Regeneration of bicortical defects in the iliac crest of estrogen-deficient sheep, using new biodegradable polyurethane bone graft substitutes

Evaluation of Vitamin D-enriched Bone Graft in Surgically-induced Critical-sized Bone Defects: An Experimental Study

BACKGROUND

Restoration of bone defects in the craniac vault may require the use of autografts, allografts, xenografts, or synthetic grafts. There are promising data that vitamin D may play a positive role in graft incorporation. The purpose of the present study is the evaluation of the impact of vitamin D addition to human-derived bone grafts in the healing of critical-sized bone defects in porcine skulls.

MATERIALS AND METHODS

Four identical critical-sized defects were created in the calvaria of 8 adult Landrace Large White pigs. The first defect was left blank as control, the second defect was filled with human-derived bone graft, the third defect was filled with human-derived bone graft enriched with a low concentration of vitamin D (2 mg/mL), and the fourth defect was filled with human-derived bone graft enriched with a high concentration of vitamin D (10 mg/mL). The animals were sacrificed after 12 weeks. Harvested tissue specimens were qualitatively evaluated by histology. New bone formation (bone volume/tissue volume) was quantitatively measured by histomorphometry.

RESULTS

Signs of bone formation were evident in all bone sockets. Mean values of the bone volume/tissue volume of the 4 defects were 10.91%, 11.05%, 10.40% and 10.87% respectively, at 12 weeks. In 5 animals, high concentration of vitamin D caused a significant improvement in bone formation in relation to controls. In 3 animals, a high concentration of vitamin D was associated with decreased bone formation compared with controls. No statistical difference was observed in the graft healing among the 4 graft sites ( P > 0.05).

CONCLUSIONS

The results of this study have shown that the addition of vitamin D to human-derived bone grafts does not have a significant effect on bone formation and graft incorporation in critical-sized bone defects of the porcine calvaria. Further high-quality studies are needed to fully elucidate the role of vitamin D in bone formation and bone graft union.

The role of Vitamin D as an adjunct for bone regeneration: A systematic review of literature

Background and objectives

In spite of bone's healing capacity, critical-size bone defect regeneration and peri-implant osseointegration are challenging. Tissue engineering provides better outcomes, but requires expensive adjuncts like stem cells, growth factors and bone morphogenic proteins. Vitamin D (Vit.D) regulates calcium and phosphorus metabolism, and helps maintain bone health. Vit.D supplements in deficient patients, accentuates bone healing and regeneration. Therefore the aim of this systematic review was to evaluate the role of adjunctive Vit.D on bone defect regeneration.

Methods

Comprehensive database search of indexed literature, published between January 1990 and June 2022, was carried out. English language articles fulfilling inclusion criteria (clinical/in vivo studies evaluating bone regeneration including osseointegration and in vitro studies assessing osteogenic differentiation, with adjunct Vit.D) were identified and screened.

Results

Database search identified 384 titles. After sequential title, abstract and full-text screening, 23 studies (in vitro - 9/in vivo - 14) were selected for review. Vit.D as an adjunct with stem cells and osteoblasts resulted in enhanced osteogenic differentiation and upregulation of genes coding for bone matrix proteins and alkaline phosphatase. When used in vivo, Vit.D resulted in early and increased new bone formation and mineralization within osseous defects, and better bone implant contact and osseointegration, around implants. Adjunct Vit.D in animals with induced systemic illnesses resulted in bone defect regeneration and osseointegration comparable to healthy animals. While systemic and local administration of Vit.D resulted in enhanced bone defect healing, outcomes were superior with systemic route.

Conclusions

Based on this review, adjunct Vit.D enhances bone defect regeneration and osseointegration. In vitro application of Vit.D to stem cells and osteoblasts enhances osteogenic differentiation. Vit.D is a potentially non-invasive and inexpensive adjunct for clinical bone regeneration and osseointegration. Long term clinical trials are recommended to establish protocols relating to type, dosage, frequency, duration and route of administration.

Possible Roles of Vitamin D in Bone Grafting

Bone grafting is one of the most commonly used options to treat large bone defects. Evidence has shown that vitamin D may affect osseointegration, a major component for successful bone grafting. In vitro studies have proved that implants coated with activated vitamin D stimulate bone production and reduce bone resorption around implants. Animal studies have noticed that oral administration of vitamin D may stimulate bone formation as well as strengthen and support the interaction between bone and implants. Vitamin D insufficiency may affect negatively the cortical peri-implant bone formation, suggesting a negative effect in graft incorporation. Few clinical studies have observed that vitamin D administration enhanced graft incorporation and bone formation, while severe vitamin D deficiency is associated with failed implant osseointegration. Even though there are encouraging results of vitamin D supplementation on graft incorporation in animal studies, the use of vitamin D as an adjuvant in bone grafting procedures cannot be fully supported at the moment. However, there is theoretical support in the use of vitamin D after surgery and the use of bone grafts to support the bone structure, relieve pain and increase graft absorption. Further experimental and clinical studies are required to support the administration of vitamin D and its analogues in such cases.

An overview of de novo bone generation in animal models

Some of the earliest success in de novo tissue generation was in bone tissue, and advances, facilitated by the use of endogenous and exogenous progenitor cells, continue unabated. The concept of one health promotes shared discoveries among medical disciplines to overcome health challenges that afflict numerous species. Carefully selected animal models are vital to development and translation of targeted therapies that improve the health and well-being of humans and animals alike. While inherent differences among species limit direct translation of scientific knowledge between them, rapid progress in ex vivo and in vivo de novo tissue generation is propelling revolutionary innovation to reality among all musculoskeletal specialties. This review contains a comparison of bone deposition among species and descriptions of animal models of bone restoration designed to replicate a multitude of bone injuries and pathology, including impaired osteogenic capacity.

