Collagen sponges for bone regeneration with rhBMP-2

Is lumbar facet fusion biomechanically equivalent to lumbar posterolateral onlay fusion?

OBJECTIVE This study was designed with the following research objectives: 1) to determine the efficacy of facet fusion with recombinant human bone morphogenetic protein-2 (rhBMP-2) on an absorbable collagen sponge (ACS) in an ovine lumbar facet fusion model; 2) to radiographically and histologically compare the efficacy of lumbar facet fusion with rhBMP-2/ACS to facet fusion with an iliac crest bone graft (ICBG); and 3) to biomechanically compare lumbar facet fusion with rhBMP-2/ACS to lumbar posterolateral fusion (PLF) with ICBG. METHODS The efficacies of the 3 treatments to induce fusion were evaluated in an instrumented ovine lumbar fusion model. Eight sheep had 10 cm³/side ICBG placed as an onlay graft for PLF at L2-3. At the adjacent L3-4 level, 0.5 cm³/side ICBG was placed for facet fusion. Finally, 0.5 cm³/side rhBMP-2/ACS (0.43 mg/ml) was placed for facet fusion at L4-5. CT scans were obtained at 2, 4, and 6 months postoperatively with 2 reviewers conducting an evaluation of the 6-month results for all treated spinal levels. All 8 sheep were killed at 6 months, and all posterolateral instrumentation was removed at this time. The spines were then sectioned through L3-4 to allow for nondestructive unconstrained biomechanical testing of the L2-3 and L4-5 segments. All treated spinal levels were analyzed using undecalcified histology with corresponding microradiography. Statistical comparisons were made between the treatment groups. RESULTS The PLF with ICBG (ICBG PLF group) and the rhBMP-2 facet fusion (rhBMP-2 Facet group) treatment groups demonstrated similar levels of stiffness, with the rhBMP-2 Facet group having on average slightly higher stiffness in all 6 loading directions. All 8 levels in the autograft facet fusion treatment group demonstrated CT radiographic and histological fusion. All 8 levels in the rhBMP-2 Facet group showed bilateral CT radiographic and histological fusion. Six of 16 rhBMP-2/ACS-treated facet defects demonstrated small intraosseous hematomas or seromas. Four of the 8 levels (50%) in the ICBG PLF treatment group demonstrated bilateral histological fusion. Three of 8 levels in the ICBG PLF treatment group showed unilateral fusion. One of the 8 levels in the ICBG PLF treatment group demonstrated bilateral histological nonfusion. CONCLUSIONS Both rhBMP-2/ACS and autograft demonstrated 100% efficacy when used for facet fusion in the instrumented ovine model. However, the ICBG PLF treatment group only demonstrated a 50% bilateral fusion rate. Biomechanically, the ICBG PLF and rhBMP-2 Facet groups demonstrated similar stiffness in all 6 loading directions, with the rhBMP-2 Facet group having on average slightly higher stiffness in all directions.

Delivery vehicle effects on bone regeneration and heterotopic ossification induced by high dose BMP-2

Bone morphogenetic protein-2 (BMP-2), delivered on absorbable collagen sponge, is frequently used to treat bone defects. However, supraphysiological BMP-2 doses are common and often associated with complications such as heterotopic ossification and inflammation, causing pain and impaired mobility. This has prompted investigations into strategies to spatially control bone regeneration, for example growth factor delivery in appropriate scaffolds. Our objective was to investigate the spatiotemporal effects of high dose BMP-2 on bone regeneration as a function of the delivery vehicle. We hypothesized that an alginate delivery system would spatially restrict bone formation compared to a collagen sponge delivery system. In vitro, BMP-2 release was accelerated from collagen sponge compared to alginate constructs. In vivo, bone regeneration was evaluated over 12weeks in critically sized rat femoral segmental defects treated with 30μg rhBMP-2 in alginate hydrogel or collagen sponge, surrounded by perforated nanofiber meshes. Total bone volume, calculated from micro-CT reconstructions, was higher in the alginate group at 12weeks. Though bone volume within the central defect region was greater in the alginate group at 8 and 12weeks, heterotopic bone volume was similar between groups. Likewise, mechanical properties from ex vivo torsional testing were comparable between groups. Histology corroborated these findings and revealed heterotopic mineralization at 2weeks post-surgery in both groups. Overall, this study recapitulated the heterotopic ossification associated with high dose BMP-2 delivery, and demonstrated that the amount and spatial pattern of bone formation was dependent on the delivery matrix.

