Precise control of synthetic hydrogel network structure via linear, independent synthesis-swelling relationships.

Hydrogel physical properties are tuned by altering synthesis conditions such as initial polymer concentration and polymer-cross-linker stoichiometric ratios. Traditionally, differences in hydrogel synthesis schemes, such as end-linked poly(ethylene glycol) diacrylate hydrogels and cross-linked poly(vinyl alcohol) hydrogels, limit structural comparison between hydrogels. In this study, we use generalized synthesis variables for hydrogels that emphasize how changes in formulation affect the resulting network structure. We identify two independent linear correlations between these synthesis variables and swelling behavior. Analysis through recently updated swollen polymer network models suggests that synthesis-swelling correlations can be used to make a priori predictions of the stiffness and solute diffusivity characteristics of synthetic hydrogels. The same experiments and analyses performed on methacrylamide-modified gelatin hydrogels demonstrate that complex biopolymer structures disrupt the linear synthesis-swelling correlations. These studies provide insight into the control of hydrogel physical properties through structural design and can be used to implement and optimize biomedically relevant hydrogels.

A shape memory foam composite with enhanced fluid uptake and bactericidal properties as a hemostatic agent.

Uncontrolled hemorrhage accounts for more than 30% of trauma deaths worldwide. Current hemostatic devices focus primarily on time to hemostasis, but prevention of bacterial infection is also critical for improving survival rates. In this study, we sought to improve on current devices used for hemorrhage control by combining the large volume-filling capabilities and rapid clotting of shape memory polymer (SMP) foams with the swelling capacity of hydrogels. In addition, a hydrogel composition was selected that readily complexes with elemental iodine to impart bactericidal properties to the device. The focus of this work was to verify that the advantages of each respective material (SMP foam and hydrogel) are retained when combined in a composite device. The iodine-doped hydrogel demonstrated an 80% reduction in bacteria viability when cultured with a high bioburden of Staphylococcus aureus. Hydrogel coating of the SMP foam increased fluid uptake by 19× over the uncoated SMP foam. The composite device retained the shape memory behavior of the foam with more than 15× volume expansion after being submerged in 37°C water for 15 min. Finally, the expansion force of the composite was tested to assess potential tissue damage within the wound during device expansion. Expansion forces did not exceed 0.6N, making tissue damage during device expansion unlikely, even when the expanded device diameter is substantially larger than the target wound site. Overall, the enhanced fluid uptake and bactericidal properties of the shape memory foam composite indicate its strong potential as a hemostatic agent to treat non-compressible wounds. STATEMENT OF SIGNIFICANCE: No hemostatic device currently used in civilian and combat trauma situations satisfies all the desired criteria for an optimal hemostatic wound dressing. The research presented here sought to improve on current devices by combining the large volume-filling capabilities and rapid clotting of shape memory polymer (SMP) foams with the swelling capacity of hydrogels. In addition, a hydrogel composition was selected that readily complexes with elemental iodine to impart bactericidal properties to the device. The focus of this work was to verify that the advantages of each respective material are retained when combined into a composite device. This research opens the door to generating novel composites with a focus on both hemostasis, as well as wound healing and microbial prevention.

Correction: In situ crosslinking of electrospun gelatin for improved fiber morphology retention and tunable degradation.

Correction for 'In situ crosslinking of electrospun gelatin for improved fiber morphology retention and tunable degradation' by A. P. Kishan et al., J. Mater. Chem. B, 2015, DOI: .

In situ crosslinking of electrospun gelatin for improved fiber morphology retention and tunable degradation.