Production of Biodegradable Palm Oil-Based Polyurethane as Potential Biomaterial for Biomedical Applications

Being biodegradable and biocompatible are crucial characteristics for biomaterial used for medical and biomedical applications. Vegetable oil-based polyols are known to contribute both the biodegradability and biocompatibility of polyurethanes; however, petrochemical-based polyols were often incorporated to improve the thermal and mechanical properties of polyurethane. In this work, palm oil-based polyester polyol (PPP) derived from epoxidized palm olein and glutaric acid was reacted with isophorone diisocyanate to produce an aliphatic polyurethane, without the incorporation of any commercial petrochemical-based polyol. The effects of water content and isocyanate index were investigated. The polyurethanes produced consisted of > 90% porosity with interconnected micropores and macropores (37-1700 µm) and PU 1.0 possessed tensile strength and compression stress of 111 kPa and 64 kPa. The polyurethanes with comparable thermal stability, yet susceptible to enzymatic degradation with 7-59% of mass loss after 4 weeks of treatment. The polyurethanes demonstrated superior water uptake (up to 450%) and did not induce significant changes in pH of the medium. The chemical changes of the polyurethanes after enzymatic degradation were evaluated by FTIR and TGA analyses. The polyurethanes showed cell viability of 53.43% and 80.37% after 1 and 10 day(s) of cytotoxicity test; and cell adhesion and proliferation in cell adhesion test. The polyurethanes produced demonstrated its potential as biomaterial for soft tissue engineering applications.

In vitro and in vivo proves of concept for the use of a chemically cross-linked poly(ester-urethane-urea) scaffold as an easy handling elastomeric biomaterial for bone regeneration

Bone loss can occur as a result of various pathologies, traumas and injuries and poor bone healing leads to functionally debilitating condition, loss of self-sufficiency and deterioration in life quality. Given the increasing incidence of facial trauma and the emergence of new procedural techniques, advanced scaffolds are currently developed as substitutes for bone tissue engineering. In this study, we investigated the capability of a chemically cross-linked ε-caprolactone-based poly(ester-urethane-urea) (PCLU) scaffold to support bone regeneration. In vitro assays demonstrated that PCLU scaffolds could be colonized by cells through direct cell seeding and cell migration from outside to scaffold inside. Moreover, PCLU scaffolds could provide a suitable environment for stem cells proliferation in a 3D spatial arrangement, and allowed osteogenic differentiation under appropriate induction. In vivo results revealed the osteogenic properties of PCLU scaffolds through a drilled-hole femoral bone defect repair improvement in rats. Using histology and microtomography analysis, we showed that PCLU scaffolds fit well the bone cavity and were eventually entrapped between the newly formed trabeculae. Finally, no sign of inflammation or rejection was noticed. We envision that PCLU scaffolds can provide the clinicians with a substitute having appropriate characteristics for the treatment of bone defects.

3D-printed flexible polymer stents for potential applications in inoperable esophageal malignancies

Palliation therapy for dysphagia using esophageal stents is the current treatment of choice for those patients with inoperable esophageal malignancies. However, the metallic and plastic stents currently used in the clinical setting may cause complications, such as tumor ingrowth and stent migration into the stomach. To effectively reduce/overcome these complications, we designed a tubular, flexible polymer stent with spirals. The parameters of the spirals were computationally optimized by using a finite element analysis. The designed polymer stents with optimized spirals were then printed by a 3D printing technique. 3D-printed tubular polymer stents without spirals served as controls. The self-expansion and anti-migration properties of the printed stent were characterized in an ex vivo normal porcine esophagus. The biodegradability test of the stent was performed in a neutral buffer and acidic gastric buffer. The cytotoxicity of the new stent was examined through the viability test of human esophagus epithelial cells. Results showed the self-expansion force of the 3D-printed polymer stent with spirals was higher than the stent without spirals. The anti-migration force of the 3D-printed stent with spirals was significantly higher than that of the stent without spirals. Furthermore, the stent with spirals significantly decreased the migration distance compared to the non-spiral 3D-printed polymer stent. Degradation study showed that the polymer materials started to degrade after six weeks and the compressive strength of the stent was not significantly decreased with time. In vitro cell viability results further indicated that the polymer stent does not have any cytotoxicity. Together, these results showed that the 3D-printed stent with spirals has potential applications in the treatment of inoperable esophageal malignancies. STATEMENT OF SIGNIFICANCE: In this study, we developed a new 3D-printed flexible tubular polymeric stent with spirals. The mechanical properties of the 3D-printed polymer stent are modulated by changing the ratios of PLA to TPU. The stent is flexible enough to be compressed in a clinically available stent delivery system, and can self-expand after it is released. The self-expansion force of the stent with spirals is higher than that of non-spiral stents. The spirals on the outside of the stent significantly increased the anti-migration force compared to non-spiral stents in an ex vivo normal pig esophagus. Together, the 3D-printed stent with spirals will bring promising potential in the treatment of inoperable esophagus malignancies or benign strictures.

Mechanically stimulated osteochondral organ culture for evaluation of biomaterials in cartilage repair studies

Surgical procedures such as microfracture or autologous chondrocyte implantation have been used to treat articular cartilage lesions; however, repair often fails in terms of matrix organization and mechanical behaviour. Advanced biomaterials and tissue engineered constructs have been developed to improve cartilage repair; nevertheless, their clinical translation has been hampered by the lack of reliable in vitro models suitable for pre-clinical screening of new implants and compounds. In this study, an osteochondral defect model in a bioreactor that mimics the multi-axial motion of an articulating joint, was developed. Osteochondral explants were obtained from bovine stifle joints, and cartilage defects of 4 mm diameter were created. The explants were used as an interface against a ceramic ball applying dynamic compressive and shear loading. Osteochondral defects were filled with chondrocytes-seeded fibrin-polyurethane constructs and subjected to mechanical stimulation. Cartilage viability, proteoglycan accumulation and gene expression of seeded chondrocytes were compared to free swelling controls. Cells within both cartilage and bone remained viable throughout the 10-day culture period. Loading did not wear the cartilage, as indicated by histological evaluation and glycosaminoglycan release. The gene expression of seeded chondrocytes indicated a chondrogenic response to the mechanical stimulation. Proteoglycan 4 and cartilage oligomeric matrix protein were markedly increased, while mRNA ratios of collagen type II to type I and aggrecan to versican were also enhanced. This mechanically stimulated osteochondral defect culture model provides a viable microenvironment and will be a useful pre-clinical tool to screen new biomaterials and biological regenerative therapies under relevant complex mechanical stimuli. STATEMENT OF SIGNIFICANCE: Articular cartilage lesions have a poor healing capacity and reflect one of the most challenging problems in orthopedic clinical practice. The aim of current research is to develop a testing system to assess biomaterials for implants, that can permanently replace damaged cartilage with the original hyaline structure and can withstand the mechanical forces long term. Here, we present an osteochondral ex vivo culture model within a cartilage bioreactor, which mimics the complex motion of an articulating joint in vivo. The implementation of mechanical forces is essential for pre-clinical testing of novel technologies in the field of cartilage repair, biomaterial engineering and regenerative medicine. Our model provides a unique opportunity to investigate healing of articular cartilage defects in a physiological joint-like environment.