STATEMENT OF SIGNIFICANCE

Alginate hydrogel-based BMP-2 delivery has induced better spatiotemporal bone regeneration in animals, compared to clinically used collagen sponge, at lower BMP-2 doses. Lack of clear dose-response relationships for BMP-2 vis-à-vis bone regeneration has contributed to the use of higher doses clinically. We investigated the potential of the alginate system, with comparatively favorable BMP-2 release-kinetics, to reduce heterotopic ossification and promote bone regeneration, when used with a high BMP-2 dose. While defect mineralization improved with alginate hydrogel, the initial high-release phase and likely early tissue exposure to BMP-2 appeared sufficient to induce heterotopic ossification. The characterization presented here should provide the framework for future evaluations of strategies to optimize bone formation and minimize adverse effects of high dose BMP-2 therapy.

Osteoinductive recombinant silk fusion proteins for bone regeneration

Protein polymers provide a unique opportunity for tunable designs of material systems due to the genetic basis of sequence control. To address the challenge of biomineralization interfaces with protein based materials, we genetically engineered spider silks to design organic-inorganic hybrid systems. The spider silk inspired domain (SGRGGLGGQG AGAAAAAGGA GQGGYGGLGSQGT)₁₅ served as an organic scaffold to control material stability and to allow multiple modes of processing, whereas the hydroxyapatite binding domain VTKHLNQISQSY (VTK), provided control over osteogenesis. The VTK domain was fused either to the N-, C- or both terminals of the spider silk domain to understand the effect of position on material properties and mineralization. The addition of the VTK domain to silk did not affect the physical properties of the silk recombinant constructs, but it had a critical role in the induction of biomineralization. When the VTK domain was placed on both the C- and N-termini the formation of crystalline hydroxyapatite was significantly increased. In addition, all of the recombinant proteins in film format supported the growth and proliferation of human mesenchymal stem cells (hMSCs). Importantly, the presence of the VTK domain enhanced osteoinductive properties up to 3-fold compared to the control (silk alone without VTK). Therefore, silk-VTK fusion proteins have been shown suitable for mineralization and functionalization for specific biomedical applications.

STATEMENT OF SIGNIFICANCE

Organic-inorganic interfaces are integral to biomaterial functions in many areas of repair and regeneration. Several protein polymers have been investigated for this purpose. Despite their success the limited options to fine-tune their material properties, degradation patterns and functionalize them for each specific biomedical application limits their application. Various studies have shown that the biological performance of such proteins can be improved by genetic engineering. The present study provides data relating protein design parameters and functional outcome quantified by biomineralization and human mesenchymal stem cell differentiation. As such, it helps the design of osteoinductive recombinant biomaterials for bone regeneration.

Chitosan as a vehicle for growth factor delivery: Various preparations and their applications in bone tissue regeneration

The replacement of conventional autografts and allografts by bone fragments constructed from alternate materials, cells, and molecules (growth factors, drugs, etc.) is an exciting prospect in the field of bone tissue engineering. Bone morphogenetic protein-2 (BMP-2) is a growth factor that has been extensively studied from this point of view. This review analyzes the relevance of chitosan and its derivatives and composites with various materials such as ceramics, heparin, silica, stem cells, titanium implants, etc., in terms of delivering BMP-2 for the purpose of bone regeneration. Chitosan offers the versatility to be modified into any shapes or sizes including conversion to nanoparticles, microspheres, nanofibers, porous scaffolds, and films. The results presented in this review clearly demonstrate that chitosan-based materials are biocompatible and have the potential to systematically and sustainably release BMP-2 where required. This release results in enhanced cell proliferation levels, enhancement of alkaline phosphatase activity, increased differentiation as well as increased mineralization under in vitro and in vivo conditions. This review also shines a spotlight on the currently developed chitosan-based products that are being used for BMP-2 delivery.

