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

Recent advancements in natural polymers-based self-healing nano-materials for wound dressing

The field of wound healing has witnessed remarkable progress in recent years, driven by the pursuit of advanced wound dressings. Traditional dressing materials have limitations like poor biocompatibility, nonbiodegradability, inadequate moisture management, poor breathability, lack of inherent therapeutic properties, and environmental impacts. There is a compelling demand for innovative solutions to transcend the constraints of conventional dressing materials for optimal wound care. In this extensive review, the therapeutic potential of natural polymers as the foundation for the development of self-healing nano-materials, specifically for wound dressing applications, has been elucidated. Natural polymers offer a multitude of advantages, possessing exceptional biocompatibility, biodegradability, and bioactivity. The intricate engineering strategies employed to fabricate these polymers into nanostructures, thereby imparting enhanced mechanical robustness, flexibility, critical for efficacious wound management has been expounded. By harnessing the inherent properties of natural polymers, including chitosan, alginate, collagen, hyaluronic acid, and so on, and integrating the concept of self-healing materials, a comprehensive overview of the cutting-edge research in this emerging field is presented in the review. Furthermore, the inherent self-healing attributes of these materials, wherein they exhibit innate capabilities to autonomously rectify any damage or disruption upon exposure to moisture or body fluids, reducing frequent dressing replacements have also been explored. This review consolidates the existing knowledge landscape, accentuating the benefits and challenges associated with these pioneering materials while concurrently paving the way for future investigations and translational applications in the realm of wound healing.

Polydopamine functionalized polyurethane shape memory sponge with controllable expansion performance triggered by near-infrared light for incompressible hemorrhage control

Uncontrolled expansion of shape memory sponges face a significant challenge in the treatment of lethal incompressible hemorrhage, which can lead to blood overflow or damage to the surrounding tissue. Herein, we developed a polydopamine functionalized polyurethane shape memory sponge (PDA-TPI-PU) with a controllable degree of expansion by near-infrared (NIR) light-triggered stimulation for the treatment of incompressible hemorrhage. The sponge has excellent liquid absorption performance and robust mechanical strength as well as good photothermal conversion ability. Under NIR light of 0.32 W/cm2, the maximum recovery rate of the fixed-shape compression sponge was 91% within 25 s in air and 80% within 25 s in blood. In the SD rat liver penetrating injury model, compared with commercial medical gelatin sponge and PVA sponge, the PDA-TPI-PU sponge could effectively control the bleeding under the NIR light irradiation and did not cause excessive compression of the wound. The sponge with these characteristics shows potential application prospects as a hemostatic material.

An All-In-One Transient Theranostic Platform for Intelligent Management of Hemorrhage

Developing theranostic devices to detect bleeding and effectively control hemorrhage in the prehospital setting is an unmet medical need. Herein, an all-in-one theranostic platform is presented, which is constructed by sandwiching silk fibroin (SF) between two silver nanowire (AgNW) based conductive electrodes to non-enzymatically diagnose local bleeding and stop the hemorrhage at the wound site. Taking advantage of the hemostatic property of natural SF, the device is composed of a shape-memory SF sponge, facilitating blood clotting, with ≈82% reduction in hemostatic time in vitro as compared with untreated blood. Furthermore, this sandwiched platform serves as a capacitive sensor that can detect bleeding and differentiate between blood and other body fluids (i.e., serum and water) via capacitance change. In addition, the AgNW electrode endows anti-infection efficiency against Escherichia coli and Staphylococcus aureus. Also, the device shows excellent biocompatibility and gradually biodegrades in vivo with no major local or systemic inflammatory responses. More importantly, the theranostic platform presents considerable hemostatic efficacy comparable with a commercial hemostat, Dengen, in rat liver bleeding models. The theranostic platform provides an unexplored strategy for the intelligent management of hemorrhage, with the potential to significantly improve patients' well-being through the integration of diagnostic and therapeutic capabilities.

Influence of the Foaming Process on the Burning Behavior of the PET-PEN Copolymer

The manufacturing process can modify the micromechanical structure, usefulness, and functionality of foams. Although one-step foaming is a simple process, controlling the morphology of the foams is difficult compared to the two-step processing method. In this study, we investigated the experimental differences in thermal and mechanical properties, particularly combustion behavior, between PET-PEN copolymers prepared by the two methods. With an increase in foaming temperature T_f, the PET-PEN copolymers became more fragile, and the breaking stress of the one-step PET-PEN foamed at the highest T_f was only 2.4% of that of the raw material. For the pristine PET-PEN, 24% of the mass was burned, leaving 76% as a molten sphere residue. The two-step MEG PET-PEN had only 1% of its mass remaining as a residue, whereas the one-step PET-PENs had between 41 and 55%. The actual mass burning rates were similar for all the samples except the raw material. The coefficient of thermal expansion of the one-step PET-PEN was about two orders of magnitude lower than that of the two-step SEG.