Synthesis and in Vitro Cytocompatibility of Segmented Poly(Ester-Urethane)s and Poly(Ester-Urea-Urethane)s for Bone Tissue Engineering

Two series of segmented polyurethanes were obtained and their mechanical and thermal properties as well as their biodegradability and cytotoxicity were evaluated. The chemical nature of the polyurethanes was varied by using either 1,4 butanediol (poly-ester-urethanes, PEUs) or l-lysine ethyl ester dihydrochloride (poly-ester-urea-urethanes, PEUUs) as chain extenders. Results showed that varying the hard segment influenced the thermal and mechanical properties of the obtained polymers. PEUs showed strain and hardness values of about 10⁻20 MPa and 10⁻65 MPa, respectively. These values were higher than the obtained values for the PEUUs due to the phase segregation and the higher crystallinity observed for the polyester-urethanes (PEUs); phase segregation was also observed and analyzed by XRD and DSC. Moreover, both series of polymers showed hydrolytic degradation when they were submerged in PBS until 90 days with 20% of weight loss. In vitro tests using a Human Osteoblastic cell line (Hob) showed an average of 80% of cell viability and good adhesion for both series of polymers.

Effect of hydroxyapatite fillers on the mechanical properties and osteogenesis capacity of bio-based polyurethane composite scaffolds

A newly designed hydroxyapatite-polyurethane (HA-PU) composite scaffold was prepared by polymerizing glyceride of castor oil (GCO) with isophorone diisocyanate (IPDI) and HA as fillers. The aim of this study was to determine the effect of HA fillers on the mechanical properties and osteogenesis capacity of the composite scaffolds. The physical and biological properties of the scaffold were evaluated by SEM observation, mechanical testing, cell culture and animal experiments. The results showed that HA fillers enhanced the mechanical properties of PU composite scaffolds such as compressive strength and elastic modulus. The mechanical properties of the scaffolds were seen to increase with increase in HA loading. The compressive strength of composite scaffold with 0 wt%, 20 wt%, 40 wt% of HA was 0.6 ± 0.1 MPa, 2.1 ± 0.1 MPa, and 4.6 ± 0.3 MPa, respectively. In vitro biodegradation studies of scaffolds were carried out. The results showed that all of the scaffolds were susceptible to cholesterol esterase (CE) -catalyzed degradation. HA-PU composite scaffolds exhibited a high affinity to osteoblastic cells and were good template for cell growth and proliferation. When implanted in bone defects of rats, PU scaffolds incorporated HA were biocompatible with the tissue host and had no immune rejection. Moreover, the higher the loading of HA in the composite scaffold, the better chances of osteogenesis. It confirmed that the prepared HA-PU composite scaffolds can be promising candidate for bone repair and bone tissue engineering.

A Novel Biodegradable Polyurethane Matrix for Auricular Cartilage Repair: An In Vitro and In Vivo Study

Auricular reconstruction poses a challenge for reconstructive and burns surgeons. Techniques involving cartilage tissue engineering have shown potential in recent years. A biodegradable polyurethane matrix developed for dermal reconstruction offers an alternative to autologous, allogeneic, or xenogeneic biologicals for cartilage reconstruction. This study assesses such a polyurethane matrix for this indication in vivo and in vitro. To evaluate intrinsic cartilage repair, three pigs underwent auricular surgery to create excisional cartilage ± perichondrial defects, measuring 2 × 3 cm in each ear, into which acellular polyurethane matrices were implanted. Biopsies were taken at day 28 for histological assessment. Porcine chondrocytes ± perichondrocytes were cultured and seeded in vitro onto 1 × 1 cm polyurethane scaffolds. The total culture period was 42 days; confocal, histological, and immunohistochemical analyses of scaffold cultures were performed on days 14, 28, and 42. In vivo, the polyurethane matrices integrated with granulation tissue filling all biopsy samples. Minimal neocartilage invasion was observed marginally on some samples. Tissue composition was identical between ears whether perichondrium was left intact, or not. In vitro, the polyurethane matrix was biocompatible with chondrocytes ± perichondrocytes and supported production of extracellular matrix and Type II collagen. No difference was observed between chondrocyte culture alone and chondrocyte/perichondrocyte scaffold coculture. The polyurethane matrix successfully integrated into the auricular defect and was a suitable scaffold in vitro for cartilage tissue engineering, demonstrating its potential application in auricular reconstruction.

Polyurethanes Crosslinked with Poly(vinyl alcohol) as a Slowly-Degradable and Hydrophilic Materials of Potential Use in Regenerative Medicine

Novel, slowly-degradable and hydrophilic materials with proper mechanical properties and surface characteristics are in great demand within the biomedical field. In this paper, the design, synthesis, and characterization of polyurethanes (PUR) crosslinked with poly(vinyl alcohol) (PVA) as a new proposition for regenerative medicine is described. PVA-crosslinked PURs were synthesized by a two-step polymerization performed in a solvent (dimethylsulfoxide, DMSO). The raw materials used for the synthesis of PVA-crosslinked PURs were poly(ε-caprolactone) (PCL), 1,6-hexamethylene diisocyanate (HDI), and PVA as a crosslinking agent. The obtained materials were studied towards their physicochemical, mechanical, and biological performance. The tests revealed contact angle of the materials surface between 38-47° and tensile strength in the range of 41-52 MPa. Mechanical characteristics of the obtained PURs was close to the characteristics of native human bone such as the cortical bone (TSb = 51-151 MPa) or the cancellous bone (TSb = 10-20 MPa). The obtained PVA-crosslinked PURs did not show significant progress of degradation after 3 months of incubation in a phosphate-buffered saline (PBS). Accordingly, the obtained materials may behave similar to slowly-degradable materials, which can provide long-term physical support in, for example, tissue regeneration, as well as providing a uniform calcium deposition on the material surface, which may influence, for example, bone restoration. A performed short-term hemocompatibility study showed that obtained PVA-crosslinked PURs do not significantly influence blood components, and a cytotoxicity test performed with the use of MG 63 cell line revealed the great cytocompatibility of the obtained materials. According to the performed studies, such PVA-crosslinked PURs may be a suitable proposition for the field of tissue engineering in regenerative medicine.