The enhancement of osteogenic capacity in a synthetic BMP-2 derived peptide coated mineralized collagen composite in the treatment of the mandibular defects

The novel synthetic peptide P17-BMP-2 could promote cell attachment and enhance osteogenic capability. A composite, comprising nano-hydroxyapatite, collagen and poly(L-lactide) (nHAC/PLLA), was an efficient scaffold for carrier of P17-BMP-2. Our aim was to investigate whether nHAC/PLLA/P17-BMP-2 accelerates the osteogenesis as a reliable method for mandibular defect healing in this study. The repair capability was assessed by the gross observation, X-ray test and histological observation in four animal experiment groups at 2 week and 4 week after surgery: Group A (control), Group B (nHAC/PLLA treatment), Group C (nHAC/PLLA with 2 mg/g P17-BMP-2 treatment) and Group D (nHAC/PLLA with 10 mg/g P17-BMP-2 treatment). The Lane-Sandhu X-ray scores of the four groups were compared among four groups as well. The results showed that the composites containing the highest content of P17- BMP-2 performed best. Therefore, the nHAC/PLLA with P17-BMP-2 composite can accelerate the osteogenesis for mandibular defect healing and could be an ideal biological material as a bone graft material option for clinical applications.

Tethering of rhBMP-2 upon calcium phosphate cement via alendronate/heparin for localized, sustained and enhanced osteoactivity

rhBMP-2 was tethered on surface of calcium phosphate cement via alendronate–heparin. This novel delivery system can concurrently satisfy high bioactive immobilization and sustainable release of rhBMP-2, and consequently induce rapid bone regeneration.

Transient Local Bone Remodeling Effects of rhBMP-2 in an Ovine Interbody Spine Fusion Model

BACKGROUND

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is a powerful osteoinductive morphogen capable of stimulating the migration of mesenchymal stem cells (MSCs) to the site of implantation and inducing the proliferation and differentiation of these MSCs into osteoblasts. Vertebral end-plate and vertebral body resorption has been reported after interbody fusion with high doses of rhBMP-2. In this study, we investigated the effects of 2 rhBMP-2 doses on peri-implant bone resorption and bone remodeling at 7 time points in an end-plate-sparing ovine interbody fusion model.

METHODS

Twenty-one female sheep underwent an end-plate-sparing discectomy followed by interbody fusion at L2-L3 and L4-L5 using a custom polyetheretherketone (PEEK) interbody fusion device. The PEEK interbody device was filled with 1 of 2 different doses of rhBMP-2 on an absorbable collagen sponge (ACS): 0.13 mg (1×) or 0.90 mg (7×). Bone remodeling and interbody fusion were assessed via high-resolution radiography and histological analyses at 1, 2, 3, 4, 8, 12, and 20 weeks postoperatively.

RESULTS

Peri-implant bone resorption peaked between 3 and 8 weeks in both the 1× and the 7× rhBMP-2/ACS-dose group. Osteoclastic activity and corresponding peri-implant bone resorption was dose-dependent, with moderate-to-marked resorption at the 7×-dose level and less resorption at the 1×-dose level. Both dose (p < 0.0007) and time (p < 0.0025) affected bone resorption significantly. Transient bone-resorption areas were fully healed by 12 weeks. Osseous bridging was seen at all but 1 spinal level at 12 and at 20 weeks.

CONCLUSIONS

In the ovine end-plate-sparing interbody fusion model, rhBMP-2 dose-dependent osteoclastic resorption is a transient phenomenon that peaks at 4 weeks postoperatively.

CLINICAL RELEVANCE

Using the U.S. Food and Drug Administration (FDA)-approved rhBMP-2 concentration and matching the volume of rhBMP-2/ACS with the volume of desired bone formation within the interbody construct may limit the occurrence of transient bone resorption.

Role of Recombinant Human Bone Morphogenetic Protein-2 on Hindfoot Arthrodesis

Despite advances in understanding bone healing physiology and surgical techniques, delayed union and nonunion still occur after the treatment of hindfoot arthrodesis. There is increasing appeal of bone morphogenetic proteins (BMPs) owing to the innate osteoinductive abilities of BMPs. Effective treatment with BMPs has been shown in animal studies. Human clinical studies have also shown success. The only study investigating the use of recombinant human BMP (rhBMP)-2 in hindfoot arthrodesis found a significant increase in fusion rate. Treatment cost effective. Complications from their use remain low. rhBMP-2 is a safe and effective bone-healing adjunct in hindfoot arthrodesis surgery.

