Characterization of the degradation mechanisms of lysine-derived aliphatic poly(ester urethane) scaffolds

An overview of polyurethane biomaterials and their use in drug delivery

Polyurethanes are a versatile and highly tunable class of materials that possess unique properties including high tensile strength, abrasion and fatigue resistance, and flexibility at low temperatures. The tunability of polyurethane properties has allowed this class of polymers to become ubiquitous in our daily lives in fields as diverse as apparel, appliances, construction, and the automotive industry. Additionally, polyurethanes with excellent biocompatibility and hemocompatibility can be synthesized, enabling their use as biomaterials in the medical field. The tunable nature of polyurethane biomaterials also makes them excellent candidates as drug delivery vehicles, which is the focus of this review. The fundamental idea we aim to highlight in this article is the structure-property-function relationships found in polyurethane systems. Specifically, the chemical structure of the polymer determines its macroscopic properties and dictates the functions for which it will perform well. By exploring the structure-property-function relationships for polyurethanes, we aim to elucidate the fundamental properties that can be tailored to achieve controlled drug release and empower researchers to design new polyurethane systems for future drug delivery applications.

Biodegradable Poly(ester) Urethane Acrylate Resins for Digital Light Processing: From Polymer Synthesis to 3D Printed Tissue Engineering Constructs

Digital light processing (DLP) is an accurate and fast additive manufacturing technique to produce a variety of products, from patient-customized biomedical implants to consumer goods. However, DLP's use in tissue engineering has been hampered due to a lack of biodegradable resin development. Herein, a library of biodegradable poly(esters) capped with urethane acrylate (with variations in molecular weight) is investigated as the basis for DLP printable resins for tissue engineering. The synthesized oligomers show good printability and are capable of creating complex structures with mechanical moduli close to those of medium-soft tissues (1-3 MPa). While fabricated films from different molecular weight resins show few differences in surface topology, wettability, and protein adsorption, the adhesion and metabolic activity of NCTC clone 929 (L929) cells and human dermal fibroblasts (HDFs) are significantly different. Resins from higher molecular weight oligomers provide greater cell adhesion and metabolic activity. Furthermore, these materials show compatibility in a subcutaneous in vivo pig model. These customizable, biodegradable, and biocompatible resins show the importance of molecular tuning and open up new possibilities for the creation of biocompatible constructs for tissue engineering.

INVESTIGATION OF THE ABILITY TO BIODEGRADABILITY OF POLYURETHANE FOAM COMPOSITE MATERIALS WITH ALBUCID AND THE DYNAMICS OF ALBUCID RELEASE IN VITRO

Studies of biodegradability of polyurethane foam (PUF) composite materials with albucid under the influence of biological medium 199 (BM 199) and saline solution for 2 weeks, 1, 3 and 6 months were conducted. IR spectroscopy, physical-mechanical tests, DSC and TGA before and after incubation in model mediums were investigated. It was found that the influence of BM 199 and saline solution on the structure and properties of composite materials with albucid is similar. According to the results of physical-mechanical studies under the influence of model mediums there are processes of biodegradation which are confirmed by a decrease in adhesive strength after incubation in BM 199 and saline solution. According to IR spectroscopy, biodegradation is accompanied by redistribution of hydrogen bonds of NH groups of the polymer matrix. The results of studies by the DSC method indicate a decrease of Tg and increase of ΔСР of PUF composites with albucid compared to the control, which is associated with increasing of segmental mobility of macromolecules under the influence of model mediums and due to the albucid release from polymer matrix. It was found that PUF and PUF composites with albucid in vitro remain heat-resistant materials, because after incubation in BM 199 and a saline solution there is an increase in T0 and Tmax by the TGA method. Studies of the dynamics of albucid release from the PUF matrix were carried out. It was found that the composite materials are capable to the prolonged release of the drug. The amount of released albucid is 36.0 % on the 60th day of the experiment, which does not exceed the therapeutic dose and has no toxic effects. Therefore, polyurethane foam composite materials with albucid can be proposed as promising materials for use as implants with prolonged action of albucid in ophthalmological surgery.

INVESTIGATION OF BIODEGRADATION AND PROPERTIES OF POLYURETHANE FOAM COMPOSITE MATERIALS WITH LYSOZYME IN VITRO

The study of biodegradation ability of polyurethane foams composite materials with lysozyme under the influence of saline solution for 2 weeks, 1, 3 and 6 months by IR spectroscopy, DSC, TGA was conducted. According to the results of IR spectroscopy under the influence of model medium there are processes of biodegradation, which are confirmed by a decrease in the intensity of the absorption band νС=О. Along with biodegradation there is a redistribution of hydrogen bonds of NH and CO groups of polymer matrix. According to DSC after incubation in saline solution there is an increase in Tg and ΔCp at the glass-transition (for polyurethane foams and composites with lysozyme in the amount of 5 wt. %), an increase in Tg and decrease in ΔCp (for composites with lysozyme in the amount of 1 and 3 wt. %), which indicates the redistribution of hydrogen bonds under the influence of saline solution and due to lysozyme release. It was found that after incubation in saline solution there is an increase in T0 and Tmax for both polyurethane foams and composite materials with lysozyme by the method of TGA. Thus, composites with lysozyme in vitro are heat-resistant materials. According to the study results of the dynamics of lysozyme release composites are capable to the prolonged release of enzyme for 5 days, the amount of which varies depending on the lysozyme content (43.85-61.97 % of the total amount of the introduced drug) and is sufficient for the manifestation of antimicrobial activity. The tissue culture method has established the biocompatibility of investigated materials. For polyurethane foam composite materials with lysozyme more active growth of fibroblastic elements than in the control and polyurethane foam and slowing down the process of cell degeneration was observed. The obtained results indicate that polyurethane foam composite materials with lysozyme are promising materials that due to the presence of the enzyme will have antimicrobial action and can be used in medical practice as polymer composites for the treatment of wounds and burns.