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.

Swellable and Thermally Responsive Hydrogel/Shape Memory Polymer Foam Composites for Sealing Lung Biopsy Tracts

Lung tissue biopsies can result in a leakage of blood (hemothorax) and air (pneumothorax) from the biopsy tract, which threatens the patient with a collapsed lung and other complications. We have developed a lung biopsy tract sealant based on a thiol-ene-crosslinked PEG hydrogel and polyurethane shape memory polymer (SMP) foam composite. After insertion into biopsy tracts, the PEG hydrogel component contributes to sealing through water-driven swelling, whereas the SMP foam contributes to sealing via thermal actuation. The gelation kinetics, swelling properties, and rheological properties of various hydrogel formulations were studied to determine the optimal formulation for composite fabrication. Composites were then fabricated via vacuum infiltration of the PEG hydrogel precursors into the SMP foam followed by thermal curing. After drying, the composites were crimped to enable insertion into biopsy tracts. Characterization revealed that the composites exhibited a slight delay in shape recovery compared to control SMP foams. However, the composites were still able to recover their shape in a matter of minutes. Cytocompatibility testing showed that leachable byproducts can be easily removed by washing and washed composites were not cytotoxic to mouse lung fibroblasts (L929s). Benchtop testing demonstrated that the composites can be easily deployed through a cannula, and the working time for deployment after exposure to water was 2 min. Furthermore, testing in an in vitro lung model demonstrated that the composites were able to effectively seal a lung biopsy tract and prevent air leakage. Collectively, these results show that the PEG hydrogel/SMP foam composites have the potential to be used as lung biopsy tract sealants to prevent pneumothorax post-lung biopsy.

Sustainable wheat gluten foams with self-expansion and water/blood-triggered shape recovery

A cheap and easily obtainable wheat gluten (WG) was used to fabricate bio-foams via a simple method of stirring, heating, and lyophilization. The foam possesses a 3D layered porous structure with interconnected channels, and the biofoam has excellent mechanical properties through glycerol plasticization and glutaraldehyde (GA) cross-linking. The water absorption and volume expansion rate can reach 793.67 ∼ 918.45% and 201.47 ∼ 239.53% respectively. In dry state, the foams had good compression resilience, and can basically recover its original shape after withstanding 60% compression strain for about 7 h. In wet state, they can withstand 10 cycles of compression test, and had good compressive resilience and durability; they also had fast liquid-triggered shape recovery performance, of which the foams can reabsorb liquid, expand, and recover its original shape within 40 seconds after withstanding 80% compression strain. In addition, The hemolysis rates of red blood cells treated with 1, 3, and 5 mg/mL of 14WG-20g-5GA foam suspension were 0.53 ± 0.12%, 2.12 ± 0.34%, and 3.97 ± 0.21%, respectively, all of which were below the permissible range for biological materials (<5%). The above-mentioned advantages made the sustainable foams be potentially useful for medical dressings, especially for the treatment of non-compressible haemorrhaging, which offered a new field of application for WG protein and its added value was also increased obviously.

A Hybrid Syntactic Foam-Based Open-Cell Foam with Reversible Actuation

Herein, we report the first hybrid open-cell foam with revisable actuation. Open-cell foams with revisable actuation are favoable for many applications. However, it is challenging to fabricate such open-cell foams with very low density. This study presents a novel concept of creating hybrid two-way (2W) shape memory open-cell foams using two-way shape-memory-polymer-based syntactic foams as the matrix. Previously, a syntactic foam prepared by incorporating hollow glass microbubbles in the cross-linked semicrystalline cis-poly(1,4-butadiene) (cPBD) was proved to have enhanced strength and specific energy output compared to the neat cPBD. Here, the same syntactic foam was used as the matrix, and the open-cell structure was produced by the salt-leaching method. The hybrid foam exhibits very attractive properties, including reversible actuation strain up to 50%, density as low as 0.07 g/cm³, energy output up to 0.23 J/g, tensile strength up to 0.84 MPa, elongation at break as high as 339%, high thermal stability with peak decomposition temperature at 450 °C, and Joule heating and strain sensing capabilities. The tensile strength and stiffness are shown to follow the well-known Gibson-Ashby model for porous materials. Combining the open-cell structure with the reversible actuation and other functionalities enables numerous potential applications for the prepared hybrid foam, including adjustable filters, insulators, sealers, and smart scaffolds.

