Chitosan Poly(vinyl alcohol) Methacrylate Hydrogels for Tissue Engineering Scaffolds.

A major challenge in tissue engineering scaffolds is controlling scaffold degradation rates during healing while maintaining mechanical properties to support tissue formation. Hydrogels are three-dimensional matrices that are widely applied as tissue scaffolds based on their unique properties that can mimic the extracellular matrix. In this study, we develop a hybrid natural/synthetic hydrogel platform to tune the properties for tissue engineering scaffold applications. We modified chitosan and poly(vinyl alcohol) (PVA) with photo-cross-linkable methacrylate functional groups and then synthesized a library of chitosan PVA methacrylate hydrogels (ChiPVAMA) with two different photoinitiators, Irgacure 2959 (I2959) and lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP). ChiPVAMA hydrogels showed tunability in degradation rates and mechanical properties based on both the polymer content and photoinitiator type. This tunability could enable their application in a range of tissue scaffold applications. In a 2D scratch wound healing assay, all hydrogel samples induced faster wound closure compared to a gauze clinical wound dressing control. NIH/3T3 cells encapsulated in hydrogels showed a high viability (∼92%) over 14 days, demonstrating the capacity of this system as a supportive cell scaffold. In addition, hydrogels containing a higher chitosan content demonstrated a high antibacterial capacity. Overall, ChiPVAMA hydrogels provide a potential tissue engineering scaffold that is tunable, degradable, and suitable for cell growth.

Antimicrobial PVA Hydrogels with Tunable Mechanical Properties and Antimicrobial Release Profiles.

Hydrogels are broadly employed in wound healing applications due to their high water content and tissue-mimicking mechanical properties. Healing is hindered by infection in many types of wound, including Crohn's fistulas, tunneling wounds that form between different portions of the digestive system in Crohn's disease patients. Owing to the rise of drug-resistant infections, alternate approaches are required to treat wound infections beyond traditional antibiotics. To address this clinical need, we designed a water-responsive shape memory polymer (SMP) hydrogel, with natural antimicrobials in the form of phenolic acids (PAs), for potential use in wound filling and healing. The shape memory properties could allow for implantation in a low-profile shape, followed by expansion and would filling, while the PAs provide localized delivery of antimicrobials. Here, we developed a urethane-crosslinked poly(vinyl alcohol) hydrogel with cinnamic (CA), p-coumaric (PCA), and caffeic (Ca-A) acid chemically or physically incorporated at varied concentrations. We examined the effects of incorporated PAs on antimicrobial, mechanical, and shape memory properties, and on cell viability. Materials with physically incorporated PAs showed improved antibacterial properties with lower biofilm formation on hydrogel surfaces. Both modulus and elongation at break could be increased simultaneously in hydrogels after both forms of PA incorporation. Cellular response in terms of initial viability and growth over time varied based on PA structure and concentration. Shape memory properties were not negatively affected by PA incorporation. These PA-containing hydrogels with antimicrobial properties could provide a new option for wound filling, infection control, and healing. Furthermore, PA content and structure provide novel tools for tuning material properties independently of network chemistry, which could be harnessed in a range of materials systems and biomedical applications.

Highly Porous Gas-Blown Hydrogels for Direct Cell Encapsulation with High Cell Viability.

Cell transplant therapies show potential as treatments for a large number of diseases. The encapsulation of cells within hydrogels is often used to mimic the extracellular matrix and protect cells from the body's immune response. However, cell encapsulation can be limited in terms of both scaffold size and cell viability due to poor nutrient and waste transport throughout the bulk of larger volume hydrogels. Strategies to address this issue include creating prevascularized or porous structured materials. For example, cell-laden hydrogels can be formed by porogen leaching or three-dimensional printing, but these techniques involve the use of multiple materials, long preparation times, and/or specialized equipment. Postfabrication cell seeding in porous scaffolds can result in inconsistent cell density throughout scaffold volumes and typically requires a bioreactor to ensure even cell distribution. In this study, we developed a highly cytocompatible direct cell encapsulation method during the rapid fabrication of porous hydrogels. Using sodium bicarbonate and citric acid as blowing agents, we employed photocurable polymers to produce highly porous materials within a matter of minutes. Cells were directly encapsulated within methacrylated poly(vinyl alcohol), poly(ethylene glycol), and gelatin hydrogels at viabilities as high as 93% by controlling solution variables, such as citric acid content, viscosity, pH, and curing time. Cell viability within the resulting porous constructs was high (>80%) over 14 days of analysis with multiple cell types. This work provides a simple, versatile, and tunable method for cell encapsulation within highly porous constructs that can be built upon in future work for the delivery of cell-based therapies. Impact Statement This simple method to obtain cell-laden hydrogel foams allows direct cell encapsulation within biomaterials without the need for porogens or microcarriers, while maintaining high cell viability. The successful encapsulation of multiple cell types into gas-blown hydrogels with varied chemistries shows the versatility of this method. While this work focuses on photocrosslinkable polymers, any quick gelling material could be used for foam fabrication in expansion of this work. The potential future impact of this work on the treatment of diseases and injuries that utilize cell therapies is wide-ranging.

