A major hindrance in engineering tissues containing highly metabolically active cells is the insufficient oxygenation of these implants, which results in dying or dysfunctional cells in portions of the graft. The development of methods to increase oxygen availability within tissue-engineered implants, particularly during the early engraftment period, would serve to allay hypoxiainduced cell death. Herein, we designed and developed a hydrolytically activated oxygen-generating biomaterial in the form of polydimethylsiloxane (PDMS)-encapsulated solid calcium peroxide, PDMS-CaO 2 . Encapsulation of solid peroxide within hydrophobic PDMS resulted in sustained oxygen generation, whereby a single disk generated oxygen for more than 6 wk at an average rate of 0.026 mM per day. The ability of this oxygen-generating material to support cell survival was evaluated using a β cell line and pancreatic rat islets. The presence of a single PDMS-CaO 2 disk eliminated hypoxia-induced cell dysfunction and death for both cell types, resulting in metabolic function and glucose-dependent insulin secretion comparable to that in normoxic controls. A single PDMS-CaO 2 disk also sustained enhanced β cell proliferation for more than 3 wk under hypoxic culture conditions. Incorporation of these materials within 3D constructs illustrated the benefits of these materials to prevent the development of detrimental oxygen gradients within large implants. Mathematical simulations permitted accurate prediction of oxygen gradients within 3D constructs and highlighted conditions under which supplementation of oxygen tension would serve to benefit cellular viability. Given the generality of this platform, the translation of these materials to other cell-based implants, as well as ischemic tissues in general, is envisioned.tissue engineering | encapsulation | bioartificial pancreas | diabetes
We demonstrate the applicability of sequential Diels-Alder and azide-alkyne [3 + 2] cycloaddition reactions (click chemistry) for the immobilization of carbohydrates and proteins onto a solid surface. An alpha,omega-poly(ethylene glycol) (PEG) linker carrying alkyne and cyclodiene terminal groups was synthesized and immobilized onto an N-(epsilon-maleimidocaproyl) (EMC)-functionalized glass slide via an aqueous Diels-Alder reaction. In the process, an alkyne-terminated PEGylated surface was provided for the conjugation of azide-containing biomolecules via click chemistry, which proceeded to completion at low temperature and in aqueous solvent. As anticipated, alkyne, azide, cyclodiene, and EMC are independently stable and do not react with common organic reagents nor functional groups in biomolecules. Given an appropriate PEG linker, sequential Diels-Alder and azide-alkyne [3 + 2] cycloaddition reactions provide an effective strategy for the immobilization of a wide range of functionally complex substances onto solid surfaces.
Encapsulation of islets of Langerhans may represent a way to transplant islets in the absence of immunosuppression. Traditional methods for encapsulation lead to diffusional limitations imposed by the size of the capsules (600-1,000 μm in diameter), which results in core hypoxia and delayed insulin secretion in response to glucose. Moreover, the large volume of encapsulated cells does not allow implantation in sites that might be more favorable to islet cell engraftment. To address these issues, we have developed an encapsulation method that allows conformal coating of islets through microfluidics and minimizes capsule size and graft volume. In this method, capsule thickness, rather than capsule diameter, is constant and tightly defined by the microdevice geometry and the rheological properties of the immiscible fluids used for encapsulation within the microfluidic system. We have optimized the method both computationally and experimentally, and found that conformal coating allows for complete encapsulation of islets with a thin (a few tens of micrometers) continuous layer of hydrogel. Both in vitro and in vivo in syngeneic murine models of islet transplantation, the function of conformally coated islets was not compromised by encapsulation and was comparable to that of unencapsulated islets. We have further demonstrated that the structural support conferred by the coating materials protected islets from the loss of function experienced by uncoated islets during ex vivo culture.cell encapsulation | polyethylene glycol | alginate | cell transplantation
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