Reactive oxygen species-degradable polythioketal urethane foam dressings to promote porcine skin wound repair

Porous, resorbable biomaterials can serve as temporary scaffolds that support cell infiltration, tissue formation, and remodeling of nonhealing skin wounds. Synthetic biomaterials are less expensive to manufacture than biologic dressings and can achieve a broader range of physiochemical properties, but opportunities remain to tailor these materials for ideal host immune and regenerative responses. Polyesters are a well-established class of synthetic biomaterials; however, acidic degradation products released by their hydrolysis can cause poorly controlled autocatalytic degradation. Here, we systemically explored reactive oxygen species (ROS)-degradable polythioketal (PTK) urethane (UR) foams with varied hydrophilicity for skin wound healing. The most hydrophilic PTK-UR variant, with seven ethylene glycol (EG7) repeats flanking each side of a thioketal bond, exhibited the highest ROS reactivity and promoted optimal tissue infiltration, extracellular matrix (ECM) deposition, and reepithelialization in porcine skin wounds. EG7 induced lower foreign body response, greater recruitment of regenerative immune cell populations, and resolution of type 1 inflammation compared to more hydrophobic PTK-UR scaffolds. Porcine wounds treated with EG7 PTK-UR foams had greater ECM production, vascularization, and resolution of proinflammatory immune cells compared to polyester UR foam-based NovoSorb Biodegradable Temporizing Matrix (BTM)-treated wounds and greater early vascular perfusion and similar wound resurfacing relative to clinical gold standard Integra Bilayer Wound Matrix (BWM). In a porcine ischemic flap excisional wound model, EG7 PTK-UR treatment led to higher wound healing scores driven by lower inflammation and higher reepithelialization compared to NovoSorb BTM. PTK-UR foams warrant further investigation as synthetic biomaterials for wound healing applications.

Bioinspired Super-Strong Aqueous Synthetic Tissue Adhesives

Existing tissue adhesives and sealants are far from satisfactory when applied on wet and dynamic tissues. Herein, we report a strategy for designing biodegradable super-strong aqueous glue (B-Seal) for surgical uses inspired by an English ivy adhesion strategy and a cement particle packing theory. B-Seal is a fast-gelling, super-strong, and elastic adhesive sealant composed of injectable water-borne biodegradable polyurethane (WPU) nanodispersions with mismatched particle sizes and counterions in its A-B formulation. B-Seal showed 24-fold greater burst pressure than DuraSeal®, 138-fold greater T-pull adhesive strength than fibrin glue, and 16-fold greater lap shear strength than fibrin glue. In vivo evaluation on a rat cerebrospinal fluid (CSF) rhinorrhea model and a porcine craniotomy model validated the safety and efficacy of B-Seal for effective CSF leak prevention and dura repair. The plant-inspired adhesion strategy combined with particle packing theory represents a new direction of designing the next-generation wet tissue adhesives for surgeries.

Preliminary in vivo study of biodegradables PLA-PEU-PLA anti-adhesion membranes in a rat Achilles tendon model of peritendinous adhesions

Peritendinous adhesions are complications known to occur up to 6 weeks after surgery and cause chronic pain and disability. Anti-adhesion barriers are currently the best option for prevention. In a...

Effects of nanocrystalline hydroxyapatite concentration and skeletal site on bone and cartilage formation in rats

Most fractures heal by a combination of endochondral and intramembranous ossification dependent upon strain and vascularity at the fracture site. Many biomaterials-based bone regeneration strategies rely on the use of calcium phosphates such as nano-crystalline hydroxyapatite (nHA) to create bone-like scaffolds. In this study, nHA was dispersed in reactive polymers to form composite scaffolds that were evaluated both in vitro and in vivo. Matrix assays, immunofluorescent staining, and Western blots demonstrated that nHA influenced mineralization and subsequent osteogenesis in a dose-dependent manner in vitro. Furthermore, nHA dispersed in polymeric composites promoted osteogenesis by a similar mechanism as particulated nHA. Scaffolds were implanted into a 2-mm defect in the femoral diaphysis or metaphysis of Sprague-Dawley rats to evaluate new bone formation at 4 and 8 weeks. Two formulations were tested: a poly(thioketal urethane) scaffold without nHA (PTKUR) and a PTKUR scaffold augmented with 22 wt% nHA (22nHA). The scaffolds supported new bone formation in both anatomic sites. In the metaphysis, augmentation of scaffolds with nHA promoted an intramembranous healing response. Within the diaphysis, nHA inhibited endochondral ossification. Immunohistochemistry was performed on cryo-sections of the bone/scaffold interface in which CD146, CD31, Endomucin, CD68, and Myeloperoxidase were evaluated. No significant differences in the infiltrating cell populations were observed. These findings suggest that nHA dispersed in polymeric composites induces osteogenic differentiation of adherent endogenous cells, which has skeletal site-specific effects on fracture healing. STATEMENT OF SIGNIFICANCE: Understanding the mechanism by which synthetic scaffolds promote new bone formation in preclinical models is crucial for bone regeneration applications in the clinic where complex fracture cases are seen. In this study, we found that dispersion of nHA in polymeric scaffolds promoted in vitro osteogenesis in a dose-dependent manner through activation of the PiT1 receptor and subsequent downstream Erk1/2 signaling. While augmentation of polymeric scaffolds with nHA enhanced intramembranous ossification in metaphyseal defects, it inhibited endochondral ossification in diaphyseal defects. Thus, our findings provide new insights into designing synthetic bone grafts that complement the skeletal site-specific fracture healing response.

Settable Polymeric Autograft Extenders in a Rabbit Radius Model of Bone Formation

Autograft (AG) is the gold standard for bone grafts, but limited quantities and patient morbidity are associated with its use. AG extenders have been proposed to minimize the volume of AG while maintaining the osteoinductive properties of the implant. In this study, poly(ester urethane) (PEUR) and poly(thioketal urethane) (PTKUR) AG extenders were implanted in a 20-mm rabbit radius defect model to evaluate new bone formation and graft remodeling. Outcomes including µCT and histomorphometry were measured at 12 weeks and compared to an AG (no polymer) control. AG control examples exhibited new bone formation, but inconsistent healing was observed. The implanted AG control was resorbed by 12 weeks, while AG extenders maintained implanted AG throughout the study. Bone growth from the defect interfaces was observed in both AG extenders, but residual polymer inhibited cellular infiltration and subsequent bone formation within the center of the implant. PEUR-AG extenders degraded more rapidly than PTKUR-AG extenders. These observations demonstrated that AG extenders supported new bone formation and that polymer composition did not have an effect on overall bone formation. Furthermore, the results indicated that early cellular infiltration is necessary for harnessing the osteoinductive capabilities of AG.