Biological Evaluation of Flexible Polyurethane/Poly l-Lactic Acid Composite Scaffold as a Potential Filler for Bone Regeneration

Degradable bone graft substitute for large-volume bone defects is a continuously developing field in orthopedics. With the advance in biomaterial in past decades, a wide range of new materials has been investigated for their potential in this application. When compared to common biopolymers within the field such as PLA or PCL, elastomers such as polyurethane offer some unique advantages in terms of flexibility. In cases of bone defect treatments, a flexible soft filler can help to establish an intimate contact with surrounding bones to provide a stable bone-material interface for cell proliferation and ingrowth of tissue. In this study, a porous filler based on segmented polyurethane incorporated with poly l-lactic acid was synthesized by a phase inverse salt leaching method. The filler was put through in vitro and in vivo tests to evaluate its potential in acting as a bone graft substitute for critical-sized bone defects. In vitro results indicated there was a major improvement in biological response, including cell attachment, proliferation and alkaline phosphatase expression for osteoblast-like cells when seeded on the composite material compared to unmodified polyurethane. In vivo evaluation on a critical-sized defect model of New Zealand White (NZW) rabbit indicated there was bone ingrowth along the defect area with the introduction of the new filler. A tight interface formed between bone and filler, with osteogenic cells proliferating on the surface. The result suggested polyurethane/poly l-lactic acid composite is a material with the potential to act as a bone graft substitute for orthopedics application.

The Influence of Calcium Glycerophosphate (GPCa) Modifier on Physicochemical, Mechanical, and Biological Performance of Polyurethanes Applicable as Biomaterials for Bone Tissue Scaffolds Fabrication

In this paper we describe the synthesis of poly(ester ether urethane)s (PEEURs) by using selected raw materials to reach a biocompatible polyurethane (PU) for biomedical applications. PEEURs were synthesized by using aliphatic 1,6-hexamethylene diisocyanate (HDI), poly(ethylene glycol) (PEG), α,ω-dihydroxy(ethylene-butylene adipate) (Polios), 1,4-butanediol (BDO) as a chain extender and calcium glycerolphosphate salt (GPCa) as a modifier used to stimulate bone tissue regeneration. The obtained unmodified (PURs) and modified with GPCa (PURs-M) PEEURs were studied by various techniques. It was confirmed that urethane prepolymer reacts with GPCa modifier. Further analysis of the obtained PURs and PURs-M by Fourier transform infrared (FTIR) and Raman spectroscopy revealed the chemical composition typical for PUs by the confirmed presence of urethane bonds. Moreover, the FTIR and Raman spectra indicated that GPCa was incorporated into the main PU chain at least at one-side. The scanning electron microscopy (SEM) analysis of the PURs-M surface was in good agreement with the FTIR and Raman analysis due to the fact that inclusions were observed only at 20% of its surface, which were related to the non-reacted GPCa enclosed in the PUR matrix as filler. Further studies of hydrophilicity, mechanical properties, biocompatibility, short term-interactions, and calcification study lead to the final conclusion that the obtained PURs-M may by suitable candidate material for further scaffold fabrication. Scaffolds were prepared by the solvent casting/particulate leaching technique (SC/PL) combined with thermally-induced phase separation (TIPS). Such porous scaffolds had satisfactory pore sizes (36–100 μm) and porosity (77–82%) so as to be considered as suitable templates for bone tissue regeneration.

Development of polyurethanes for bone repair

The purpose of this paper is to review recent developments on polyurethanes aimed at the design, synthesis, modifications, and biological properties in the field of bone tissue engineering. Different polyurethane systems are presented and discussed in terms of biodegradation, biocompatibility and bioactivity. A comprehensive discussion is provided of the influence of hard to soft segments ratio, catalysts, stiffness and hydrophilicity of polyurethanes. Interaction with various cells, behavior in vivo and current strategies in enhancing bioactivity of polyurethanes are described. The discussion on the incorporation of biomolecules and growth factors, surface modifications, and obtaining polyurethane-ceramics composites strategies is held. The main emphasis is placed on the progress of polyurethane applications in bone regeneration, including bone void fillers, shape memory scaffolds, and drug carrier.

Ascorbic Acid in Polyurethane Systems for Tissue Engineering

The introduction of the paper was devoted to the main items of tissue engineering (TE) and the way of porous structure obtaining as scaffolds. Furthermore, the significant role of the scaffold design in TE was described. It was shown, that properly designed polyurethanes (PURs) find application in TE due to the proper physicochemical, mechanical and biological properties. Then the use of L-ascorbic acid (L-AA) in PUR systems for TE was described. L-AA has been applied in this area due to its suitable biological characteristics and antioxidative properties. Moreover, L-AA influences tissue regeneration due to improving collagen synthesis, which is a primary component of the extracellular matrix (ECM). Modification of PUR with L-AA leads to the materials with higher biocompatibility and such system is promising for TE applications.

Elastomeric enriched biodegradable polyurethane sponges for critical bone defects: a successful case study reducing donor site morbidity

Bone substitutes have been a critical issue as the natural source can seldom provide enough bone to support full healing. No bone substitute complies with all necessary functions and characteristics that an autograft does. Polyurethane sponges have been used as a surgical alternative to cancellous bone grafts for critical bone defect donor sites. Critical bone defects were created on the tibial tuberosity and iliac crest using an ovine model. In group I (control-untreated), no bone regeneration was observed in any animal. In group II (defects left empty but covered with a microporous polymeric membrane), the new bone bridged the top ends in all animals. In groups III and IV, bone defects were implanted with polyurethane scaffolds modified with biologically active compounds, and bone regeneration was more efficient than in group II. In groups III and IV there were higher values of bone regeneration specific parameters used for evaluation (P < 0.05) although the comparison between these groups was not possible. The results obtained in this study suggest that biodegradable polyurethane substitutes modified with biologically active substances may offer an alternative to bone graft, reducing donor site morbidity associated with autogenous cancellous bone harvesting.