Polylactic Acid Chemical Foaming Assisted by Solid-State Processing: Solid-State Shear Pulverization and Cryogenic Milling

A chemical foaming process of polylactic acid (PLA) was developed via the solid-state processing methods of solid-state shear pulverization (SSSP) and cryogenic milling. Based on the ability of solid-state processing to enhance the crystallization kinetics of PLA, chemical foaming agents (CFA) are first compounded before foaming via compression molding. Specifically, the effects of the pre-foaming solid-state processing method and CFA concentration were investigated. Density reduction, mechanical properties, thermal behavior, and cell density of PLA foams are characterized. Solid-state processing of PLA before foaming greatly increases the extent of PLA foaming by achieving void fractions approximately twice that of the control foams. PLA's improved ability to crystallize is displayed through both dynamic mechanical analysis and differential scanning calorimetry. The solid-state-processed foams display superior mechanical robustness and undergo low stress relaxation. The cell density of the PLA foams also increases with solid-state processing, especially through SSSP. Additionally, crosslinking of PLA during the pre-foaming processing step is found to result in the greatest enhancement of crystallization but decreased void fraction and foam effectiveness. Overall, SSSP and cryogenic milling show significant promise in improving chemical foaming in alternative biopolymers.

Shape memory polymer foams with tunable interconnectivity using off-the-shelf foaming components

The ability to easily and safety tune pore structures of gas-blown polyurethane shape memory polymer (SMP) foams could improve their outcomes as hemostatic dressings or tissue engineering scaffolds and enable overall commercialization efforts. Incorporating physical blowing agents into the polymer mix can be used to tune pore size and interconnectivity without altering foam chemistry. Enovate (HFC-254fa) is a commonly used physical blowing agent in gas-blown foams, but the Environmental Protection Agency (EPA) considers its use unacceptable because it is a hydrofluorocarbon that contributes to global warming. Here, off-the-shelf solvents accepted for use by the EPA, acetone, dimethyoxymethane (methylal), and methyl formate, were used as physical blowing agents by adding small volumes during foam fabrication. Increasing the physical blowing agent volume resulted in greater pore interconnectivity while maintaining SMP foam chemical and thermal properties. Pore size and interconnectivity also impacted cell and blood interactions with the foams. This work provides a safe and easy method for tuning SMP foam interconnectivity to aid in future commercialization efforts in a range of potential biomedical applications.

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.

Photopatterning Crystal Orientation in Shape-Morphing Polymers

Shape-morphing polymers have gained particular attention due to their unique capability of shape transformation under numerous external stimuli such as light, pH, and temperature. Their shape-morphing properties can be used in various applications such as robotics, artificial muscles, and biomedical devices. To take advantage of the stimuli-responsive properties of the smart polymers in such applications, programming shape change precisely through a facile synthetic procedure is essential. Programmable shape-morphing is readily obtained in hydrogels and liquid crystal polymer networks, but shape programming of semicrystalline polymers usually relies on low-resolution mechanical deformation. In this paper, a semicrystalline shape-morphing polymer with a controlled shape programmability was developed via photopatterning crystal orientation using a spatially controlled photopolymerization technique. The semicrystalline polymer network forms aligned crystallites at the boundaries between dark and bright regions during photopolymerization using a projector, which introduces an anisotropic stimulus response in the films. The semicrystalline polymer films with photoaligned crystallites expand 9-15% in the direction perpendicular to the patterned lines when heated above the melting temperature. Furthermore, spatially patterning the crystal orientation enables the formation of various complex 3D structures including a helical coil, a coil with a handedness inversion, a cone, a saddle, and a twisting flower. Finally, the magnitude of the shape transformation was controlled by varying the polymerization temperatures, and the actuation temperature was tuned by changing the amount of crystallinity in the polymer films. The simplicity and ease of control of our approach to program complex 3D structures from 2D semicrystalline polymer films make it a promising system for the aforementioned applications.

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.

Hemostatic materials in wound care

Blood plays an essential role in the human body. Hemorrhage is a critical cause of both military and civilian casualties. The human body has its own hemostatic mechanism that involves complex processes and has limited capacity. However, in emergency situations such as battlefields and hospitals, when the hemostatic mechanism of the human body itself cannot stop bleeding effectively, hemostatic materials are needed for saving lives. In this review, the hemostatic mechanisms and performance of the most commonly used hemostatic materials, (including fibrin, collagen, zeolite, gelatin, alginate, chitosan, cellulose and cyanoacrylate) and the commercial wound dressings based on these materials, will be discussed. These materials may have limitations, such as poor tissue adhesion, risk of infection and exothermic reactions, that may lessen their hemostatic efficacy and cause secondary injuries. High-performance hemostatic materials, therefore, have been designed and developed to improve hemostatic efficiency in clinical use. In this review, hemostatic materials with advanced performances, such as antibacterial capacity, superhydrophobicity/superhydrophilicity, superelasticity, high porosity and/or biomimicry, will be introduced. Future prospects of hemostatic materials will also be discussed in this review.

