Proangiogenic alginate-g-pyrrole hydrogel with decoupled control of mechanical rigidity and electrically conductivity.

BACKGROUND: An electrically conductive hydrogel has emerged to regulate cellular secretion activities with electrical stimulation. However, the electrical conductivity of typical hydrogel systems decreases with increasing elastic modulus of the hydrogels because of decreased transport of ions through a polymeric cross-linked mesh. METHOD: This study hypothesized that the inverse dependency between electrical conductivity and elastic modulus would be made through the cross-linking of conductive monomer-units conjugated to a hydrophilic polymeric backbone. This hypothesis was examined through the cross-linking of pyrrole groups that were conjugated to an alginate backbone, termed alginate-g-pyrrole. RESULTS: Hydrogels with increased degrees of pyrrole substitution exhibited a simultaneous increase in the gels mechanical rigidity and electrical conductivity. The resulting hydrogel could control the adhesion and vascular endothelial growth factor secretion of cells via applied electrical stimulation. CONCLUSIONS: This material design principle will be broadly useful to fabricating materials used for various actuation, cell culture, and biomedical applications.

The spatiotemporal control of erosion and molecular release from micropatterned poly(ethylene glycol)-based hydrogel.

Hydrogels have been extensively studied as a carrier of various hydrophilic molecular compounds and cells for local delivery and subsequent controlled release. One of key design parameters in the hydrogel assembly is an ability to control spatiotemporal gel degradation, in order to tailor release rates of multiple drugs and also regulate phenotypic activities of co-cultured cells. To achieve this goal, this study presents a simple but innovative implantable, microfabricated hydrogel patch that undergoes micropatterned surface erosion at controlled rates and subsequently discharges two molecular compounds of interests at desired rates. This device was prepared by first fabricating a non-degradable poly(ethylene glycol) dimethacrylate (PEGDMA) hydrogel patch containing micro-pockets of controlled spacing and subsequently filling micro-pockets with a hydrogel of poly(ethylene imine) (PEI) and PEG diacrylate (PEGDA) that was tailored to degrade at controlled rates. Separate incorporation of vascular endothelial growth factor (VEGF)121 and VEGF165, known to orchestrate vascular development, into the PEI-PEGDA gel and PEGDMA hydrogel resulted in enhanced neovascularization at the implantation sites due to bimodal, sequential release of two VEGF isoforms. We believe that the hydrogel patch fabricated in this study will be highly useful to better understand a broad array of complex biological processes and also improve the efficacy of molecular cargos in varied applications.

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.

Microfabrication of proangiogenic cell-laden alginate-g-pyrrole hydrogels.

Cells have been extensively studied for their uses in various therapies because of their capacities to produce therapeutic proteins and recreate new tissues. It has often been suggested that the efficacy of cell therapies can greatly be improved through the ability to localize and regulate cellular activities at a transplantation site; however, the technologies for this control are lacking. Therefore, this study reports a cell-Laden hydrogel patch engineered to support the proliferation and angiogenic growth factor expression of cells adhered to their surfaces, and to further promote neovascularization. Hydrogels consisting of alginate chemically linked with pyrrole units, termed alginate-g-pyrrole, were prepared through an oxidative cross-linking reaction between pyrrole units. Fibroblasts adhered to the alginate-g-pyrrole hydrogels, and exhibited increased proliferation and overall vascular endothelial growth factor (VEGF) expression, compared to those on pyrrole-free hydrogels. Furthermore, the alginate-g-pyrrole hydrogel surfaces were modified to present microposts, subsequently increasing the amount of pyrrole units on their surfaces. Cells adhered to the microfabricated gel surfaces exhibited increased proliferation and overall VEGF expression proportional to the density of the microposts. The resulting micropatterned alginate-g-pyrrole hydrogels exhibited increases in the size and density of mature blood vessels when implanted on chick chorioallantoic membranes (CAMs). The hydrogel system developed in this study will be broadly useful for improving the efficacy of a wide array of cell-based wound healing and tissue regenerative therapies.

Modulating the rigidity and mineralization of collagen gels using poly(lactic-co-glycolic acid) microparticles.

Extensive efforts have been made to prepare osteoconductive collagen gels for the regeneration of normal bone and the pathological examination of diseased bone; however, collagen gels are often plagued by limited controllability of their rigidity and mineral deposition. This study reports a simple but efficient strategy that tunes the mechanical properties of, and apatite formation in, collagen gels by incorporating hydrolyzable poly(lactic-co-glycolic acid) (PLGA) microparticles within the gels. The PLGA microparticles are associated with the collagen fibrils and increased both the gel's elasticity and rigidity while minimally influencing its permeability. As compared with pure collagen gels, the PLGA microparticle-filled collagen gels, termed PLGA-Col hydrogels, significantly enhanced the deposition of apatite-like minerals within the gels when incubated in simulated body fluid or encapsulated with mesenchymal stem cells (MSCs) undergoing osteogenic differentiation. Finally, PLGA-Col hydrogels mineralized by differentiated MSCs led to an enhanced formation of bone-like tissues within the hydrogels. Overall, the PLGA-Col hydrogel system developed in this study will serve to improve the quality of osteoconductive matrices for both fundamental and clinical studies that are relevant to bone repair, regeneration, and pathogenesis.

Ellipsoidal Polyaspartamide Polymersomes with Enhanced Cell-Targeting Ability.

