Maintaining functional islets through encapsulation in an injectable saccharide-peptide hydrogel.

Islet transplantation offers a promising treatment for type 1 diabetes (T1D). However, a major hurdle in this treatment is the rapid loss of functional islets during culture and after transplantation. The liver site, currently utilized for transplantation, is suboptimal for achieving long-term insulin independence due to a rapid islet loss followed by a chronic decline in islet function after transplantation. Herein, we report a synthetic saccharide-peptide (SP) hydrogel that allows suspending islets in liquid and injecting for in situ polymerization without forming islet clumps, indicating its potential in extrahepatic islet transplantation. In vitro, rat islets in SP hydrogel maintained a 3D structure and high glucose-stimulated insulin release similar to that observed in freshly isolated islets for 4 weeks, while control islets cultured in suspension lost their 3D structure and insulin release responses by 2 weeks. Biocompatibility of SP hydrogel was shown by the absence of cytokine mRNA activation in peripheral blood mononuclear cells (PBMCs) exposed to hydrogel in vitro and by the absence of cellular infiltrates in and around the hydrogel implanted subcutaneously. Syngeneic Lewis rat islets transplanted in SP hydrogel in various extrahepatic sites stained strongly for insulin, and more effectively reversed diabetes than unencapsulated islets when transplanted in an omental pocket. In conclusion, the SP hydrogel is non-cytotoxic and supports normal islet structure and function both in vitro and in vivo. Specifically, the ability of the hydrogel to separate individual islets after transplantation is important for maintaining their function in vivo. This important property, combined with the versatility and biocompatibility, makes our SP hydrogel a promising synthetic scaffold that can facilitate transplantation of organized heterogeneous cells to preserve their micro-structure and function.

Biodegradable and biocompatible synthetic saccharide-Peptide hydrogels for three-dimensional stem cell culture.

Saccharide-peptide hydrogels have been developed in our laboratory as new synthetic extracellular matrices for regenerative medicine applications. In this work, we have expanded on our previously reported system and applied copolymerization of cysteine (Cys) and vinyl sulfone (VS)-functionalized saccharide-peptide polymers via Michael-type addition for encapsulation and 3D culture of cells. Specifically, our aims were to (1) develop a novel hydrogel platform, which could be applied for encapsulating and culturing mesenchymal stem cells (MSCs) in a 3D environment, (2) characterize the tunable properties of the hydrogel, specifically, degradation, mechanical, and gel network properties, and (3) determine the biocompatibility of the saccharide-peptide hydrogel material with MSCs. Hydrogel mechanical properties were tunable by varying the VS:Cys ratio (= 0.5, 1, or 2) as well as the pH (6, 7, or 8) of the cross-linking components. Stiffer gels were formed at VS:Cys = 1 and pH 6 or 7. Gels formed at pH 8 or with excess Cys (VS:Cys = 0.5) or VS (VS:Cys = 2) were significantly softer. Cross-linking pH and VS:Cys ratio also had an effect on the degradation behavior of the VS:Cys gels, with higher cross-linking pH resulting in an accelerated loss of mass. On the basis of environmental scanning electron microscopy (ESEM) analysis and fluorescence microscopy, all hydrogels appeared to exhibit porous gel networks. MSCs cultured in monolayer and exposed to soluble Cys or VS copolymers (0.1-5 mg/mL) did not exhibit measurable cytotoxicity. In addition, MSCs were cultured in 3D for up to 14 days in vitro without deleterious effects on cell viability. In summary, we have established and characterized a tunable 3D saccharide-peptide hybrid copolymer hydrogel platform for culturing MSCs. Future studies will focus on utilizing the hydrogel system for controlling the differentiation of MSCs.

De novo design of saccharide-peptide hydrogels as synthetic scaffolds for tailored cell responses.

A new class of functional saccharide-peptide copolymer-based hydrogels was synthesized and investigated as synthetic extracellular matrices for regenerative medicine applications. The polymer was composed entirely of natural building blocks, namely, galactaric acid and lysine on the backbone, with tyrosine grafted onto the side chain as a handle for enzyme-catalyzed hydrogelation. The resulting hydrogels are degradable under simulated physiological conditions and exhibit minimal cytotoxicity on dermal fibroblast and PC-12 cells. As a demonstration of the versatility of the system, the mechanical properties of the gels can be independently controlled without changing the polymer chemical composition. Using an identical copolymer solution, by simply allowing different lengths of cross-linking time, a series of hydrogels was obtained with different mechanical moduli at constant chemical structure. The moduli of the resulting hydrogels varied stepwise from 1.7, 4.1, 6.9, and 12.5 kPa to allow for systematic studies on the effects of modulus on cell behavior. It was exciting to observe that a simple change in hydrogel physical properties could induce a direct phenotypic change in cell adhesion and proliferation. Depending on the substrate mechanical modulus, the cell morphology changed and proliferation rate differed by an order of magnitude for different cell lines. These data suggest our saccharide-peptide hydrogels as promising synthetic extracellular matrices for cell culture and tissue regeneration.

A novel time-varying poly lactic-co glycolic acid external sheath for vein grafts designed under physiological loading.

Changes in dimensional and mechanical properties of degradable sheaths in poly lactic-co glycolic acid (PLGA) have been researched extensively. Composite PLGA having variable resorption rates in multiple layers under physiological loading has not been reported. Our novel design of a PLGA sheath is composed of 3 layers with different degradation rates (i.e., the innermost layer degrades the fastest, followed by the middle, while the outer layer degrades the slowest). In the presence of physiological luminal pressure, diameter is greater, thickness is less, resorption rate is greater, pore size is greater, and incremental modulus is greater than in nonpressurized sheaths. Furthermore, the ratio of the pore size to the sheath radius affects the dimensional changes of the sheath in the radial direction. In addition to changing the pore size-to-sheath radius ratio, the dimensional changes can be manipulated by choosing different glycolic and lactic acid ratios for the different layers. The application of this novel PLGA design for gradual arterialization of vein grafts is contemplated.