Strategies for delivering bone morphogenetic protein for bone healing

Bone morphogenetic proteins (BMPs) are the most significant growth factors that belong to the Transforming Growth Factor Beta (TGF-β) super-family. Though more than twenty members of this family have been identified so far in humans, Food and Drug Administration (FDA) approved two growth factors: BMP-2 and BMP-7 for treatments of spinal fusion and long-bone fractures with collagen carriers. Currently BMPs are clinically used in spinal fusion, oral and maxillofacial surgery and also in the repair of long bone defects. The efficiency of BMPs depends a lot on the selection of suitable carriers. At present, different types of carrier materials are used: natural and synthetic polymers, calcium phosphate and ceramic-polymer composite materials. Number of research articles has been published on the minute intricacies of the loading process and release kinetics of BMPs. Despite the significant evidence of its potential for bone healing demonstrated in animal models, future clinical investigations are needed to define dose, scaffold and route of administration. The efficacy and application of BMPs in various levels with a proper carrier and dose is yet to be established. The present article collates various aspects of success and limitation and identifies the prospects and challenges associated with the use of BMPs in orthopaedic surgery.

Scientific evidence on the use of recombinant human bone morphogenetic protein-2 (rhBMP-2) in oral and maxillofacial surgery

PURPOSE

This study aimed to identify the main indications for the use of recombinant human bone morphogenetic protein-2 (rhBMP-2) for bone repair and maintenance in the maxilla and mandible through a review of clinical trials evaluating the viability of using rhBMP-2 to delay the installation of dental implants, thus allowing satisfactory bone formation and long-term osseointegration.

METHODS

Literature search of the PubMed/Medline databases was performed using the following MeSH index terms-"bone morphogenetic protein 2" and "dentistry". Only clinical trials necessarily published in English, related to dentistry, and focused on bone reconstruction in critical defects, post-extraction alveoli, increasing the atrophic alveolar ridge, or surgery for maxillary sinus elevation were included, regardless of the age, sex, ethnicity, associated morbidities, or period of publication.

RESULTS

Of the 17 studies identified based on the search filters, 2 were excluded. Therefore, 15 studies were finally included in this review.

CONCLUSIONS

Based on the results of our review, we concluded that the use of rhBMP-2 for the preservation of the alveolar ridge after tooth extraction or for increasing the local defects is safe and viable. The use of rhBMP-2/Bio-Oss® for the elevation of the maxillary sinus membrane is unnecessary; however, it can improve and accelerate the maturation process in cases of guided bone regeneration in peri-implant defects. Compounds comprising rhBMP-2, allogenic bone, and plasma-rich platelet (PRP) can act as autograft substitutes in mandibular critical defects.

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

Enhancement of posterolateral lumbar spine fusion using recombinant human bone morphogenetic protein-2 and mesenchymal stem cells delivered in fibrin glue

Mesenchymal stem cells have shown great potential for accelerating bone healing. In the present study, we evaluate the efficacy of fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 composite for posterolateral spinal fusion in a rabbit model. Forty adult rabbits underwent posterolateral intertransverse fusion at the L5–L6 level. The animals were randomly divided into four groups based on the implant material: fibrin glue, fibrin glue/mesenchymal stem cells composite, fibrin glue-recombinant human bone morphogenetic protein-2 (fibrin glue/recombinant human bone morphogenetic protein-2) composite, and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 composite. After six weeks, the rabbits were euthanized for manual palpation, radiographic examination, biomechanical testing, and histology. Manual palpation results showed that the fusion rate for fibrin glue, fibrin glue/mesenchymal stem cells, fibrin glue/recombinant human bone morphogenetic protein-2, and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 was 0, 0, 40%, and 70%, respectively. Moreover, fusion rate determined by radiographic examination for fibrin glue, fibrin glue/mesenchymal stem cells, fibrin glue/recombinant human bone morphogenetic protein-2, and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 was 0, 0, 40%, and 80%, respectively. Gray analysis showed that fibrin glue/recombinant human bone morphogenetic protein-2 group had higher ossification area and density than fibrin glue group; and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 group had higher ossification area and density than fibrin glue/recombinant human bone morphogenetic protein-2 group. Formation of continuous bone masses between L5 and L6 level in mesenchymal stem cells/recombinant human bone morphogenetic protein-2/fibrin glue group was further confirmed by computed tomography scanning and three-dimensional reconstruction. Biomechanical testing demonstrated that the fusion strength (flexion, extension, lateral bending, and axial rotation) in fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 group is significantly higher than that in fibrin glue/recombinant human bone morphogenetic protein-2 group. The formation of mature bone tissues between transverse processes of the fused specimens from both fibrin glue/recombinant human bone morphogenetic protein-2, and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 groups was confirmed by HE staining, and quantitative real-time polymerase chain reaction results showed the upregulation of CD31, type I collagen, osteocalcin, and osteonectin in the fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 group. In conclusion, our findings show that mesenchymal stem cells delivered with recombinant human bone morphogenetic protein-2 using fibrin glue as carrier are more effective in enhancing spine fusion than recombinant human bone morphogenetic protein-2 without mesenchymal stem cells in the rabbit model.