Antimicrobial Shape Memory Polymer Hydrogels for Chronic Wound Dressings.

Chronic wounds can remain open for several months and have high risks of amputation due to infection. Dressing materials to treat chronic wounds should be conformable for irregular wound geometries, maintain a moist wound bed, and reduce infection risks. To that end, we developed cytocompatible shape memory polyurethane-based poly(ethylene glycol) (PEG) hydrogels that allow facile delivery to the wound site. Plant-based phenolic acids were physically incorporated onto the hydrogel scaffolds to provide antimicrobial properties. These materials were tested to confirm their shape memory properties, cytocompatibility, and antibacterial properties. The incorporation of phenolic acids provides a new mechanism for tuning intermolecular bonding in the hydrogels and corollary mechanical and shape memory properties. Phenolic acid-containing hydrogels demonstrated an increased shape recovery ratio (1.35× higher than the control formulation), and materials with cytocompatibility >90% were identified. Antimicrobial properties were retained over 20 days in hydrogels with higher phenolic acid content. Phenolic acid retention and antimicrobial efficacy were dependent upon phenolic acid structures and interactions with the polymer backbone. This novel hydrogel system provides a platform for future development as a chronic wound dressing material that is easy to implant and reduces infection risks.

Magnetically Actuated Shape Memory Polymers for On-Demand Drug Delivery.

Repeated use of intravenous infusions to deliver drugs can cause nerve damage, pain, and infection. There is an unmet need for a drug delivery method that administers drugs on demand for prolonged use. Here, we developed magnetically responsive shape memory polymers (SMPs) to enhance control over drug release. Iron oxide magnetic nanoparticles (mnps) were synthesized and incorporated into previously developed SMPs to enable magnetically induced shape memory effects that can be activated remotely via the application of an alternating magnetic field. These materials were tested for their shape memory properties (dynamic mechanical analysis), cytocompatibility (3T3 fibroblast viability), and tunable drug delivery rates (UV−VIS to evaluate the release of incorporated doxorubicin, 6-mercaptopurine, and/or rhodamine). All polymer composites had >75% cytocompatibility over 72 h. Altering the polymer chemistry and mnp content provided methods to tune drug release. Namely, linear polymers with higher mnp content had faster drug release. Highly cross-linked polymer networks with lower mnp content slowed drug release. Shape memory properties and polymer/drug interactions provided additional variables to tune drug delivery rates. Polymers that were fixed in a strained secondary shape had a slower release rate compared with unstrained polymers, and hydrophobic drugs were released more slowly than hydrophilic drugs. Using these design principles, a single material with gradient chemistry and dual drug loading was synthesized, which provided a unique mechanism to deliver two drugs from a single scaffold with distinct delivery profiles. This system could be employed in future work to provide controlled release of selected drug combinations with enhanced control over release as compared with previous approaches.

Hemostatic shape memory polymer foams with improved survival in a lethal traumatic hemorrhage model.