Self-Assembling Peptide Nanofibrous Hydrogel on Immediate Hemostasis and Accelerative Osteosis

The use of local agents to achieve hemostasis of bone that does not interfere with repair and recovery is a complex and emergency subject in surgery. In this study, the dual functional biodegradable self-assembling nanopeptide (SAP) RADA16-I was synthesized by solid phase synthesis and was shown to exhibit immediate hemostasis and accelerative osteosis. The RADA16-I showed good performance as a hemostatic agent, which was investigated by comparison with the effects of bone wax in the ilium bone defect model of New Zealand rabbits. The RADA16-I exhibited efficient function of bone regeneration in both radiographic analysis and histological examination, while the bone wax inhibited osteogenesis. Moreover, in in vivo experiment, the RADA16-I was shown to exhibit excellent biocompatibility, while the group with bone wax showed a severe inflammatory response at the interface with bone. Thus, the RADA16-I is proven to be an excellent biocompatible material with effective dual function of hemostasis and osteosis.

Preliminary In Vitro Assessment of Stem Cell Compatibility with Cross-Linked Poly(ε-caprolactone urethane) Scaffolds Designed through High Internal Phase Emulsions

By using a high internal phase emulsion process, elastomeric poly(ε-caprolactone urethane) (PCLU) scaffolds were designed with pores size ranging from below 150 μm to 1800 μm and a porosity of 86% making them suitable for bone tissue engineering applications. Moreover, the pores appeared to be excellently interconnected, promoting cellularization and future bone ingrowth. This study evaluated the in vitro cytotoxicity of the PCLU scaffolds towards human mesenchymal stem cells (hMSCs) through the evaluation of cell viability and metabolic activity during extract test and indirect contact test at the beginning of the scaffold lifetime. Both tests demonstrated that PCLU scaffolds did not induce any cytotoxic response. Finally, direct interaction of hMSCs and PCLU scaffolds showed that PCLU scaffolds were suitable for supporting the hMSCs adhesion and that the cells were well spread over the pore walls. We conclude that PCLU scaffolds may be a good candidate for bone tissue regeneration applications using hMSCs.

Association of vitamin D3 with alveolar bone regeneration in dogs

Designed sockets prepared on the mandibles of nine Beagle dogs were divided into three groups: Calcitriol +Alloplast, Alloplast and Empty. Five of the nine dogs received Vit.D₃ and calcium supplement (Vit.D/Ca group), while the other four dogs without supplements were assigned to Non-Vit.D/Ca group. After 4 weeks, the extent of vertical ridge resorption (VRR), bone density (density), new bone formation (NBF) and implant stability quotient (ISQ) were measured. Following systemic Vit.D/Ca administration, the Empty subgroup showed significant differences from the Calcitriol + Alloplast subgroup on variants NBF/Density/VRR and the Alloplast subgroup on items NBF/Density/ISQ/VRR. Alternatively, the Calcitriol + Alloplast subgroup revealed higher values of NBF/Density/ISQ (P < 0.001) and a lower VRR value (P = 0.001) than the Alloplast subgroup. Although there were no significant differences in NBF (P = 0.349), density (P = 0.796), ISQ (P = 0.577) and VRR (0.979) comparisons on alloplast treatment between the Vit.D/Ca and Non-Vit.D/Ca groups, local application with Calcitriol + Alloplast demonstrated better NBF/Density/ISQ (P = 0.02 to <0.001) effects than which of Alloplast subgroups. Consequently, the results showed that both systemic and local vitamin D₃ treatment might accelerate bone regeneration in dogs. Within the using dose, systemic vitamin D₃ treatment displayed a superior stimulating effect than local vitamin D₃ application did.

Alveolar bone regeneration in response to local application of calcitriol in vitamin D deficient rats

AIM

Vitamin D deficiency is considered to diminish bone regeneration. Yet, raising the serum levels takes months. A topic application of the active vitamin D metabolite, calcitriol, may be an effective approach. Thus, it becomes important to know the effect of vitamin D deficiency and local application on alveolar bone regeneration.

MATERIAL AND METHODS

Sixty rats were divided into three groups; two vitamin depletion groups and a control group. Identical single defects (2 mm diameter) were created in the maxilla and mandible treated with calcitriol soaked collagen in one deficiency group while in the other two groups not. Histomorphometric analysis and micro CTs were performed after 1 and 3 weeks. Serum levels of 25(OH)D3 and PTH were determined.

RESULTS

Bone formation rate significantly increased within the observation period in all groups. Bone regeneration was higher in the maxilla than in the mandible. However, bone regeneration was lower in the control group compared to vitamin depletion groups, with no significant effects by local administration of calcitriol (micro CT mandible p = 0.003, maxilla p < 0.001; histomorphometry maxilla p = 0.035, mandible p = 0.18).

CONCLUSION

Vitamin D deficiency not necessarily impairs bone regeneration in the rat jaw and a single local calcitriol application does not enhance healing.

Biodegradable poly(ester urethane) urea scaffolds for tissue engineering: Interaction with osteoblast-like MG-63 cells

Porous three-dimensional scaffolds with potential for application as cancellous bone graft substitutes were prepared from aliphatic segmented poly(ester urethane) urea using the phase-inverse technique. Proton nuclear magnetic resonance, size-exclusion chromatography, electron spectroscopy for chemical analysis, secondary ion mass spectrometry, infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, computed tomography and mechanical tests were carried out, to characterize the scaffolds' physicochemical properties. Human osteosarcoma MG-63 cells were seeded into the scaffolds for 1, 2, 3 and 4weeks to evaluate their potential to support attachment, growth and proliferation of osteogenic cells. The scaffold-cell interaction was assessed by analysis of DNA content, total protein amount, alkaline phosphatase activity and WST-1 assay. The scaffolds supported cell attachment, growth and proliferation over the whole culture period of 4weeks (DNA, total protein amount). There was, however, a reduction in the WST-1 assay values at 4weeks, which might suggest a reduction in the rate of cell proliferation at this time.