Antagonism of the Met5-enkephalin-opioid growth factor receptor-signaling axis promotes MSC to differentiate into osteoblasts

Chronic opioid therapy is associated with bone loss. This led us to hypothesize that the opioid antagonists, that include naloxone, would stimulate bone formation by regulating MSC differentiation. The opioid growth factor receptor (OGFR) is a non-canonical opioid receptor that binds naloxone with high affinity whereas the native opioid growth factor, met5-enkephalin (met5), binds both the OGFR and the canonical delta opioid receptor (OPRD). Naloxone and an shRNA OGFR lentivirus were employed to disrupt the OGFR-signaling axis in cultured MSC. In parallel, naloxone was administered to bone marrow using a mouse unicortical defect model. OPRD, OGFR, and the met5-ligand were highly expressed in MSC and osteoblasts. A pulse-dose of naloxone increased mineral formation in MSC cultures in contrast to MSC treated with continuous naloxone or OGFR deficient MSC. Importantly, SMAD1 and SMAD8/9 expression increased after a pulse dose of naloxone whereas SMAD1, SMAD7, and ID1 were increased in the OGFR deficient MSC. Inhibited OGFR signaling decreased proliferation and increased p21 expression. The addition of naloxone to the unicortical defect resulted in increased bone formation within the defect. Our data suggest that novel mechanism through which signaling through the OGFR regulates osteogenesis via negative regulation of SMAD1 and p21. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1195-1205, 2016.

BMP-2 delivered via sucrose acetate isobutyrate (SAIB) improves bone repair in a rat open fracture model

Human bone morphogenetic proteins (BMPs) are an alternative to bone graft for the treatment of high-energy open fractures. The standard delivery system for BMP-2 is a porous collagen sponge, but we have previously found that the biocompatible, high viscosity carrier, Sucrose acetate isobutyrate (SAIB) is an effective and potentially less invasive alternative. The efficacy of SAIB as a BMP-2 delivery system was examined in an open fracture model featuring a femoral osteotomy with periosteal stripping in 9-week-old male Sprague Dawley rats. SAIB containing BMP-2 (SAIB/BMP-2) was delivered into the fracture site during surgery and an additional group was further co-treated with zoledronic acid and hydroxyapatite nanoparticles (SAIB/BMP-2/HA/ZA). These were compared to untreated fractures and SAIB carrier alone (negative controls), and BMP-2 loaded collagen sponge (positive control). The rate of radiographic union and the biomechanical properties of the healed fractures were compared after 6-week. Untreated and SAIB-treated fractures showed poor repair, with 53% and 64%, respectively, not bridged at 6 week. In contrast, collagen/BMP-2, SAIB/BMP-2, and SAIB/BMP-2/HA/ZA showed significantly increased union (100%, 100%, and 94%, respectively, p < 0.05). Four-point bend testing revealed that collagen/BMP-2 and SAIB/BMP-2/HA/ZA restored the strength of fractured femora to that of intact femora by 6 week, whereas untreated and SAIB remained less than intact controls by 60% and 67%, respectively (p < 0.05). Overall, the SAIB/BMP-2/HA/ZA formulation was comparable to BMP-2 infused collagen sponge in terms of promoting open fractures repair, but with the additional potential for less invasive delivery. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1168-1176, 2016.