Degradation resistance of PVDF mesh in vivo in comparison to PP mesh

Mesh implant has been applied in hernia repair and urogynecological reconstruction. Polypropylene (PP) is now the most widely used material for non-resorbable mesh implants. A degradation phenomenon of PP mesh, which is apparent on the mesh surface as cracking, flaking and peeling, was discovered in the 1990's. This phenomenon of mesh implant has drawn attention because of mesh-related litigations. Polyvinylidene fluoride (PVDF), due to its high biocompatible performance, has been used since 2003 as an alternative material for non-resorbable mesh implants. Till now, no such degradation phenomenon of PVDF mesh has been reported, although limited study on PVDF mesh is available. In this paper, we researched the degradation of PVDF meshes taking the degradation of PP mesh as a reference. The meshes analysed in this study were received from a previous animal experiment. To expose the surface of explanted meshes, a tissue removing method with protease was used and the result of this cleaning process was tested by X-ray Photoelectron Spectroscopy (XPS). The morphological condition of the mesh surface was compared using Scanning Electron Microscopy (SEM) and the chemical condition concerning degradation was analysed through Fourier Transform Infrared Spectroscopy (FTIR). The surface condition of PVDF mesh after 3-, 6-, 12- and 24-month implantation was illustrated and compared with two types of PP meshes. XPS revealed an absence of nitrogen, confirming the successful removal of tissue residues using protease. SEM results presented no notable morphological surface change of the PVDF mesh and progressive surface cracking processes over time of both types of PP meshes. FTIR spectra of the implanted PVDF meshes had no considerable difference from the spectrum of the pristine mesh, while FTIR spectra of both types of PP meshes had extra chemical functional groups (carbonyl (CO) and hydroxyl (-OH) groups) increasing with implantation time, indicating progressive degradation. This study highlights the morphological and chemical stability of the PVDF mesh and demonstrates that the PVDF mesh is more resistant to degradation in comparison to the other two types of PP meshes.

Design Challenges in Polymeric Scaffolds for Tissue Engineering

Numerous surgical procedures are daily performed worldwide to replace and repair damaged tissue. Tissue engineering is the field devoted to the regeneration of damaged tissue through the incorporation of cells in biocompatible and biodegradable porous constructs, known as scaffolds. The scaffolds act as host biomaterials of the incubating cells, guiding their attachment, growth, differentiation, proliferation, phenotype, and migration for the development of new tissue. Furthermore, cellular behavior and fate are bound to the biodegradation of the scaffold during tissue generation. This article provides a critical appraisal of how key biomaterial scaffold parameters, such as structure architecture, biochemistry, mechanical behavior, and biodegradability, impart the needed morphological, structural, and biochemical cues for eliciting cell behavior in various tissue engineering applications. Particular emphasis is given on specific scaffold attributes pertaining to skin and brain tissue generation, where further progress is needed (skin) or the research is at a relatively primitive stage (brain), and the enumeration of some of the most important challenges regarding scaffold constructs for tissue engineering.

Recent Developments in Polyurethane-Based Materials for Bone Tissue Engineering

To meet the needs of clinical medicine, bone tissue engineering is developing dynamically. Scaffolds for bone healing might be used as solid, preformed scaffolding materials, or through the injection of a solidifiable precursor into the defective tissue. There are miscellaneous biomaterials used to stimulate bone repair including ceramics, metals, naturally derived polymers, synthetic polymers, and other biocompatible substances. Combining ceramics and metals or polymers holds promise for future cures as the materials complement each other. Further research must explain the limitations of the size of the defects of each scaffold, and additionally, check the possibility of regeneration after implantation and resistance to disease. Before tissue engineering, a lot of bone defects were treated with autogenous bone grafts. Biodegradable polymers are widely applied as porous scaffolds in bone tissue engineering. The most valuable features of biodegradable polyurethanes are good biocompatibility, bioactivity, bioconductivity, and injectability. They may also be used as temporary extracellular matrix (ECM) in bone tissue healing and regeneration. Herein, the current state concerning polyurethanes in bone tissue engineering are discussed and introduced, as well as future trends.

Nanocrystalline hydroxyapatite-poly(thioketal urethane) nanocomposites stimulate a combined intramembranous and endochondral ossification response in rabbits

Resorbable bone cements are replaced by bone osteoclastic resorption and osteoblastic new bone formation near the periphery. However, the ideal bone cement would be replaced by new bone through processes similar to fracture repair, which occurs through a variable combination of endochondral and intramembranous ossification. In this study, nanocrystalline hydroxyapatite (nHA)-poly(thioketal urethane) (PTKUR) cements were implanted in femoral defects in New Zealand White rabbits to evaluate ossification at 4, 12, and 18 months. Four formulations were tested: an injectable, flowable cement and three moldable putties with varying ratios of calcium phosphate to sucrose granules. New bone formation and resorption of the cement by osteoclasts occurred near the periphery. Stevenel's Blue and Safranin O staining revealed infiltration of chondrocytes into the cements and ossification of the cartilaginous intermediate. These findings suggest that nHA-PTKUR cements support combined intramembranous and endochondral ossification, resulting in enhanced osseointegration of the cement that could potentially improve patient outcomes.

Synthesis and characterization of biodegradable linear shape memory polyurethanes with high mechanical performance by incorporating novel long chain diisocyanates

Novel long chain diisocyanates were developed for synthesis of biodegradable linear shape memory polyurethanes demonstrating high mechanical performance.

Preliminary design of a new degradable medical device to prevent the formation and recurrence of intrauterine adhesions

Intrauterine adhesions lead to partial or complete obliteration of the uterine cavity and have life-changing consequences for women. The leading cause of adhesions is believed to be loss of stroma resulting from trauma to the endometrium after surgery. Adhesions are formed when lost stroma is replaced by fibrous tissue that join the uterine walls. Few effective intrauterine anti-adhesion barriers for gynecological surgery exist. We designed a degradable anti-adhesion medical device prototype to prevent adhesion formation and recurrence and restore uterine morphology. We focused on ideal degradation time for complete uterine re-epithelialization for optimal anti-adhesion effect and clinical usability. We developed a triblock copolymer prototype [poly(lactide) combined with high molecular mass poly(ethylene oxide)]. Comparative pre-clinical studies demonstrated in vivo anti-adhesion efficacy. Ease of introduction and optimal deployment in a human uterus confirmed clinical usability. This article provides preliminary data to develop an intrauterine medical device and conduct a clinical trial.