A porous tissue engineering scaffold selectively degraded by cell-generated reactive oxygen species

Biodegradable tissue engineering scaffolds are commonly fabricated from poly(lactide-co-glycolide) (PLGA) or similar polyesters that degrade by hydrolysis. PLGA hydrolysis generates acidic breakdown products that trigger an accelerated, autocatalytic degradation mechanism that can create mismatched rates of biomaterial breakdown and tissue formation. Reactive oxygen species (ROS) are key mediators of cell function in both health and disease, especially at sites of inflammation and tissue healing, and induction of inflammation and ROS are natural components of the in vivo response to biomaterial implantation. Thus, polymeric biomaterials that are selectively degraded by cell-generated ROS may have potential for creating tissue engineering scaffolds with better matched rates of tissue in-growth and cell-mediated scaffold biodegradation. To explore this approach, a series of poly(thioketal) (PTK) urethane (PTK-UR) biomaterial scaffolds were synthesized that degrade specifically by an ROS-dependent mechanism. PTK-UR scaffolds had significantly higher compressive moduli than analogous poly(ester urethane) (PEUR) scaffolds formed from hydrolytically-degradable ester-based diols (p < 0.05). Unlike PEUR scaffolds, the PTK-UR scaffolds were stable under aqueous conditions out to 25 weeks but were selectively degraded by ROS, indicating that their biodegradation would be exclusively cell-mediated. The in vitro oxidative degradation rates of the PTK-URs followed first-order degradation kinetics, were significantly dependent on PTK composition (p < 0.05), and correlated to ROS concentration. In subcutaneous rat wounds, PTK-UR scaffolds supported cellular infiltration and granulation tissue formation, followed first-order degradation kinetics over 7 weeks, and produced significantly greater stenting of subcutaneous wounds compared to PEUR scaffolds. These combined results indicate that ROS-degradable PTK-UR tissue engineering scaffolds have significant advantages over analogous polyester-based biomaterials and provide a robust, cell-degradable substrate for guiding new tissue formation.

Biomimetic polyurethanes in nano and regenerative medicine

Nature's inspiration is a promising tool to design new biomaterials especially for frontier technological areas such as tissue engineering and nanomedicine.

Progress and challenges in biomaterials used for bone tissue engineering: bioactive glasses and elastomeric composites

Driven by the increasing economic burden associated with bone injury and disease, biomaterial development for bone repair represents the most active research area in the field of tissue engineering. This article provides an update on recent advances in the development of bioactive biomaterials for bone regeneration. Special attention is paid to the recent developments of sintered Na-containing bioactive glasses, borate-based bioactive glasses, those doped with trace elements (such as Cu, Zn, and Sr), and novel elastomeric composites. Although bioactive glasses are not new to bone tissue engineering, their tunable mechanical properties, biodegradation rates, and ability to support bone and vascular tissue regeneration, as well as osteoblast differentiation from stem and progenitor cells, are superior to other bioceramics. Recent progresses on the development of borate bioactive glasses and trace element-doped bioactive glasses expand the repertoire of bioactive glasses. Although boride and other trace elements have beneficial effects on bone remodeling and/or associated angiogenesis, the risk of toxicity at high levels must be highly regarded in the design of new composition of bioactive biomaterials so that the release of these elements must be satisfactorily lower than their biologically safe levels. Elastomeric composites are superior to the more commonly used thermoplastic-matrix composites, owing to the well-defined elastic properties of elastomers which are ideal for the replacement of collagen, a key elastic protein within the bone tissue. Artificial bone matrix made from elastomeric composites can, therefore, offer both sound mechanical integrity and flexibility in the dynamic environment of injured bone.

Vascularized bone tissue engineering: approaches for potential improvement

Significant advances have been made in bone tissue engineering (TE) in the past decade. However, classical bone TE strategies have been hampered mainly due to the lack of vascularization within the engineered bone constructs, resulting in poor implant survival and integration. In an effort toward clinical success of engineered constructs, new TE concepts have arisen to develop bone substitutes that potentially mimic native bone tissue structure and function. Large tissue replacements have failed in the past due to the slow penetration of the host vasculature, leading to necrosis at the central region of the engineered tissues. For this reason, multiple microscale strategies have been developed to induce and incorporate vascular networks within engineered bone constructs before implantation in order to achieve successful integration with the host tissue. Previous attempts to engineer vascularized bone tissue only focused on the effect of a single component among the three main components of TE (scaffold, cells, or signaling cues) and have only achieved limited success. However, with efforts to improve the engineered bone tissue substitutes, bone TE approaches have become more complex by combining multiple strategies simultaneously. The driving force behind combining various TE strategies is to produce bone replacements that more closely recapitulate human physiology. Here, we review and discuss the limitations of current bone TE approaches and possible strategies to improve vascularization in bone tissue substitutes.

Controlled release of triamcinolone acetonide from polyurethane implantable devices: application for inhibition of inflammatory-angiogenesis

The purpose of this study was to develop triamcinolone acetonide-loaded polyurethane implants (TA PU implants) for the local treatment of different pathologies including arthritis, ocular and neuroinflammatory disorders. The TA PU implants were characterized by FTIR, SAXS and WAXS. The in vitro and in vivo release of TA from the PU implants was evaluated. The efficacy of TA PU implants in suppressing inflammatory-angiogenesis in a murine sponge model was demonstrated. FTIR results revealed no chemical interactions between polymer and drug. SAXS results indicated that the incorporation of the drug did not disturb the polymer morphology. WAXS showed that the crystalline nature of the TA was preserved after incorporation into the PU. The TA released from the PU implants efficiently inhibited the inflammatory-angiogenesis induced by sponge discs in an experimental animal model. Finally, TA PU implants could be used as local drug delivery systems because of their controlled delivery of TA.

Reconstruction of iliac crest with bovine cancellous allograft after bone graft harvest for symphysis pubis arthrodesis

PURPOSE

The aim of this study was to evaluate the safety and efficiency, as well as the incorporation characteristics of a specific type of xenograft used for iliac crest defects post-harvesting tri-cortical iliac crest bone graft.

METHODS

Sixteen patients diagnosed with chronic anterior pelvic pain were operated for pubic symphysis fusion. The tri-cortical graft harvested from the iliac crest was inserted into the pubic symphysis and compressed with a reconstruction plate. The defect in the iliac crest was filled with a block of cancellous bovine substitute (Tutobone®). The length of iliac crest defect, time to fusion of pubic symphysis, time to incorporation of the graft and complications were recorded. The postoperative pain and patients' satisfaction were evaluated.