Although there are many hemostatic agents available for use on the battlefield, uncontrolled hemorrhage is still the primary cause of preventable death. Current hemostatic dressings include QuikClot® Combat Gauze (QCCG) and XStat®, which have inadequate success in reducing mortality. To address this need, a new hemostatic material was developed using shape memory polymer (SMP) foams, which demonstrate biocompatibility, rapid clotting, and shape recovery to fill the wound site. SMP foam hemostatic efficacy was examined in a lethal, noncompressible porcine liver injury model over 6 h following injury. Wounds were packed with SMP foams, XStat, or QCCG and compared in terms of time to bleeding cessation, total blood loss, and animal survival. The hemostatic material properties and in vitro blood interactions were also characterized. SMP foams decreased blood loss and active bleeding time in comparison with XStat and QCCG. Most importantly, SMP foams increased the 6 h survival rate by 50% and 37% (vs. XStat and QCCG, respectively) with significant increases in survival times. Based upon in vitro characterizations, this result is attributed to the low stiffness and shape filling capabilities of SMP foams. This study demonstrates that SMP foams have promise for improving upon current clinically available hemostatic dressings and that hemostatic material properties are important to consider in designing devices for noncompressible bleeding control. STATEMENT OF SIGNIFICANCE: Uncontrolled hemorrhage is the leading cause of preventable death on the battlefield, and it accounts for approximately 1.5 million deaths each year. New biomaterials are required for improved hemorrhage control, particularly in noncompressible wounds in the torso. Here, we compared shape memory polymer (SMP) foams with two clinical dressings, QuikClot Combat Gauze and XStat, in a pig model of lethal liver injury. SMP foam treatment reduced bleeding times and blood loss and significantly improved animal survival. After further material characterization, we determined that the improved outcomes with SMP foams are likely due to their low stiffness and controlled shape change after implantation, which enabled their delivery to the liver injuries without inducing further wound tearing. Overall, SMP foams provide a promising option for hemorrhage control.

Biostable Shape Memory Polymer Foams for Smart Biomaterial Applications.

Polyurethane foams provide a wide range of applications as a biomaterial system due to the ability to tune their physical, chemical, and biological properties to meet the requirements of the intended applications. Another key parameter that determines the usability of this biomaterial is its degradability under body conditions. Several current approaches focus on slowing the degradation rate for applications that require the implant to be present for a longer time frame (over 100 days). Here, biostable shape memory polymer (SMP) foams were synthesized with added ether-containing monomers to tune the degradation rates. The physical, thermal and shape memory properties of these foams were characterized along with their cytocompatibility and blood interactions. Degradation profiles were assessed in vitro in oxidative (3% H2O2; real-time) and hydrolytic media (0.1 M NaOH; accelerated) at 37 °C. The resulting foams had tunable degradation rates, with up 15% mass remaining after 108 days, and controlled erosion profiles. These easy-to-use, shape-filling SMP foams have the potential for various biomaterial applications where longer-term stability without the need for implant removal is desired.

Shape Memory Polymer Foams with Tunable Degradation Profiles.

Uncontrolled hemorrhage is the leading cause of preventable death on the battlefield and results in ∼1.5 million deaths each year. The primary current treatment options are gauze and/or tourniquets, which are ineffective for up to 80% of wounds. Additionally, most hemostatic materials must be removed from the patient within <12 h, which limits their applicability in remote scenarios and can cause additional bleeding upon removal. Here, degradable shape memory polymer (SMP) foams were synthesized to overcome these limitations. SMP foams were modified with oxidatively labile ether groups and hydrolytically labile ester groups to degrade after implantation. Foam physical, thermal, and shape memory properties were assessed along with cytocompatibility and blood interactions. Degradation profiles were obtained in vitro in oxidative and hydrolytic media (3% H2O2 (oxidation) and 0.1 M NaOH (hydrolysis) at 37 °C). The resulting foams had tunable, clinically relevant degradation rates, with complete mass loss within 30-60 days. These SMP foams have potential to provide an easy-to-use, shape-filling hemostatic dressing that can be left in place during traumatic wound healing with future potential use in regenerative medicine applications.

Characterization of shape memory polymer foam hemostats in in vitro hemorrhagic wound models.

Shape memory polymer foam hemostats are a promising option for future hemorrhage control in battlefield wounds. To enable their use as hemostatic devices, they must be optimized in terms of formulation and architecture, and their safety and efficacy must be characterized in animal models. Relevant in vitro models can be used for device optimization to help mitigate the excess use of animals and reduce costs of clinical translation. In this work, a simplified gunshot wound model and a grade V liver injury model were constructed. The models were used to characterize the effects of shape memory polymer foam hemostat geometry on wall pressures, application/removal times, hemorrhage (fluid loss), and fluid absorption in comparison with clinical controls. It was found that there is no benefit in over-sizing the hemostatic device relative to wound volume and that geometry effects are dependent upon the wound type. These models provide a rapid means for elucidation of promising hemostat geometries and formulations for use in future in vivo testing.