Poly(Thioketal Urethane) Autograft Extenders in an Intertransverse Process Model of Bone Formation

IMPACT STATEMENT

The development of autograft extenders is a significant clinical need in bone tissue engineering. We report new settable poly(thioketal urethane)-based autograft extenders that have bone-like mechanical properties and handling properties comparable to calcium phosphate bone cements. These settable autograft extenders remodeled to form new bone in a biologically stringent intertransverse process model of bone formation that does not heal when treated with calcium phosphate bone void fillers or cements alone. This is the first study to report settable autograft extenders with bone-like strength and handling properties comparable to ceramic bone cements, which have the potential to improve treatment of bone fractures and other orthopedic conditions.

Settable polymer/ceramic composite bone grafts stabilize weight-bearing tibial plateau slot defects and integrate with host bone in an ovine model

Bone fractures at weight-bearing sites are challenging to treat due to the difficulty in maintaining articular congruency. An ideal biomaterial for fracture repair near articulating joints sets rapidly after implantation, stabilizes the fracture with minimal rigid implants, stimulates new bone formation, and remodels at a rate that maintains osseous integrity. Consequently, the design of biomaterials that mechanically stabilize fractures while remodeling to form new bone is an unmet challenge in bone tissue engineering. In this study, we investigated remodeling of resorbable bone cements in a stringent model of mechanically loaded tibial plateau defects in sheep. Nanocrystalline hydroxyapatite-poly(ester urethane) (nHA-PEUR) hybrid polymers were augmented with either ceramic granules (85% β-tricalcium phosphate/15% hydroxyapatite, CG) or a blend of CG and bioactive glass (BG) particles to form a settable bone cement. The initial compressive strength and fatigue properties of the cements were comparable to those of non-resorbable poly(methyl methacrylate) bone cement. In animals that tolerated the initial few weeks of early weight-bearing, CG/nHA-PEUR cements mechanically stabilized the tibial plateau defects and remodeled to form new bone at 16 weeks. In contrast, cements incorporating BG particles resorbed with fibrous tissue filling the defect. Furthermore, CG/nHA-PEUR cements remodeled significantly faster at the full weight-bearing tibial plateau site compared to the mechanically protected femoral condyle site in the same animal. These findings are the first to report a settable bone cement that remodels to form new bone while providing mechanical stability in a stringent large animal model of weight-bearing bone defects near an articulating joint.

Injectable, compression-resistant polymer/ceramic composite bone grafts promote lateral ridge augmentation without protective mesh in a canine model

OBJECTIVE

The objective of this study was to test the hypothesis that a compression-resistant bone graft augmented with recombinant human morphogenetic protein-2 (rhBMP-2) will promote lateral ridge augmentation without the use of protective mesh in a canine model.

MATERIALS & METHODS

Compression-resistant (CR) bone grafts were evaluated in a canine model of lateral ridge augmentation. Bilateral, right trapezoidal prism-shaped defects (13-14 mm long × 8-9 mm wide × 3-4 mm deep at the base) in 13 hounds (two defects per hound) were treated with one of four groups: (i) absorbable collagen sponge + 400 μg rhBMP-2/ml (ACS, clinical control) protected by titanium mesh, (ii) CR without rhBMP-2 (CR, negative control), (iii) CR + 200 μg rhBMP-2 (CR-L), or (iv) CR + 400 μg rhBMP-2 (CR-H). All animals were euthanized after 16 weeks. Ridge height and width and new bone formation were assessed by μCT, histology, and histomorphometry. The release kinetics of rhBMP-2 from CR bone grafts in vitro and in vivo in a femoral condyle defect model in rabbits was also evaluated.

RESULTS

All four bone grafts promoted new bone formation (11-31.6 volume%) in the lateral ridge defects. For CR grafts, ridge height and width increased in a dose-responsive manner with increasing rhBMP-2 concentration. Ridge height and width measured for CR-H without the use of protective mesh was comparable to that measured for ACS with a protective mesh.

CONCLUSIONS

At the same dose of rhBMP-2, an injectable, compression-resistant bone graft resulted in a comparable volume of new bone formation with the clinical control (ACS). These findings highlight the potential of compression-resistant bone grafts without the use of protective mesh for lateral ridge augmentation.

Hydrolytic Degradation and Mechanical Stability of Poly(ε-Caprolactone)/Reduced Graphene Oxide Membranes as Scaffolds for In Vitro Neural Tissue Regeneration

The present work studies the functional behavior of novel poly(ε-caprolactone) (PCL) membranes functionalized with reduced graphene oxide (rGO) nanoplatelets under simulated in vitro culture conditions (phosphate buffer solution (PBS) at 37 °C) during 1 year, in order to elucidate their applicability as scaffolds for in vitro neural regeneration. The morphological, chemical, and DSC results demonstrated that high internal porosity of the membranes facilitated water permeation and procured an accelerated hydrolytic degradation throughout the bulk pathway. Therefore, similar molecular weight reduction, from 80 kDa to 33 kDa for the control PCL, and to 27 kDa for PCL/rGO membranes, at the end of the study, was observed. After 1 year of hydrolytic degradation, though monomers coming from the hydrolytic cleavage of PCL diffused towards the PBS medium, the pH was barely affected, and the rGO nanoplatelets mainly remained in the membranes which envisaged low cytotoxic effect. On the other hand, the presence of rGO nanomaterials accelerated the loss of mechanical stability of the membranes. However, it is envisioned that the gradual degradation of the PCL/rGO membranes could facilitate cells infiltration, interconnectivity, and tissue formation.