RESULTS

The median age of patients was 36.5 years (range 27-75). Fusion was obtained in 15 patients (94 %). The median time to fusion was four months (range three to seven). The length of the iliac crest bone defect ranged from 40 to 70 mm. Integration of the bovine substitute was achieved in 15 patients (94 %) over a median period of three months (range two to six). The postoperative median pain score was 2 (range 1-5). Twelve patients (75 %) reported good satisfaction. No major complications or allergic reactions were observed.

CONCLUSIONS

The xenograft used in this study provided a safe and effective method of reconstruction of iliac crest donor site defects. It has satisfactory incorporation, high biocompatibility and no signs of inflammatory reactions. This new technique is simple and easily reproducible in routine clinical practice.

The biocompatibility and antimicrobial activity of nanocomposites from polyurethane and nano silicate platelets

Nanocomposites from a polyether-type waterborne polyurethane (PU) and 0.1 wt % of silicate materials were prepared. The individual silicate materials were natural clays (montmorillonite and mica), their exfoliated clays [nano silicate platelets (NSP) and nano mica platelets], and NSP modified with C18 fatty amine (NSP-S). The physico-chemcical properties and antimicrobial activity of the nanocomposites were characterized in vitro. The biostability and biocompatibility of the nanocomposites were evaluated in vivo. The nanocomposites exhibited various surface morphologies with phase separation of hard and soft domains in nanometric scales. The nanocomposite containing NSP (PU-NSP) showed better endothelial cell attachment and gene expression. The better biocompatibility of PU-NSP and PU-NSP-S was evidenced by the lower thickness of foreign body capsules in rat subcutaneous implantation. PU-NSP had the least surface degradation in vivo as demonstrated by the electron microscopy and infrared spectroscopy. This may be associated with the different surface structure. PU-NSP and PU-NSP-S showed strong bacteriostatic effects, which suggested that the nano clay in the polymer matrix may still interact with the microbes.

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

Characterization of the degradation mechanism of polymeric scaffolds and delivery systems for regenerative medicine is essential to assess their clinical applicability. Key performance criteria include induction of a minimal, transient inflammatory response and controlled degradation to soluble non-cytotoxic breakdown products that are cleared from the body by physiological processes. Scaffolds fabricated from biodegradable poly(ester urethane)s (PEURs) undergo controlled degradation to non-cytotoxic breakdown products and support the ingrowth of new tissue in preclinical models of tissue regeneration. While previous studies have shown that PEUR scaffolds prepared from lysine-derived polyisocyanates degrade faster under in vivo compared to in vitro conditions, the degradation mechanism is not well understood. In this study, we have shown that PEUR scaffolds prepared from lysine triisocyanate (LTI) or a trimer of hexamethylene diisocyanate (HDIt) undergo hydrolytic, esterolytic, and oxidative degradation. Hydrolysis of ester bonds to yield α-hydroxy acids is the dominant mechanism in buffer, and esterolytic media modestly increase the degradation rate. While HDIt scaffolds show a modest (<20%) increase in degradation rate in oxidative medium, LTI scaffolds degrade six times faster in oxidative medium. Furthermore, the in vitro rate of degradation of LTI scaffolds in oxidative medium approximates the in vivo rate in rat excisional wounds, and histological sections show macrophages expressing myeloperoxidase at the material surface. While recent preclinical studies have underscored the potential of injectable PEUR scaffolds and delivery systems for tissue regeneration, this promising class of biomaterials has a limited regulatory history. Elucidation of the macrophage-mediated oxidative mechanism by which LTI scaffolds degrade in vivo provides key insights into the ultimate fate of these materials when injected into the body.

Evaluation of a new press-fit in situ setting composite porous scaffold for cancellous bone repair: towards a "surgeon-friendly" bone filler?

In this study, a composite porous material obtained by coating a poly(ester urethane) foam with a calcium phosphate cement is proposed as novel cancellous bone filler with easy handling, in situ hardening and press-fitting properties. The coating can be applied to the foam in the surgical theater, allowing refinement of scaffold shape to the needs of the ongoing surgery. An innovative experiment was developed in order to determine the setting curve of the composite scaffold as well as the time of manipulation available to the surgeon without risk of material damage. This composite material is soft and can be press-fit in a cavity without damaging the scaffold in the first 5 min after coating application. The composite scaffold hardens quickly (22 min) and, once the cement has set, its compressive strength and fracture energy are increased by over an order of magnitude as compared to the initial poly(ester urethane) foam. This set of interesting properties makes calcium phosphate cement-coated elastomeric scaffolds a new promising strategy for cancellous bone filling.

Candidate bone-tissue-engineered product based on human-bone-derived cells and polyurethane scaffold

Biodegradable polyurethanes (PURs) have recently been investigated as candidate materials for bone regenerative medicine. There are promising reports documenting the biocompatibility of selected PURs in vivo and the tolerance of certain cells toward PURs in vitro - potentially to be used as scaffolds for tissue-engineered products (TEPs). The aim of the present study was to take a step forward and create a TEP using human osteogenic cells and a polyurethane scaffold, and to evaluate the quality of the obtained TEP in vivo. Human-bone-derived cells (HBDCs) were seeded and cultured on polyurethane scaffolds in a bioreactor for 14 days. The TEP examination in vitro was based on the evaluation of cell number, cell phenotype and cell distribution within the scaffold. TEPs and control samples (scaffolds without cells) were implanted subcutaneously into SCID mice for 4 and 13 weeks. Explants harvested from the animals were examined using histological and immunohistochemical methods. They were also tested in mechanical trials. It was found that dynamic conditions for cell seeding and culture enable homogeneous distribution, maintaining the proliferative potential and osteogenic phenotype of the HBDCs cultured on polyurethane scaffolds. It was also found that HBDCs implanted as a component of TEP survived and kept their ability to produce the specific human bone extracellular matrix, which resulted in higher mechanical properties of the harvested explants when preseeded with HBDCs. The whole system, including the investigated PUR scaffold and the method of human cell seeding and culture, is recommended as a candidate bone TEP.

