Highly Cross-Linked Shape Memory Polymers with Tunable Oxidative and Hydrolytic Degradation Rates and Selected Products Based on Succinic Acid

Bioactive Polyurethane Shape Memory Polymer Foam Dressings with Enhanced Blood and Cell Interactions for Improved Wound Healing

Polyurethane (PUr) shape memory polymer (SMP) foams have demonstrated excellent bleeding control in traumatic wounds. Unlike the current clinically available treatment options, PUr SMP foams can address noncompressible bleeds, are safe for prolonged use, and are highly tunable, offering broad functionalities like biodegradation and antimicrobial properties. Despite their hemostatic efficacy, PUrs are entirely synthetic, which limits their long-term healing capacity if left in a wound to degrade. This work employed methods for facile incorporation of bioactive collagen and gelatin into PUr foams postfabrication to enhance their clotting efficacy and drive cell interactions. The procoagulant nature of collagen and gelatin increased the clotting accomplished by the PUr SMP foams. Additionally, the bioactive PUr SMP foams promoted cell attachment, spreading, and proliferation on foam pores, which could facilitate tissue migration into the scaffold and promote wound repair. Overall, a bioactive PUr SMP foam dressing could significantly improve traumatic wound healing outcomes.

Rapid synthesis of degradable ester/thioether monomers and their incorporation into thermoset polyurethane foams for traumatic wound healing

Polyurethane (PUr) foam hemostatic dressings are highly effective at controlling bleeding in traumatic wounds, but their traditionally slow degradation rate requires dressing removal, which could result in wound rebleeding. Incorporating degradable linkages into the PUr network can provide a biodegradable dressing that could be left in place during healing, eliminating rebleeding upon removal and providing scaffolding for new tissue ingrowth with no remains of the applied dressing after healing. In this work, a library of degradable PUr foams was synthesized from degradable monomers based on hydrolytically labile esters and oxidatively labile thioethers using rapid click-chemistry reactions. In a twelve-week in vitro degradation study in 3% hydrogen peroxide and 0.1 M sodium hydroxide, incorporation of degradable monomers resulted in significantly increased PUr foam mass loss, offering biodegradable foam dressings that could better match the rate of traumatic wound healing. Changes to foam chemical, mechanical, thermal, and physical properties throughout degradation were also analyzed. Furthermore, the degradable PUr foams had increased platelet interactions, which could improve foam-induced clotting for a more effective hemostatic dressing. Overall, a biodegradable PUr foam hemostatic dressing could significantly improve healing outcomes in traumatic wounds. STATEMENT OF SIGNIFICANCE: A simple, solvent-free, rapid synthesis technique was developed to provide degradable polythiol monomers for use in polyurethane synthesis. The degradable monomers were incorporated into hemostatic polyurethane foams to provide materials with tunable degradation rates within clinically-relevant time frames. The resulting foams and their degradation byproducts were cytocompatible and hemocompatible, and foams made with the new degradable monomers had enhanced blood clotting, enabling their future use as hemostatic dressings.

In vitro and in vivo degradation correlations for polyurethane foams with tunable degradation rates

Polyurethane foams present a tunable biomaterial platform with potential for use in a range of regenerative medicine applications. Achieving a balance between scaffold degradation rates and tissue ingrowth is vital for successful wound healing, and significant in vivo testing is required to understand these processes. Vigorous in vitro testing can minimize the number of animals that are required to gather reliable data; however, it is difficult to accurately select in vitro degradation conditions that can effectively mimic in vivo results. To that end, we performed a comprehensive in vitro assessment of the degradation of porous shape memory polyurethane foams with tunable degradation rates using varying concentrations of hydrogen peroxide to identify the medium that closely mimics measured in vivo degradation rates. Material degradation was studied over 12 weeks in vitro in 1%, 2%, or 3% hydrogen peroxide and in vivo in subcutaneous pockets in Sprague Dawley rats. We found that the in vitro degradation conditions that best predicted in vivo degradation rates varied based on the number of mechanisms by which the polymer degraded and the polymer hydrophilicity. Namely, more hydrophilic materials that degrade by both hydrolysis and oxidation require lower concentrations of hydrogen peroxide (1%) to mimic in vivo rates, while more hydrophobic scaffolds that degrade by oxidation alone require higher concentrations of hydrogen peroxide (3%) to model in vivo degradation. This information can be used to rationally select in vitro degradation conditions that accurately identify in vivo degradation rates prior to characterization in an animal model.

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.

4D polycarbonates via stereolithography as scaffolds for soft tissue repair

3D printing has emerged as one of the most promising tools to overcome the processing and morphological limitations of traditional tissue engineering scaffold design. However, there is a need for improved minimally invasive, void-filling materials to provide mechanical support, biocompatibility, and surface erosion characteristics to ensure consistent tissue support during the healing process. Herein, soft, elastomeric aliphatic polycarbonate-based materials were designed to undergo photopolymerization into supportive soft tissue engineering scaffolds. The 4D nature of the printed scaffolds is manifested in their shape memory properties, which allows them to fill model soft tissue voids without deforming the surrounding material. In vivo, adipocyte lobules were found to infiltrate the surface-eroding scaffold within 2 months, and neovascularization was observed over the same time. Notably, reduced collagen capsule thickness indicates that these scaffolds are highly promising for adipose tissue engineering and repair.