Electrospinning is a popular technique to fabricate tissue engineering scaffolds due to the exceptional tunability of the fiber morphology, which can be used to control the scaffold mechanical properties, degradation rate, and cell behavior. Recent work has focused on electrospinning natural polymers such as gelatin to improve the regeneration potential of these grafts. Gelatin scaffolds must be crosslinked to avoid rapid dissolution upon implantation with current crosslinking strategies requiring additional post-processing steps. Despite the strong dependence of scaffold properties on fiber morphology, there has been minimal emphasis on retaining the original fiber morphology of electrospun gelatin scaffolds after implantation. This work describes a method for in situ crosslinking of gelatin to produce electrospun fibers with improved fiber morphology retention after implantation. A double barrel syringe with an attached mixing head and a diisocyanate crosslinker were utilized to generate electrospun scaffolds that crosslink during the electrospinning process. These in situ crosslinked fiber meshes retained morphology after 1 week incubation in water at 37 °C; whereas, uncrosslinked meshes lost the fibrous morphology within 24 hours. Degree of crosslinking was quantified and relationships between the crosslinker ratio and enzymatic degradation rate were evaluated. The degradation rate decreased with increased crosslinker ratio, resulting in a highly tunable system. Additionally, tensile testing under simulated physiological conditions indicated that increased crosslinker ratios resulted in increases in initial modulus and tensile strength. Overall, this in situ crosslinking technique provides a method to crosslink gelatin during electrospinning and can be used to tune the degradation rate of resulting scaffolds while enabling improved fiber morphology retention after implantation.

Determination of the in vivo degradation mechanism of PEGDA hydrogels.

Poly(ethylene glycol) (PEG) hydrogels are one of the most extensively utilized biomaterials systems due to their established biocompatibility and highly tunable properties. It is widely acknowledged that traditional acrylate-derivatized PEG (PEGDA) hydrogels are susceptible to slow degradation in vivo and are therefore unsuitable for long-term implantable applications. However, there is speculation whether the observed degradation is due to hydrolysis of endgroup acrylate esters or oxidation of the ether backbone, both of which are possible in the foreign body response to implanted devices. PEG diacrylamide (PEGDAA) is a polyether-based hydrogel system with similar properties to PEGDA but with amide linkages in place of the acrylate esters. This provides a hydrolytically-stable control that can be used to isolate the relative contributions of hydrolysis and oxidation to the in vivo degradation of PEGDA. Here we show that PEGDAA hydrogels remained stable over 12 weeks of subcutaneous implantation in a rat model while PEGDA hydrogels underwent significant degradation as indicated by both increased swelling ratio and decreased modulus. As PEGDA and PEGDAA have similar susceptibility to oxidation, these results demonstrate for the first time that the primary in vivo degradation mechanism of PEGDA is hydrolysis of the endgroup acrylate esters. Additionally, the maintenance of PEGDAA hydrogel properties in vivo indicates their suitability for long-term implants. These studies serve to elucidate key information about a widely used biomaterial system to allow for better implantable device design and to provide a biostable replacement option for PEGDA in applications that require long-term stability.

Drying and storage effects on poly(ethylene glycol) hydrogel mechanical properties and bioactivity.

Hydrogels based on poly(ethylene glycol) (PEG) are increasingly used in biomedical applications because of their ability to control cell-material interactions by tuning hydrogel physical and biological properties. Evaluation of stability after drying and storage are critical in creating an off-the-shelf biomaterial that functions in vivo according to original specifications. However, there has not been a study that systematically investigates the effects of different drying conditions on hydrogel compositional variables. In the first part of this study, PEG-diacrylate hydrogels underwent common processing procedures (vacuum-drying, lyophilizing, hydrating then vacuum-drying), and the effect of this processing on the mechanical properties and swelling ratios was measured. Significant changes in compressive modulus, tensile modulus, and swelling ratio only occurred for select processed hydrogels. No consistent trends were observed after processing for any of the formulations tested. The effect of storage conditions on cell adhesion and spreading on collagen- and streptococcal collagen-like protein (Scl2-2)-PEG-diacrylamide hydrogels was then evaluated to characterize bioactivity retention after storage. Dry storage conditions preserved bioactivity after 6 weeks of storage; whereas, storage in PBS significantly reduced bioactivity. This loss of bioactivity was attributed to ester hydrolysis of the protein linker, acrylate-PEG-N-hydroxysuccinimide. These studies demonstrate that these processing methods and dry storage conditions may be used to prepare bioactive PEG hydrogel scaffolds with recoverable functionality after storage.

Multilayer vascular grafts based on collagen-mimetic proteins.