Thiol-containing degradable poly(thiourethane-urethane)s for tissue engineering

Poly(thiourethane-urethane)s with varying amounts of sulphur were synthesised by a two-step polycondensation consisting of the sequential addition of 1,6-hexamethylene diisocyanate and bis(2-mercaptoethyl) ether in a poly(epsilon-caprolactone) diol solution. Polymers prepared had high weight-average molecular weight and typical microdomains separation, as shown by size-exclusion chromatography and thermal analysis. Polymer surfaces were characterized by X-ray photoelectron spectroscopy and atomic force microscopy. The quantification of thiol groups at the surface was assessed using a fluorescent assay. Thiol concentration ranged between 7 and 14 nmol/cm, and was directly related to the amount of sulphur introduced in the polymerization and the macromolecule chains orientation at the surfaces. A preliminary in vitro degradation study and a proliferation assay were performed. The poly(thiourethane-urethane)s may have important applications as biodegradable and biocompatible materials for cartilage and bone tissue engineering. The surface thiol groups add the prospect of further functionalization. This is an important benefit compared to biodegradable poly(urethane)s that usually present low biological activity.

Evaluation of a novel biodegradable polymer for the generation of a dermal matrix

Dermal skin substitutes can be used to overcome the immediate problem of donor site shortage in the treatment of major skin loss conditions, such as burn injury. In this study, the biocompatibility, safety, and potential of three variants of NovoSorb (a family of novel biodegradable polyurethanes) as dermal scaffolds were determined in a series of in vitro and in vivo systems. All three polymers exhibited minimal cytotoxic effects on human skin cells, allowing keratinocytes, dermal fibroblasts, and microvascular endothelial cells to grow normally in coculture. Subcutaneous implantation of the polymers in rats demonstrated no systemic toxic effects of the materials or their degradation products. The anticipated local foreign body reaction compared favorably with commercially available medical sutures. Assessment of a three-dimensional polymer matrix followed. The success of sequential culturing of dermal fibroblasts and keratinocytes within the matrix indicated that the generation of a cultured skin substitute is achievable. The polymeric matrix also provided a scaffold for the guided formation of a cultured microvasculature. When engrafted onto a surgically created full-thickness sheep wound, the noncellular matrix integrated, healed with an epidermis supported by a basement membrane, and was capable of withstanding wound contraction. The resistance to contraction compared favorably with a commercially available collagen-based dermal matrix (Integra). These results suggest that the NovoSorb matrix could form the basis of an elegant two-stage burn treatment strategy, with an initial noncellular biodegradable temporizing matrix to stabilize the wound bed followed by the application of cultured skin substitute.

Microporous biodegradable polyurethane membranes for tissue engineering

Microporous membranes with controlled pore size and structure were produced from biodegradable polyurethane based on aliphatic diisocyanate, poly(epsilon-caprolactone) diol and isosorbide chain extender using the modified phase-inversion technique. The following parameters affecting the process of membrane formation were investigated: the type of solvent, solvent-nonsolvent ratio, polymer concentration in solution, polymer solidification time, and the thickness of the polymer solution layer cast on a substrate. The experimental systems evaluated were polymer-N,N-dimethylformamide-water, polymer-N,N-dimethylacetamide-water and polymer-dimethylsulfoxide-water. From all three systems evaluated the best results were obtained for the system polymer-N,N-dimethylformamide-water. The optimal conditions for the preparation of microporous polyurethane membranes were: polymer concentration in solution 5% (w/v), the amount of nonsolvent 10% (v/v), the cast temperature 23 degrees C, and polymer solidification time in the range of 24-48 h depending on the thickness of the cast polymer solution layer. Membranes obtained under these conditions had interconnected pores, well defined pore size and structure, good water permeability and satisfactory mechanical properties to allow for suturing. Potential applications of these membranes are skin wound cover and, in combination with autogenous chondrocytes, as an "artificial periosteum" in the treatment of articular cartilage defects.

In vivo biocompatibility and vascularization of biodegradable porous polyurethane scaffolds for tissue engineering

Scaffolds for tissue engineering should be biocompatible and stimulate rapid blood vessel ingrowth. Herein, we analyzed in vivo the biocompatibility and vascularization of three novel types of biodegradable porous polyurethane scaffolds. The polyurethane scaffolds, i.e., PU-S, PU-M and PU-F, were implanted into dorsal skinfold chambers of BALB/c mice. Using intravital fluorescence microscopy we analyzed vascularization of the implants and venular leukocyte-endothelial cell interaction in the surrounding host tissue over a 14 day period. Incorporation of the scaffolds was analyzed by histology, and a WST-1 assay was performed to evaluate their cell biocompatibility in vitro. Our results indicate that none of the polyurethane scaffolds was cytotoxic. Accordingly, rolling and adherent leukocytes in venules of the dorsal skinfold chamber were found in a physiological range after scaffold implantation and did not significantly differ between the groups, indicating a good in vivo biocompatibility. However, the three scaffolds induced a weak angiogenic response with a microvessel density of only approximately 47-60 and approximately 3-10cm/cm(2) in the border and centre zones of the scaffolds at day 14 after implantation. Histology demonstrated that the scaffolds were incorporated in a granulation tissue, which exhibited only a few blood vessels and inflammatory cells. In conclusion, PU-S, PU-M and PU-F scaffolds may be used to generate tissue constructs which do not induce a strong inflammatory reaction after implantation into patients. However, the scaffolds should be further modified or conditioned in order to accelerate and improve the process of vascularization.

Biodegradable polyurethanes: synthesis and applications in regenerative medicine

Biostable polyurethanes (PURs) have been incorporated in biomedical devices since the 1960s. The mechanisms of degradation and compositions with improved in vivo biostability have been investigated extensively. In recent years, biodegradable PURs have been investigated for applications in regenerative medicine. In contrast to the biostable implants, these biomaterials are designed to undergo controlled degradation in vivo and promote ingrowth of new tissue. Tissue-engineered scaffolds have been fabricated from biodegradable PURs using a variety of techniques, including thermally induced phase separation, salt leaching, wet spinning, electrospinning, and carbon dioxide foaming. These materials have been reported to support the ingrowth of cells and tissue in vitro and in vivo, and undergo controlled degradation to noncytotoxic decomposition products. Due to their tunable biological, mechanical, and physicochemical properties, biodegradable PURs present compelling future opportunities as scaffolds for regeneration of tissue. This review article summarizes recent advances made in the synthesis of biodegradable PURs and the application of these materials as scaffolds for regenerative medicine.