Shape memory scaffolds are needed for minimally invasive tissue repair and void filling. Here the authors report on the development of 4D printed polycarbonate-based scaffolds with surface degradation properties which fill voids without deforming tissue and allow for tissue ingrowth with reduced immune response.

Shape Memory Poly(β-hydroxythioether) Foams for Oil Remediation in Aquatic Environments

Shape memory poly(β-hydroxythioether) foams were produced using organobase catalyzed reactions between epoxide and thiol monomers, allowing for the rapid formation of porous media within approximately 5 min, confirmed using both rheology and physical foam blowing. The porous materials possess ultralow densities (0.022 g × cm^-3) and gel fractions of approximately 93%. Thermomechanical characterizations of the materials revealed glass transition temperatures tunable from approximately 50 to 100 °C, elastic moduli of approximately 2 kPa, and complete strain recovery upon heating of the sample above its glass transition temperature. The foams were characterized for their ability to take up oil from an aqueous multilayered ideal environment, revealing more than 2000% mass of oil (relative to the foam mass) could be collected. Importantly, while post-fabrication functionalization was possible with isocyanate chemistry followed by addition of hexadecanethiol or 3,3-bis(hexadecylthio)propan-1-ol, the oil collection efficiency of the system was not significantly enhanced, indicating that these materials, as porous media, possess unique attributes that make them appealing for environmental remediation without the need for costly modifications or manipulations.

In vitro cytocompatibility testing of oxidative degradation products

Implantable medical devices must undergo thorough evaluation to ensure safety and efficacy before use in humans. If a device is designed to degrade, it is critical to understand the rate of degradation and the degradation products that will be released. Oxidative degradation is typically modeled in vitro by immersing materials or devices in hydrogen peroxide, which can limit further analysis of degradation products in many cases. Here we demonstrate a novel approach for testing the cytocompatibility of degradation products for oxidatively-degradable biomaterials where the materials are exposed to hydrogen peroxide, and then catalase enzyme is used to convert the hydrogen peroxide to water and oxygen so that the resulting aqueous solution can be added to cell culture media. To validate our results, expected degradation products are also synthesized then added to cell culture media. We used these methods to evaluate the cytocompatibility of degradation products from an oxidatively-degradable shape memory polyurethane designed in our lab and found that the degradation of these polymers is unlikely to cause a cytotoxic response in vivo based on the guidance provided by ISO 10993-5. These methods may also be applicable to other biocompatibility tests such as tests for mutagenicity or systemic toxicity, and evaluations of cell proliferation, migration, or gene and protein expression.

Catalyzed HTPB/HDI-Trimer Curing Reactions and Influence on Pot Life

In this research, rheokinetics is used to study the curing reaction of hydroxyl terminated polybutadiene (HTPB) and trimer of hexamethylene-1,6-diisocyanate (HDI-trimer) with 1,4-diazabicyclo[2.2.2]octane (DABCO) as catalyst under different catalyst mass fraction. The results show that the pot life of the system depends on the catalyst mass fraction in the binder system. Furthermore, with increased catalyst mass fraction, the fitting diagram obtained by plotting ln (viscosity) versus curing time shows a better linear relationship. Therefore, the amount of catalyst required to achieve a certain pot life can be calculated through the formula. It is worth mentioning that the applicable pot life equations are proposed in the paper. From the equations, we find that under isothermal curing conditions at 35 °C, when the mass fraction of DABCO was 0.216 wt.%, the pot life of the HTPB/HDI-trimer binder system reaches 4 h.

Partially-Perforated Self-Reinforced Polyurea Foams

This paper reports the unique microstructure of polyurea foams that combines the advantages of open and closed cell polymeric foams, which were synthesized through a self-foaming process. The latter was the result of aggressive mechanical mixing of diamine curative, isocyanate, and deionized water at ambient conditions, which can be adjusted on-demand to produce variable density polyurea foam. The spherical, semi-closed microcellular structure has large perforations on the cell surface resulting from the concurrent expansion of neighboring cells and small holes at the bottom surface of the cells. This resulted in a partially perforated microcellular structure of polyurea foam. As a byproduct of the manufacturing process, polyurea microspheres nucleate and deposit on the inner cell walls of the foam, acting as a reinforcement. Since cell walls and the microspheres are made of polyurea, the resulting reinforcement effect overcomes the fundamental interfacial issue of different adjacent materials. The partially perforated, self-reinforced polyurea foam is compared to the performance of traditional counterparts in biomechanical impact scenarios. An analytical model was developed to explicate the stiffening effect associated with the reinforcing microspheres. The model results indicate that the reinforced microcell exhibited, on average, ~30% higher stiffness than its barren counterpart.

3D Printing for the Clinic: Examining Contemporary Polymeric Biomaterials and Their Clinical Utility

The advent of additive manufacturing offered the potential to revolutionize clinical medicine, particularly with patient-specific implants across a range of tissue types. However, to date, there are very few examples of polymers being used for additive processes in clinical settings. The state of the art with regards to 3D printable polymeric materials being exploited to produce novel clinically relevant implants is discussed here. We focus on the recent advances in the development of implantable, polymeric medical devices and tissue scaffolds without diverging extensively into bioprinting. By introducing the major 3D printing techniques along with current advancements in biomaterials, we hope to provide insight into how these fields may continue to advance while simultaneously reviewing the ongoing work in the field.