Surface delivery of tunable doses of BMP-2 from an adaptable polymeric scaffold induces volumetric bone regeneration

The rapid and effective bone regeneration of large non-healing defects remains challenging. Bioactive proteins, such as bone morphogenetic protein (BMP)-2, are proved their osteoinductivity, but their clinical use is currently limited to collagen as biomaterial. Being able to deliver BMP-2 from any other biomaterial would broaden its clinical use. This work presents a novel means for repairing a critical size volumetric bone femoral defect in the rat by combining a osteoinductive surface coating (2D) to a polymeric scaffold (3D hollow tube) made of commercially-available PLGA. Using a polyelectrolyte film as BMP-2 carrier, we tune the amount of BMP-2 loaded in and released from the polyelectrolyte film coating over a large extent by controlling the film crosslinking level and initial concentration of BMP-2 in solution. Using microcomputed tomography and quantitative analysis of the regenerated bone growth kinetics, we show that the amount of newly formed bone and kinetics can be modulated: an effective and fast repair was obtained in 1-2 weeks in the best conditions, including complete defect bridging, formation of vascularized and mineralized bone tissue. Histological staining and high-resolution computed tomography revealed the presence of bone regeneration inside and around the tube with spatially distinct organization for trabecular-like and cortical bones. The amount of cortical bone and its thickness increased with the BMP-2 dose. In view of the recent developments in additive manufacturing techniques, this surface-coating technology may be applied in combination with various types of polymeric or metallic scaffolds to offer new perspectives of bone regeneration in personalized medicine.

PLGA-linked alendronate enhances bone repair in diaphysis defect model

Alendronate (ALN) is known as an anti-resorptive drug for the treatment of osteoporosis. Recently, ALN was found to stimulate osteogenic differentiation in mesenchymal stem cells and enhance new bone formation in calvarial bone defects. Previous in vitro and in vivo studies found that the effective concentration of ALN was approximately 1-10   μm. In the present study, a poly (lactic-co-glycolic acid) (PLGA) cross-linked ALN (PLGA-ALN) with a short-term controlled-release property for local application to enhance bone repair was developed. An in vitro drug-release kinetic test showed that PLGA-ALN microspheres released an effective concentration (50-100 nm) of ALN for 9 days. The effect of PLGA-ALN on bone repair was tested in a rat femoral bone defect model. The biomechanical study results showed that the maximal strength, stiffness and energy absorption were significantly increased in the PLGA-ALN group compared with the PLGA group. The microstructure of the newly formed bone at the defect site was analysed using microcomputed tomography. The PLGA-ALN group significantly improved the trabecular bone volume at the defect site compared with the PLGA group. The fibril collagen and immunolocalized bone morphogenetic protein 2 were evident in the newly formed trabecular bone in the PLGA-ALN group. Local use of newly developed PLGA-ALN-enhanced bone repair was attributable to increasing bone matrix formation, which improved the ultrastructure of the newly formed bone and thus increased the biomechanical properties of the repaired bone. It is suggested that PLGA-ALN may be a potential bone graft substitute to enhance bone repair. Copyright © 2016 John Wiley & Sons, Ltd.

Bone regenerative effect of recombinant human bone morphogenetic protein-2 after cyst enucleation

Background

The aim of this study is to quantitatively evaluate the effect of rhBMP-2 for repair of bone defects after cyst enucleation using the osteogenesis index (OI).

Methods

Under general anesthesia, 10 patients (12 lesions) underwent oral or maxillofacial surgery for cyst enucleation. Postoperatively, 12 lesions were divided into two groups: group A (six lesions) was treated with absorbable collagen sponge (ACS) in combination with rhBMP-2, and group B (six lesions) was treated with ACS alone. After 3 months, cone-beam computed tomographic scans were obtained to measure changes in the volume of the lesions. We then calculated the OI of each group at two different Hounsfield units to determine any statistically significant difference between these two groups (Mann–Whitney U test).

Results

As tested at the level of new bone, the mean OI was 72.37 % in group A and 55.08 % in group B —a statistically significant difference (p = 0.041). As tested at the level of mature bone, the mean OI was 27.47 % in group A and 18.88 % in group B, but the difference was not statistically significant (p = 0.394).