Characterization of Phenolic Acid Antimicrobial and Antioxidant Structure-Property Relationships.

Keywords: phenolic acids; antimicrobial; antioxidant.

Biodegradable shape memory polymer foams with appropriate thermal properties for hemostatic applications.

Shape memory polymer (SMP) foams are a promising material for hemostatic dressings due to their biocompatibility, high surface area, excellent shape recovery, and ability to quickly initiate blood clotting. Biodegradable SMP foams could eliminate the need for a secondary removal procedure of hemostatic material from the patients' wound, further facilitating wound healing. In this study, we developed hydrolytically and oxidatively biodegradable SMP foams by reacting polyols (triethanolamine or glycerol) with 6-aminocaproic acid or glycine to generate foaming monomers with degradable ester bonds. These monomers were used in foam synthesis to provide highly crosslinked SMP foam structures. The ester-containing foams showed clinically relevant thermal properties that were comparable to controls and excellent shape recovery within eight min. Triethanolamine-based ester-containing foams showed interconnected porous structure along with increased mechanical strength. Faster hydrolytic and oxidative biodegradation rates were achieved in ester-containing foams in comparison to controls. These biodegradable SMP foams with clinically applicable thermal properties possess great potential as an effective hemostatic device for use in hospitals or on battlefields.

Effects of Sterilization on Shape Memory Polyurethane Embolic Foam Devices.

Polyurethane shape memory polymer (SMP) foams have been developed for various embolic medical devices due to their unique properties in minimally invasive biomedical applications. These polyurethane materials can be stored in a secondary shape, from which they can recover their primary shape after exposure to an external stimulus, such as heat and water exposure. Tailored actuation temperatures of SMPs provide benefits for minimally invasive biomedical applications, but incur significant challenges for SMP-based medical device sterilization. Most sterilization methods require high temperatures or high humidity to effectively reduce the bioburden of the device, but the environment must be tightly controlled after device fabrication. Here, two probable sterilization methods (nontraditional ethylene oxide (ntEtO) gas sterilization and electron beam irradiation) are investigated for SMP medical devices. Thermal characterization of the sterilized foams indicated that ntEtO gas sterilization significantly decreased the glass transition temperature. Further material characterization was undertaken on the electron beam (ebeam) sterilized samples, which indicated minimal changes to the thermomechanical integrity of the bulk foam and to the device functionality.

Particulate Release From Nanoparticle-Loaded Shape Memory Polymer Foams.

Highly porous, open-celled shape memory polymer (SMP) foams are being developed for a number of vascular occlusion devices. Applications include abdominal aortic and neurovascular aneurysm or peripheral vascular occlusion. A major concern with implanting these high surface area materials in the vasculature is the potential to generate unacceptable particulate burden, in terms of number, size, and composition. This study demonstrates that particulate numbers and sizes in SMP foams are in compliance with limits stated by the most relevant standard and guidance documents. Particulates were quantified in SMP foams as made, postreticulation, and after incorporating nanoparticles intended to increase material toughness and improve radiopacity. When concentrated particulate treatments were administered to fibroblasts, they exhibited high cell viability (100%). These results demonstrate that the SMP foams do not induce an unacceptable level of risk to potential vascular occlusion devices due to particulate generation.

Cold Plasma Reticulation of Shape Memory Embolic Tissue Scaffolds.

Polyurethane shape memory polymer (SMP) foams are proposed for use as thrombogenic scaffolds to improve the treatment of vascular defects, such as cerebral aneurysms. However, gas blown SMP foams inherently have membranes between pores, which can limit their performance as embolic tissue scaffolds. Reticulation, or the removal of membranes between adjacent foam pores, is advantageous for improving device performance by increasing blood permeability and cellular infiltration. This work characterizes the effects of cold gas plasma reticulation processes on bulk polyurethane SMP films and foams. Plasma-induced changes on material properties are characterized using scanning electron microscopy, uniaxial tensile testing, goniometry, and free strain recovery experiments. Device specific performance is characterized in terms of permeability, platelet attachment, and cell-material interactions. Overall, plasma reticulated SMP scaffolds show promise as embolic tissue scaffolds due to increased bulk permeability, retained thrombogenicity, and favorable cell-material interactions.