Porcine Ischemic Wound-Healing Model for Preclinical Testing of Degradable Biomaterials

Impaired wound healing that mimics chronic human skin pathologies is difficult to achieve in current animal models, hindering testing and development of new therapeutic biomaterials that promote wound healing. In this article, we describe a refinement and simplification of the porcine ischemic wound model that increases the size and number of experimental sites per animal. By comparing three flap geometries, we adopted a superior configuration (15 × 10 cm) that enabled testing of twenty 1 cm² wounds in each animal: 8 total ischemic wounds within 4 bipedicle flaps and 12 nonischemic wounds. The ischemic wounds exhibited impaired skin perfusion for ∼1 week. To demonstrate the utility of the model for comparative testing of tissue regenerative biomaterials, we evaluated the healing process in wounds implanted with highly porous poly (thioketal) urethane (PTK-UR) scaffolds that were fabricated through reaction of reactive oxygen species (ROS)-cleavable PTK macrodiols with isocyanates. PTK-lysine triisocyanate (LTI) scaffolds degraded significantly in vitro under both oxidative and hydrolytic conditions whereas PTK-hexamethylene diisocyanate trimer (HDIt) scaffolds were resistant to hydrolytic breakdown and degraded exclusively through an ROS-dependent mechanism. Upon placement into porcine wounds, both types of PTK-UR materials fostered new tissue ingrowth over 10 days in both ischemic and nonischemic tissue. However, wound perfusion, tissue infiltration and the abundance of pro-regenerative, M2-polarized macrophages were markedly lower in ischemic wounds independent of scaffold type. The PTK-LTI implants significantly improved tissue infiltration and perfusion compared with analogous PTK-HDIt scaffolds in ischemic wounds. Both LTI and HDIt-based PTK-UR implants enhanced M2 macrophage activity, and these cells were selectively localized at the scaffold/tissue interface. In sum, this modified porcine wound-healing model decreased animal usage, simplified procedures, and permitted a more robust evaluation of tissue engineering materials in preclinical wound healing research. Deployment of the model for a relevant biomaterial comparison yielded results that support the use of the PTK-LTI over the PTK-HDIt scaffold formulation for future advanced therapeutic studies.

Composite elastomeric polyurethane scaffolds incorporating small intestinal submucosa for soft tissue engineering

Although soft tissue replacement has been clinically successful in many cases, the corresponding procedure has many limitations including the lack of resilience and mechanical integrity, significant donor-site morbidity, volume loss with time, and fibrous capsular contracture. These disadvantages can be alleviated by utilizing bio-absorbable scaffolds with high resilience and large strain, which are capable of stimulating natural tissue regeneration. Hence, the chemically crosslinked tridimensional scaffolds obtained by incorporating water-based polyurethane (PU) (which was synthesized from polytetramethylene ether glycol, isophorone diisocyanate, and 2,2-bis(hydroxymethyl) butyric acid) into a bioactive extracellular matrix consisting of small intestinal submucosa (SIS) have been tested in this study to develop a new approach for soft tissue engineering. After characterizing the structure and properties of the produced PU/SIS composites, the strength, Young's modulus, and resilience of wet PU/SIS samples were compared with those of crosslinked PU. In addition, the fabricated specimens were investigated using human umbilical vein endothelial cells to evaluate their ability to enhance cell attachment and proliferation. As a result, the synthesized PU/SIS samples exhibited high resilience and were capable of enhancing cell viability with no evidence of cytotoxicity. Subcutaneous implantation in animals and the subsequent testing conducted after 2, 4, and 8weeks indicated that sound implant integration and vascularization occurred inside the PU/SIS composites, while the presence of SIS promoted cell infiltration, angiogenesis, and ultimately tissue regeneration. The obtained results revealed that the produced PU/SIS composites were characterized by high bioactivity and resilience, and, therefore, could be used for soft tissue engineering applications.

STATEMENT OF SIGNIFICANCE

Hybrid composites containing synthetic polymers with high mechanical strength and naturally derived components, which create a bio-mimetic environment, are one of the most promising biomaterials. Although synthetic polymer/ECM composites have been previously used for soft tissue repair, their resilience properties were not investigated in sufficient detail, while the development of elastic composites composed of synthetic polymers and ECMs in nontoxic aqueous solutions remains a rather challenging task. In this study, porous PU/SIS composites were fabricated in a non-toxic manner; the obtained materials exhibited sufficient mechanical support, which promote cell growth, angiogenesis, and tissue regeneration. The described method can be adapted for the development of scaffolds with various acellular matrices and subsequently used during the restoration of particular types of tissue.

Polyimide Aerogels Using Triisocyanate as Cross-linker

A family of polyimide (PI)-based aerogels is produced using Desmodur N3300A, an inexpensive triisocyanate, as the cross-linker. The aerogels are prepared by cross-linking amine end-capped polyimide oligomers with the triisocyanate. The polyimide oligomers are formulated using 2,2'-dimethylbenzidine, 4,4'-oxydianiline, or mixtures of both diamines, combined with 3,3',4,4'-biphenyltetracarboxylic dianhydride, and are chemically imidized at room temperature. Depending on the backbone chemistry, chain length, and polymer concentration, density of the aerogels ranged from 0.06 to 0.14 g/cm³ and Brunauer-Emmett-Teller surface areas ranged from 350 to 600 m²/g. Compressive moduli of these aerogels were as high as 225 MPa, which are comparable to, or higher than, those previously reported prepared with similar backbone structures but with other cross-linkers. Because of their lower cost and commercial availability as cross-linker, the aerogels may have further potential as insulation for building and construction, clothing, sporting goods, and automotive applications, although lower-temperature stability may limit their use in some aerospace applications.

Resorbable Nanocomposites with Bone-Like Strength and Enhanced Cellular Activity

Bone cements for treatment of fractures at weight-bearing sites are subjected to dynamic physiological loading from daily activities. An ideal bone cement rapidly sets after injection, exhibits bone-like strength, stimulates osteogenic differentiation of endogenous cells, and resorbs at a rate aligned with patient biology. However, currently available materials fall short of these targeted properties. Nanocrystalline hydroxyapatite (nHA) enhances osteogenic differentiation, new bone formation, and osteoclast differentiation activity compared to amorphous or micron-scale crystalline hydroxyapatite. However, the brittle mechanical properties of nHA precludes its use in treatment of weight-bearing bone defects. In this study, we report settable nHA-poly(ester urethane) (PEUR) nanocomposites synthesized from nHA, lysine triisocyanate (LTI), and poly(caprolactone) triol via a solvent-free process. The nanocomposites are easily mixed and injected using a double-barrel syringe, exhibit mechanical properties exceeding those of conventional bone cements, enhance mineralization of osteoprogenitor cells in vitro, and undergo osteoclast-mediated degradation in vitro. This combination of properties cannot be achieved using other technologies, which underscores the potential of nHA-PEUR nanocomposites as a new approach for promoting bone healing at weight-bearing sites.