A major roadblock in the development of an off-the-shelf, small-caliber vascular graft is achieving rapid endothelialization of the conduit while minimizing the risk of thrombosis, intimal hyperplasia, and mechanical failure. To address this need, a collagen-mimetic protein derived from group A Streptococcus, Scl2.28 (Scl2), was conjugated into a poly(ethylene glycol) (PEG) hydrogel to generate bioactive hydrogels that bind to endothelial cells (ECs) and resist platelet adhesion. The PEG-Scl2 hydrogel was then reinforced with an electrospun polyurethane mesh to achieve suitable biomechanical properties. In the current study, initial evaluation of this multilayer design as a potential off-the-shelf graft was conducted. First, electrospinning parameters were varied to achieve composite burst pressure, compliance, and suture retention strength that matched reported values of saphenous vein autografts. Composite stability following drying, sterilization, and physiological conditioning under pulsatile flow was then demonstrated. Scl2 bioactivity was also maintained after drying and sterilization as indicated by EC adhesion and spreading. Evaluation of platelet adhesion, aggregation, and activation indicated that PEG-Scl2 hydrogels had minimal platelet interactions and thus appear to provide a thromboresistant blood contacting layer. Finally, evaluation of EC migration speed demonstrated that PEG-Scl2 hydrogels promoted higher migration speeds than PEG-collagen analogs and that migration speed was readily tuned by altering protein concentration. Collectively, these results indicate that this multilayer design warrants further investigation and may have the potential to improve on current synthetic options.

Compositional control of poly(ethylene glycol) hydrogel modulus independent of mesh size.

Poly(ethylene glycol) (PEG) hydrogels are of great interest in tissue engineering because of their established biocompatibility, high permeability, and tunable material properties. However, rational design of PEG hydrogel scaffold properties has been inhibited by the interdependence of key material properties such as modulus and mesh size. This study examined the effect of an acrylated 4-arm PEG cross-linker on gel modulus and mesh size as a means of inducing local increases in cross-link density to decouple these two parameters. It was determined that adding the 4-arm PEG cross-linker to PEG hydrogels resulted in statistically significant increases in both tensile and compressive modulus while having minimal effects on overall gel mesh size. The incorporation of the 4-arm PEG cross-linker also broadened the range of achievable mechanical properties. This study provides the methodology to independently tune PEG hydrogel modulus and mesh size, which may be utilized in future investigations of the individual and combined effects of PEG hydrogel modulus and mesh size on cell behavior and viability. It also presents a more finely tunable hydrogel scaffold with utility in a broad range of tissue engineering applications.

Bioactive hydrogels based on Designer Collagens.

Designer Collagens are based on streptococcal collagen-like (Scl) proteins that form a triple helix similar to mammalian collagens but that are non-platelet aggregating. In contrast to the numerous cell-binding sites on collagen, Scl2 from Streptococcus pyogenes serotype M28 does not contain any known cell-binding sites and thus provides a blank slate in terms of cellular interactions. In the current study, Scl2 protein was modified to include receptor binding motifs that interact with alpha1 and/or alpha2 integrin subunits. The modfied Scl2 proteins have been demonstrated to mediate differential endothelial cell (EC) and smooth muscle cell (SMC) adhesion via these integrins and to retain the non-platelet aggregating properties of the "parent" Scl2. Thromboresistant scaffolds which selectively bind ECs vs. SMCs would be desirable for vascular repair or replacement. Despite the potential of these Scl proteins in vascular applications, the utility of this recombinant protein family is currently limited to coatings due to the inability of Scl proteins to assemble into stable three-dimensional networks. To address this limitation, the Scl2 proteins were functionalized with photocrosslinking sites to enable incorporation into a hydrogel matrix. Characterization studies confirmed that the functionalization of the Scl2 proteins did not disrupt triple helix conformation, integrin binding or cell adhesion. Bioactive hydrogels were fabricated by combining the functionalized Scl2 proteins with poly(ethylene glycol) diacrylate (PEGDA) and photocrosslinking. EC and SMC adhesion studies confirmed cell-specific adhesion due to selective integrin binding to the two receptor binding motifs investigated. These results serve to highlight the potential of this novel biomaterial platform in the development of improved tissue engineered vascular grafts.