Wound Healing: From Passive to Smart Dressings

The universal increase in the number of patients with nonhealing skin wounds imposes a huge social and economic burden on the patients and healthcare systems. Although, the application of traditional wound dressings contributes to an effective wound healing outcome, yet, the complexity of the healing process remains a major health challenge. Recent advances in materials and fabrication technologies have led to the fabrication of dressings that provide proper conditions for effective wound healing. The 3D-printed wound dressings, biomolecule-loaded dressings, as well as smart and flexible bandages are among the recent alternatives that have been developed to accelerate wound healing. Additionally, the new generation of wound dressings contains a variety of microelectronic sensors for real-time monitoring of the wound environment and is able to apply required actions to support the healing progress. Moreover, advances in manufacturing flexible microelectronic sensors enable the development of the next generation of wound dressing substrates, known as electronic skin, for real-time monitoring of the whole physiochemical markers in the wound environment in a single platform. The current study reviews the importance of smart wound dressings as an emerging strategy for wound care management and highlights different types of smart dressings for promoting the wound healing process.

Smart scaffolds: shape memory polymers (SMPs) in tissue engineering

Smart scaffolds based on shape memory polymer (SMPs) have been increasingly studied in tissue engineering. The unique shape actuating ability of SMP scaffolds has been utilized to improve delivery and/or tissue defect filling. In this regard, these scaffolds may be self-deploying, self-expanding, or self-fitting. Smart scaffolds are generally thermoresponsive or hydroresponsive wherein shape recovery is driven by an increase in temperature or by hydration, respectively. Most smart scaffolds have been directed towards regenerating bone, cartilage, and cardiovascular tissues. A vast variety of smart scaffolds can be prepared with properties targeted for a specific tissue application. This breadth of smart scaffolds stems from the variety of compositions employed as well as the numerous methods used to fabricated scaffolds with the desired morphology. Smart scaffold compositions span across several distinct classes of SMPs, affording further tunability of properties using numerous approaches. Specifically, these SMPs include those based on physically cross-linked and chemically cross-linked networks and include widely studied shape memory polyurethanes (SMPUs). Various additives, ranging from nanoparticles to biologicals, have also been included to impart unique functionality to smart scaffolds. Thus, given their unique functionality and breadth of tunable properties, smart scaffolds have tremendous potential in tissue engineering.

Fabrication of Biomedical Scaffolds Using Biodegradable Polymers

Degradable polymers are used widely in tissue engineering and regenerative medicine. Maturing capabilities in additive manufacturing coupled with advances in orthogonal chemical functionalization methodologies have enabled a rapid evolution of defect-specific form factors and strategies for designing and creating bioactive scaffolds. However, these defect-specific scaffolds, especially when utilizing degradable polymers as the base material, present processing challenges that are distinct and unique from other classes of materials. The goal of this review is to provide a guide for the fabrication of biodegradable polymer-based scaffolds that includes the complete pathway starting from selecting materials, choosing the correct fabrication method, and considering the requirements for tissue specific applications of the scaffold.

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.

Blood-clotting model and simulation analysis of polyvinyl alcohol-chitosan composite hemostatic materials

The blood-clotting performance and characteristics of hemostatic materials are critical for their development and actual application. Based on the theory of porous media and characteristics of a non-Newtonian fluid, this study proposed an adsorption factor to characterize the porosity generation and blood coagulation process of hemostatic materials. On this basis, we constructed a physical model of blood coagulation in a porous medium integrated with the power-law fluid model to study the proposed poly(vinyl alcohol)-chitosan (PVA-CS) composite hemostatic material. Moreover, we simulated the dynamic blood flow process and blood coagulation process in the PVA-CS hemostatic material by introducing the physical model. The simulation results show that the blood begins to coagulate, which affects the porosity and permeability of the blood-containing area, resulting in changing the porosity after blood flowed into the hemostatic material. The porosity, permeability, and blood flow rate will approach zero until the generated blood coagulation entirely blocked the porous medium. Besides, simulation can provide the pressure and velocity distribution varying in the coagulation process of hemostatic materials. The temperature will also influence the hemostatic performance of the PVA-CS material. In all, the proposed simulation method enabled the coagulation mechanism of PVA-CS to be revealed from the perspective of blood flow in porous media combined with the adsorption factor.

Smart materials in cardiovascular implants: Shape memory alloys and shape memory polymers

Smart materials have intrinsic properties that change in a controlled fashion in response to external stimuli. Currently, the only smart materials with a significant clinical impact in cardiovascular implant design are shape memory alloys, particularly Nitinol. Recent prodigious progress in material science has resulted in the development of sophisticated shape memory polymers. In this article, we have reviewed the literature and outline the characteristics, advantages, and disadvantages of shape memory alloys and shape memory polymers which are relevant to clinical cardiovascular applications, and describe the potential of these smart materials for applications in coronary stents and transcatheter valves.