Conclusions

The application of rhBMP-2 after maxillofacial cyst enucleation accelerated new bone formation in the bone defects. Thus, the use of rhBMP-2 in combination with ACS may be considered an alternative to conventional bone grafting in some patients with postoperative bone defects.

Electronic supplementary material

The online version of this article (doi:10.1186/s40902-016-0070-4) contains supplementary material, which is available to authorized users.

Co‑delivery and controlled release of stromal cell‑derived factor‑1α chemically conjugated on collagen scaffolds enhances bone morphogenetic protein‑2‑driven osteogenesis in rats

There has been considerable focus in investigations on the delivery systems and clinical applications of bone morphogenetic protein-2 (BMP-2) for novel bone formation. However, current delivery systems require high levels of BMP-2 to exert a biological function. There are several concerns in using of high levels of BMP-2, including safety and the high cost of treatment. Therefore, the development of strategies to decrease the levels of BMP-2 required in these delivery systems is required. In our previous studies, a controlled-release system was developed, which used Traut's reagent and the cross-linker, 4-(N-maleimi-domethyl) cyclohexane-1-carboxylic acid 3-sulfo-N-hydroxysuccinimide ester sodium salt (Sulfo-SMCC), to chemically conjugate BMP-2 directly on collagen discs. In the current study, retention efficiency and release kinetics of stromal cell-derived factor-1α (SDF-1α) cross-linked on collagen scaffolds were detected. In addition, the osteogenic activity of SDF-1α and suboptimal doses of BMP-2 cross-linked on collagen discs following subcutaneous implantation in rats were evaluated. Independent two-tailed t-tests and one-way analysis of variance were used for analysis. In the present study, the controlled release of SDF-1α chemically conjugated on collagen scaffolds was demonstrated. By optimizing the concentrations of Traut's reagent and the Sulfo-SMCC cross-linker, a significantly higher level of SDF-1α was covalently retained on the collagen scaffold, compared with that retained using a physical adsorption method. Mesenchymal stem cell homing indicated that the biological function of the SDF-1α cross-linked on the collagen scaffolds remained intact. In rats, co-treatment with SDF-1α and a suboptimal dose of BMP-2 cross-linked on collagen scaffolds using this chemically conjugated method induced higher levels of ectopic bone formation, compared with the physical adsorption method. No ectopic bone formation was observed following treatment with a suboptimal dose of BMP-2 alone. Therefore, the co-delivery of SDF-1α and a suboptimal dose of BMP-2 chemically conjugated on collagen scaffolds for the treatment of bone injuries reduced the level of BMP-2 required, reducing the risks of side effects.

A Biphasic Calcium Sulphate/Hydroxyapatite Carrier Containing Bone Morphogenic Protein-2 and Zoledronic Acid Generates Bone

In orthopedic surgery, large amount of diseased or injured bone routinely needs to be replaced. Autografts are mainly used but their availability is limited. Commercially available bone substitutes allow bone ingrowth but lack the capacity to induce bone formation. Thus, off-the-shelf osteoinductive bone substitutes that can replace bone grafts are required. We tested the carrier properties of a biphasic, calcium sulphate and hydroxyapatite ceramic material, containing a combination of recombinant human bone morphogenic protein-2 (rhBMP-2) to induce bone, and zoledronic acid (ZA) to delay early resorption. In-vitro, the biphasic material released 90% of rhBMP-2 and 10% of ZA in the first week. No major changes were found in the surface structure using scanning electron microscopy (SEM) or in the mechanical properties after adding rhBMP-2 or ZA. In-vivo bone formation was studied in an abdominal muscle pouch model in rats (n = 6/group). The mineralized volume was significantly higher when the biphasic material was combined with both rhBMP-2 and ZA (21.4 ± 5.5 mm³) as compared to rhBMP-2 alone (10.9 ± 2.1 mm³) when analyzed using micro computed tomography (μ-CT) (p < 0.01). In the clinical setting, the biphasic material combined with both rhBMP-2 and ZA can potentially regenerate large volumes of bone.