In situ foamable, degradable polyurethane as biomaterial for soft tissue repair

Degradable foams which can be inserted endoscopically as liquid or pasty mixtures into soft tissue defects possess a promising potential for the surgical treatment of such defects. The defects can be sealed under in situ foaming and simultaneous material expansion. We developed an in situ foamable (l-lactide-co-ε-caprolactone)-based, star-shaped prepolymer by ring opening polymerization of l-lactide and ε-caprolactone in the presence of meso-erythritol as starter. By conversion of the terminal hydroxyl groups of the formed oligoester with lysine diisocyanate ethyl ester (LDI) an isocyanate-endcapped, reactive prepolymer has been received. Foaming can be initiated by addition of 1,4-diazabicyclo[2,2,2]octane (DABCO), water, LDI and DMSO. By varying the composition of these additives, the foaming and curing time could be varied within a clinically acceptable range. A porosity of approximately 90%, and an average tensile strength of 0.3MPa with elongations of 90% were determined for the foams. In vitro cytotoxicity on cured foams was assayed on 3T3 fibroblasts and demonstrated an excellent cytocompatibility. This was also confirmed in an in vivo study using an established rat model, where prefabricated foams and in situ hardening material were inserted into subdermal skin incisions in parallel. The feature of chronic inflammation was only weakly developed in both groups and slightly more pronounced and persisted for longer time in the group of in situ foamed material. In both groups the foreign materials were vascularized, degraded and substituted by connective tissue. The results encourage to proceed with trials where the materials are used to fill more heavily loaded defects.

Thermally induced shape memory behavior, enzymatic degradation and biocompatibility of PLA/TPU blends: "Effects of compatibilization"

Poly(lactic acid) (PLA)/thermoplastic polyurethane (TPU) blends were melt-mixed and compatibilized to investigate their biocompatibility, biodegradability and thermally induced shape memory properties. The blend compositions were PLA/TPU: 80/20 (20TPU) and PLA/TPU: 50/50 (50TPU). 1,4-phenylene diisocyanate (PDI) was used in order to compatibilize the components reactively. The PDI composition was 0.5, 1, 3% by weight. Biodegradability was assessed by enzymatic degradation tests. Biocompatibility was investigated through in-vitro cell-culture experiments. Shape memory tests exhibited that 20TPU blends have higher recovery ratio than that of 50TPU blends. It was observed that the shape recovery ratio was enhanced by the addition of PDI. The highest shape recovery ratio was obtained at 3%PDI in 20TPU blends. Enzymatic biodegradability tests showed that the increasing TPU content decreased the biodegradability of the blends. It was found that compatibilization slowed down the enzymatic degradation of PLA/TPU blends. In-vitro cell-culture experiments indicated that all blends were biocompatible, and no evidence of cytotoxicity was observed.

Oxidation and degradation of polypropylene transvaginal mesh

Polypropylene (PP) transvaginal mesh (TVM) repair for stress urinary incontinence (SUI) has shown promising short-term objective cure rates. However, life-altering complications have been associated with the placement of PP mesh for SUI repair. PP degradation as a result of the foreign body reaction (FBR) has been proposed as a contributing factor to mesh complications. We hypothesized that PP oxidizes under in vitro conditions simulating the FBR, resulting in degradation of the PP. Three PP mid-urethral slings from two commercial manufacturers were evaluated. Test specimens (n = 6) were incubated in oxidative medium for up to 5 weeks. Oxidation was assessed by Fourier Transform Infrared Spectroscopy (FTIR), and degradation was evaluated by scanning electron microscopy (SEM). FTIR spectra of the slings revealed evidence of carbonyl and hydroxyl peaks after 5 weeks of incubation time, providing evidence of oxidation of PP. SEM images at 5 weeks showed evidence of surface degradation, including pitting and flaking. Thus, oxidation and degradation of PP pelvic mesh were evidenced by chemical and physical changes under simulated in vivo conditions. To assess changes in PP surface chemistry in vivo, fibers were recovered from PP mesh explanted from a single patient without formalin fixation, untreated (n = 5) or scraped (n = 5) to remove tissue, and analyzed by X-ray photoelectron spectroscopy. Mechanical scraping removed adherent tissue, revealing an underlying layer of oxidized PP. These findings underscore the need for further research into the relative contribution of oxidative degradation to complications associated with PP-based TVM devices in larger cohorts of patients.

Precise control of drug loading and release of an NSAID–polymer conjugate for long term osteoarthritis intra-articular drug delivery

The synthesis of biocompatible polymer diclofenac conjugates (PDCs) that have a high drug loading and high degree of control over diclofenac (DCF) release is described.

Biodegradable polymer scaffolds

Tissue engineering aims to repair the damaged tissue by transplantation of cells or introducing bioactive factors in a biocompatible scaffold. In recent years, biodegradable polymer scaffolds mimicking the extracellular matrix have been developed to promote the cell proliferation and extracellular matrix deposition. The biodegradable polymer scaffolds thus act as templates for tissue repair and regeneration. This article reviews the updated information regarding various types of natural and synthetic biodegradable polymers as well as their functions, physico-chemical properties, and degradation mechanisms in the development of biodegradable scaffolds for tissue engineering applications, including their combination with 3D printing.

Oxidatively Degradable Poly(thioketal urethane)/Ceramic Composite Bone Cements with Bone-Like Strength

Synthetic bone cements are commonly used in orthopaedic procedures to aid in bone regeneration following trauma or disease. Polymeric cements like PMMA provide the mechanical strength necessary for orthopaedic applications, but they are not resorbable and do not integrate with host bone. Ceramic cements have a chemical composition similar to that of bone, but their brittle mechanical properties limit their use in weight-bearing applications. In this study, we designed oxidatively degradable, polymeric bone cements with mechanical properties suitable for bone tissue engineering applications. We synthesized a novel thioketal (TK) diol, which was crosslinked with a lysine triisocyanate (LTI) prepolymer to create hydrolytically stable poly(thioketal urethane)s (PTKUR) that degrade in the oxidative environment associated with bone defects. PTKUR films were hydrolytically stable for up to 6 months, but degraded rapidly (<1 week) under simulated oxidative conditions in vitro. When combined with ceramic micro- or nanoparticles, PTKUR cements exhibited working times comparable to calcium phosphate cements and strengths exceeding those of trabecular bone. PTKUR/ceramic composite cements supported appositional bone growth and integrated with host bone near the bone-cement interface at 6 and 12 weeks post-implantation in rabbit femoral condyle plug defects. Histological evidence of osteoclast-mediated resorption of the cements was observed at 6 and 12 weeks. These findings demonstrate that a PTKUR bone cement with bone-like strength can be selectively resorbed by cells involved in bone remodeling, and thus represent an important initial step toward the development of resorbable bone cements for weight-bearing applications.

