and three-dimensional (3D) cultures using hydrogels formed from calcium cross-linked alginate molecules conjugated with synthetic oligopeptides containing the Arg-Gly-Asp sequence (RGD peptides). The RGD peptide density (N(RGD)) in gel matrices was controlled by mixing alginate molecules modified with RGD peptides and unmodified alginate molecules at varied ratios. The mechanical stiffness was controlled with the cross-linking density of gel matrices. Interestingly, the RGD peptide density modulated cell proliferation in both 2D and 3D cultures as well as the number and size of SSC colonies formed in 3D cultures. In contrast, cell proliferation was minimally influenced by mechanical stiffness in 2D cultures. Overall, the results of this study elucidate an important factor regulating SSC proliferation and also present a bioactive hydrogel that can be used as a 3D synthetic basement membrane. In addition, the results of this study will be broadly useful in controlling the proliferation of various stem cells.

Hydrogels used for cell-based drug delivery.

Stem cells, progenitor cells, and lineage-committed cells are being considered as a new generation of drug depots for the sustained release of therapeutic biomolecules. Hydrogels are often used in conjunction with the therapeutic secreting cells to provide a physical barrier to protect the cells from hostile extrinsic factors. Although the hydrogels significantly improve the therapeutic efficacy of transplanted cells, there have been no successful products commercialized based on these technologies. Recently, biomaterials are increasingly designed to provide cells with both a physical barrier and an extracellular matrix to further improve the secretion of therapeutic proteins from cells. This review will discuss (1) the cell encapsulation process, (2) the immunogenicity of the encapsulating hydrogel, (3) the transport properties of the hydrogel, (4) the hydrogel mechanical properties, and will propose new strategies to improve the hydrogel and cell interaction for successful cell-based drug delivery strategies.