Nano-sized polymersomes functionalized with peptides or proteins are being increasingly studied for targeted delivery of diagnostic and therapeutic molecules. Earlier computational studies have suggested that ellipsoidal nanoparticles, compared to spherical ones, display enhanced binding efficiency with target cells, but this has not yet been experimentally validated. We hypothesize that hydrophilic polymer chains coupled to vesicle-forming polymers would result in ellipsoidal polymersomes. In addition, ellipsoidal polymersomes modified with cell adhesion peptides bind with target cells more actively than spherical ones. We examine this hypothesis by substituting polyaspartamide with octadecyl chains and varying numbers of poly(ethylene glycol) (PEG) chains. Increasing the degree of substitution of PEG from 0.5 to 1.0 mol% drives the polymer to self-assemble into an ellipsoidal polymersome with an aspect ratio of 2.1. Further modification of these ellipsoidal polymersomes with peptides containing an Arg-Gly-Asp sequence (RGD peptides) lead to a significant increase in the rate of association and decrease in the rate of dissociation with a substrate coated with αvß3 integrins. In addition, in a circulation-mimicking flow, the ellipsoidal polymersomes linked with RGD peptides adhere to target tissues more favorably than their spherical equivalents do. Overall, the results of this study will greatly serve to improve the efficiency of targeted delivery of a wide array of polymersomes loaded with various biomedical modalities.

A cell-instructive hydrogel to regulate malignancy of 3D tumor spheroids with matrix rigidity.

Three dimensional (3D) tumor spheroid models are becoming important biomedical tools for both fundamental and applied cancer studies, but current models do not account for different levels of cancer malignancy. Several studies have reported that the mechanical rigidity of a hydrogel plays a significant role in regulating the phenotypes of cancer cells adhered to the gel surface. This finding suggests that matrix rigidity should also modulate the malignancy of 3D tumor spheroids. However, the role of matrix stiffness is often confounded by concurrent changes in 3D matrix permeability. This study reports an advanced strategy to assemble 3D liver tumor spheroids with controlled intercellular organization, phenotypes, and angiogenic activities using hydrogels with controlled stiffness and minimal differences in molecular diffusivity. The elastic moduli of cell-encapsulated collagen gels were increased by stiffening interconnected collagen fibers with varied amounts of poly(ethylene glycol) di-(succinic acid N-hydroxysuccinimidyl ester). Interestingly, hepatocellular carcinoma cells encapsulated in a fat-like, softer hydrogel formed malignant cancer spheroids, while cells cultured in a liver-like, stiffer gel formed compact hepatoids with suppressed malignancy. Overall, both the hydrogel and the 3D tumor spheroids developed in this study will be greatly useful to better understand and regulate the emergent behaviors of various cancer cells.

Tuning the non-equilibrium state of a drug-encapsulated poly(ethylene glycol) hydrogel for stem and progenitor cell mobilization.

Injectable and biodegradable hydrogels have been increasingly studied for sustained drug delivery in various molecular therapies. However, it remains a challenge to attain desired delivery rate at injection sites due to local tissue pressures exerted on the soft hydrogels. Furthermore, there is often limited controllability of stiffness and degradation rates, which are key factors required for achieving desired drug release rate and therapeutic efficacy. This study presents a stiff and metastable poly(ethylene glycol) diacrylate (PEGDA)-poly(ethylene imine) (PEI) hydrogel which exhibits an elastic modulus equivalent to bulk plastic materials, and controllable degradation rate independent of its initial elastic modulus. Such unique stiffness was attained from the highly branched architecture of PEI, and the decoupled controllability of degradation rate was achieved by tuning the non-equilibrium swelling of the hydrogel. Furthermore, a single intramuscular administration of granulocyte colony stimulating factor (GCSF)-encapsulated PEGDA-PEI hydrogel extended the mobilization of mononuclear cells to four days. A larger yield of expanded CD34+ and CD31+ endothelial progenitor cells (EPCs) was also obtained as compared to the daily bolus administration. Overall, the hydrogel created in this study will be useful for the controlled and sustained delivery of a wide array of drug molecules.

Three dimensionally flocculated proangiogenic microgels for neovascularization.

Microparticles encapsulating regenerative medicines have been used in tissue engineering because of their several advantages, including non-invasive drug delivery and controllable drug release rates. However, microparticles implanted in tissue defects are readily displaced by external mechanical forces, decreasing their regenerative efficacy. We hypothesized that a drug-encapsulated colloidal gel formed through colloidal attraction between microparticles would resist displacement at an implant site, and subsequently improve therapeutic efficacy. This hypothesis was examined using a colloidal gel formed from the mixing of negatively charged microgels composed of poly(ethylene glycol) (PEG) and poly(sodium acrylate), and positively charged microgels composed of PEG and poly(vinyl benzyl trimethyl ammonium chloride). The structural strength of the colloidal gel could be tuned with the zeta potential and volumetric ratios of the oppositely charged microgels. Furthermore, the implantation of the colloidal gel, encapsulating vascular endothelial growth factor, significantly increased the vascular density while limiting host inflammation, as compared with the implantation of unary microgel suspensions. This study demonstrates an enhancement in the efficacy of microparticle drug delivery systems by tuning rheological properties of suspensions, which should be useful for the design of a wide array of particulate systems for both tissue engineering and drug delivery.