Local Delivery of PHD2 siRNA from ROS-Degradable Scaffolds to Promote Diabetic Wound Healing

Small interfering RNA (siRNA) delivered from reactive oxygen species-degradable tissue engineering scaffolds promotes diabetic wound healing in rats. Porous poly(thioketal-urethane) scaffolds implanted in diabetic wounds locally deliver siRNA that inhibits the expression of prolyl hydroxylase domain protein 2, thereby increasing the expression of progrowth genes and increasing vasculature, proliferating cells, and tissue development in diabetic wounds.

Rapidly Biodegrading PLGA-Polyurethane Fibers for Sustained Release of Physicochemically Diverse Drugs

Sustained release of physicochemically diverse drugs from electrospun fibers remains a challenge and precludes the use of fibers in many medical applications. Here, we synthesize a new class of polyurethanes with poly(lactic-co-glycolic acid) (PLGA) moieties that degrade faster than polyurethanes based on polycaprolactone. The new polymers, with varying hard to soft segment ratios and fluorobenzene pendant group content, were electrospun into nanofibers and loaded with four physicochemically diverse small molecule drugs. Polymers were characterized using GPC, XPS, and 19F NMR. The size and morphology of electrospun fibers were visualized using SEM, and drug/polymer compatibility and drug crystallinity were evaluated using DSC. We measured in vitro drug release, polymer degradation and cell-culture cytotoxicity of biodegradation products. We show that these newly synthesized PLGA-based polyurethanes degrade up to 65-80% within 4 weeks and are cytocompatible in vitro. The drug-loaded electrospun fibers were amorphous solid dispersions. We found that increasing the hard to soft segment ratio of the polymer enhances the sustained release of positively charged drugs, whereas increasing the fluorobenzene pendant content caused more rapid release of some drugs. In summary, increasing the hard segment or fluorobenzene pendant content of segmented polyurethanes containing PLGA moieties allows for modulation of physicochemically diverse drug release from electrospun fibers while maintaining a biologically relevant biodegradation rate.

Remodeling of injectable, low-viscosity polymer/ceramic bone grafts in a sheep femoral defect model

Ceramic/polymer composite bone grafts offer the potential advantage of combining the osteoconductivity of ceramic component with the ductility of polymeric component, resulting in a graft that meets many of the desired properties for bone void fillers (BVF). However, the relative contributions of the polymer and ceramic components to bone healing are not well understood. In this study, we compared remodeling of low-viscosity (LV) ceramic/poly(ester urethane) composites to a ceramic BVF control in a sheep femoral condyle plug defect model. LV composites incorporating either ceramic (LV/CM) or allograft bone (LV/A) particles were evaluated. We hypothesized that LV/CM composites which have the advantageous handling properties of injectability, flowability, and settability would heal comparably to the CM control, which was evaluated for up to 2 years to study its long-term degradation properties. Remodeling of LV/CM was comparable to that observed for the CM control, as evidenced by new bone formation on the surface of the ceramic particles. At early time points (4 months), LV/CM composites healed similar to the ceramic clinical control, while LV/A components showed more variable healing due to osteoclast-mediated resorption of the allograft particles. At longer time points (12-15 months), healing of LV/CM composites was more variable due to the nonhomogeneous distribution and lower concentration of the ceramic particles compared to the ceramic clinical control. Resorption of the ceramic particles was almost complete at 2 years. This study highlights the importance of optimizing the loading and distribution of ceramic particles in polymer/ceramic composites to maximize bone healing. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2333-2343, 2017.

Effects of Recombinant Human Bone Morphogenetic Protein-2 Dose and Ceramic Composition on New Bone Formation and Space Maintenance in a Canine Mandibular Ridge Saddle Defect Model

Treatment of mandibular osseous defects is a significant clinical challenge. Maintenance of the height and width of the mandibular ridge is essential for placement of dental implants and restoration of normal dentition. While guided bone regeneration using protective membranes is an effective strategy for maintaining the anatomic contour of the ridge and promoting new bone formation, complications have been reported, including wound failure, seroma, and graft exposure leading to infection. In this study, we investigated injectable low-viscosity (LV) polyurethane/ceramic composites augmented with 100 μg/mL (low) or 400 μg/mL (high) recombinant human bone morphogenetic protein-2 (rhBMP-2) as space-maintaining bone grafts in a canine mandibular ridge saddle defect model. LV grafts were injected as a reactive paste that set in 5-10 min to form a solid porous composite with bulk modulus exceeding 1 MPa. We hypothesized that compression-resistant LV grafts would enhance new bone formation and maintain the anatomic contour of the mandibular ridge without the use of protective membranes. At the rhBMP-2 dose recommended for the absorbable collagen sponge carrier in dogs (400 μg/mL), LV grafts maintained the width and height of the host mandibular ridge and supported new bone formation, while at suboptimal (100 μg/mL) doses, the anatomic contour of the ridge was not maintained. These findings indicate that compression-resistant bone grafts with bulk moduli exceeding 1 MPa and rhBMP-2 doses comparable to that recommended for the collagen sponge carrier support new bone formation and maintain ridge height and width in mandibular ridge defects without protective membranes.

Synthesis of polyester urethane urea and fabrication of elastomeric nanofibrous scaffolds for myocardial regeneration

Fabrication of bioactive scaffolds is one of the most promising strategies to reconstruct the infarcted myocardium. In this study, we synthesized polyester urethane urea (PEUU), further blended it with gelatin and fabricated PEUU/G nanofibrous scaffolds. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC) and X-ray diffraction were used for the characterization of the synthesized PEUU and properties of nanofibrous scaffolds were evaluated using scanning electron microscopy (SEM), ATR-FTIR, contact angle measurement, biodegradation test, tensile strength analysis and dynamic mechanical analysis (DMA). In vitro biocompatibility studies were performed using cardiomyocytes. DMA analysis showed that the scaffolds could be reshaped with cyclic deformations and might remain stable in the frequencies of the physiological activity of the heart. On the whole, our study suggests that aligned PEUU/G 70:30 nanofibrous scaffolds meet the required specifications for cardiac tissue engineering and could be used as a promising construct for myocardial regeneration.

Substrate modulus of 3D-printed scaffolds regulates the regenerative response in subcutaneous implants through the macrophage phenotype and Wnt signaling

The growing need for therapies to treat large cutaneous defects has driven recent interest in the design of scaffolds that stimulate regenerative wound healing. While many studies have investigated local delivery of biologics as a restorative approach, an increasing body of evidence highlights the contribution of the mechanical properties of implanted scaffolds to wound healing. In the present study, we designed poly(ester urethane) scaffolds using a templated-Fused Deposition Modeling (t-FDM) process to test the hypothesis that scaffolds with substrate modulus comparable to that of collagen fibers enhance a regenerative versus a fibrotic response. We fabricated t-FDM scaffolds with substrate moduli varying from 5 to 266 MPa to investigate the effects of substrate modulus on healing in a rat subcutaneous implant model. Angiogenesis, cellular infiltration, collagen deposition, and directional variance of collagen fibers were maximized for wounds treated with scaffolds having a substrate modulus (Ks = 24 MPa) comparable to that of collagen fibers. The enhanced regenerative response in these scaffolds was correlated with down-regulation of Wnt/β-catenin signaling in fibroblasts, as well as increased polarization of macrophages toward the restorative M2 phenotype. These observations highlight the substrate modulus of the scaffold as a key parameter regulating the regenerative versus scarring phenotype in wound healing. Our findings further point to the potential use of scaffolds with substrate moduli tuned to that of the native matrix as a therapeutic approach to improve cutaneous healing.

Hydrolytic and oxidative degradation of electrospun supramolecular biomaterials: In vitro degradation pathways

The emerging field of in situ tissue engineering (TE) of load bearing tissues places high demands on the implanted scaffolds, as these scaffolds should provide mechanical stability immediately upon implantation. The new class of synthetic supramolecular biomaterial polymers, which contain non-covalent interactions between the polymer chains, thereby forming complex 3D structures by self assembly. Here, we have aimed to map the degradation characteristics of promising (supramolecular) materials, by using a combination of in vitro tests. The selected biomaterials were all polycaprolactones (PCLs), either conventional and unmodified PCL, or PCL with supramolecular hydrogen bonding moieties (either 2-ureido-[1H]-pyrimidin-4-one or bis-urea units) incorporated into the backbone. As these materials are elastomeric, they are suitable candidates for cardiovascular TE applications. Electrospun scaffold strips of these materials were incubated with solutions containing enzymes that catalyze hydrolysis, or solutions containing oxidative species. At several time points, chemical, morphological, and mechanical properties were investigated. It was demonstrated that conventional and supramolecular PCL-based polymers respond differently to enzyme-accelerated hydrolytic or oxidative degradation, depending on the morphological and chemical composition of the material. Conventional PCL is more prone to hydrolytic enzymatic degradation as compared to the investigated supramolecular materials, while, in contrast, the latter materials are more susceptible to oxidative degradation. Given the observed degradation pathways of the examined materials, we are able to tailor degradation characteristics by combining selected PCL backbones with additional supramolecular moieties. The presented combination of in vitro test methods can be employed to screen, limit, and select biomaterials for pre-clinical in vivo studies targeted to different clinical applications.

Controllable degradation kinetics of POSS nanoparticle-integrated poly(ε-caprolactone urea)urethane elastomers for tissue engineering applications

Biodegradable elastomers are a popular choice for tissue engineering scaffolds, particularly in mechanically challenging settings (e.g. the skin). As the optimal rate of scaffold degradation depends on the tissue type to be regenerated, next-generation scaffolds must demonstrate tuneable degradation patterns. Previous investigations mainly focussed on the integration of more or less hydrolysable components to modulate degradation rates. In this study, however, the objective was to develop and synthesize a family of novel biodegradable polyurethanes (PUs) based on a poly(ε-caprolactone urea)urethane backbone integrating polyhedral oligomeric silsesquioxane (POSS-PCLU) with varying amounts of hard segments (24%, 28% and 33% (w/v)) in order to investigate the influence of hard segment chemistry on the degradation rate and profile. PUs lacking POSS nanoparticles served to prove the important function of POSS in maintaining the mechanical structures of the PU scaffolds before, during and after degradation. Mechanical testing of degraded samples revealed hard segment-dependent modulation of the materials’ viscoelastic properties, which was attributable to (i) degradation-induced changes in the PU crystallinity and (ii) either the presence or absence of POSS. In conclusion, this study presents a facile method of controlling degradation profiles of PU scaffolds used in tissue engineering applications.

Injected biodegradable polyurethane scaffolds support tissue infiltration and delay wound contraction in a porcine excisional model

The filling of wound cavities with new tissue is a challenge. We previously reported on the physical properties and wound healing kinetics of prefabricated, gas-blown polyurethane (PUR) scaffolds in rat and porcine excisional wounds. To address the capability of this material to fill complex wound cavities, this study examined the in vitro and in vivo reparative characteristics of injected PUR scaffolds employing a sucrose porogen. Using the porcine excisional wound model, we compared reparative outcomes to both preformed and injected scaffolds as well as untreated wounds at 9, 13, and 30 days after scaffold placement. Both injected and preformed scaffolds delayed wound contraction by 19% at 9 days and 12% at 13 days compared to nontreated wounds. This stenting effect proved transient since both formulations degraded by day 30. Both types of scaffolds significantly inhibited the undesirable alignment of collagen and fibroblasts through day 13. Injected scaffolds were highly compatible with sentinel cellular events of normal wound repair cell proliferation, apoptosis, and blood vessel density. The present study provides further evidence that either injected or preformed PUR scaffolds facilitate wound healing, support tissue infiltration and matrix production, delay wound contraction, and reduce scarring in a clinically relevant animal model, which underscores their potential utility as a void-filling platform for large cutaneous defects. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1679-1690, 2016.