Acellular hydroxyapatite-collagen scaffolds support angiogenesis and osteogenic gene expression in an ectopic murine model: Effects of hydroxyapatite volume fraction

Acellular hydroxyapatite (HA) reinforced collagen scaffolds were previously reported to induce angiogenesis and osteogenesis after ectopic implantation but the effect of the HA volume fraction was not investigated. Therefore, the objective of this study was to investigate the effect of HA volume fraction on in vivo angiogenesis and osteogenesis in acellular collagen scaffolds containing 0, 20, and 40 vol % HA after subcutaneous ectopic implantation for up to 12 weeks in mice. Endogenous cell populations were able to completely and uniformly infiltrate the entire scaffold within 6 weeks independent of the HA content, but the cell density was increased in scaffolds containing HA versus collagen alone. Angiogenesis, remodeling of the original scaffold matrix, mineralization, and osteogenic gene expression were evident in scaffolds containing HA, but were not observed in collagen scaffolds. Moreover, HA promoted a dose-dependent increase in measured vascular density, cell density, matrix deposition, and mineralization. Therefore, the results of this study suggest that HA promoted the recruitment and differentiation of endogenous cell populations to support angiogenic and osteogenic activity in collagen scaffolds after subcutaneous ectopic implantation. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2178-2188, 2016.

Demineralized dentin matrix combined with recombinant human bone morphogenetic protein-2 in rabbit calvarial defects

OBJECTIVES

The aim of this study was to compare the osteogenic effects of demineralized dentin matrix (DDM) combined with recombinant human bone morphogenetic protein-2 (rhBMP-2) in rabbit calvarial defects with DDM and anorganic bovine bone (ABB) combined with rhBMP-2.

MATERIALS AND METHODS

Four round defects with 8-mm diameters were created in each rabbit calvaria. Each defect was treated with one of the following: 1) DDM, 2) ABB/rhBMP-2, or 3) DDM/rhBMP-2. The rhBMP-2 was combined with DDM and ABB according to a stepwise dry and dip lyophilizing protocol. Histological and microcomputed tomography (µCT) analyses were performed to measure the amount of bone formation and bone volume after 2- and 8-week healing intervals.

RESULTS

Upon histological observation at two weeks, the DDM and ABB/rhBMP-2 groups showed osteoconductive bone formation, while the DDM/rhBMP-2 group showed osteoconductive and osteoinductive bone formation. New bone formation was higher in DDM/rhBMP-2, DDM and ABB decreasing order. The amounts of bone formation were very similar at two weeks; however, at eight weeks, the DDM/rhBMP-2 group showed a two-fold greater amount of bone formation compared to the DDM and ABB/rhBMP-2 groups. The µCT analysis showed markedly increased bone volume in the DDM/rhBMP-2 group at eight weeks compared with that of the DDM group. Notably, there was a slight decrease in bone volume in the ABB/rhBMP-2 group at eight weeks. There were no significant differences among the DDM, ABB/rhBMP-2, and DDM/rhBMP-2 groups at two or eight weeks.

CONCLUSION

Within the limitations of this study, DDM appears to be a suitable carrier for rhBMP-2 in orthotopic sites.

Evaluation of Functionalized Porous Titanium Implants for Enhancing Angiogenesis in Vitro

Implant constructs supporting angiogenesis are favorable for treating critically-sized bone defects, as ingrowth of capillaries towards the center of large defects is often insufficient. Consequently, the insufficient nutritional supply of these regions leads to impaired bone healing. Implants with specially designed angiogenic supporting geometry and functionalized with proangiogenic cytokines can enhance angiogenesis. In this study, Vascular Endothelial Growth Factor (VEGF) and High Mobility Group Box 1 (HMGB1) were used for incorporation into poly-ε-caprolactone (PCL)-coated porous titanium implants. Bioactivity of released factors and influence on angiogenesis of functionalized implants were evaluated using a migration assay and angiogenesis assays. Both implants released angiogenic factors, inducing migration of endothelial cells. Also, VEGF-functionalized PCL-coated titanium implants enhanced angiogenesis in vitro. Both factors were rapidly released in high doses from the implant coating during the first 72 h.

Response of human macrophages to wound matrices in vitro

Chronic wounds remain a major burden to the global healthcare system. Myriad wound matrices are commercially available but their mechanisms of action are poorly understood. Recent studies have shown that macrophages are highly influenced by their microenvironment, but it is not known how different biomaterials affect this interaction. Here, it was hypothesized that human macrophages respond differently to changes in biomaterial properties in vitro with respect to phenotype, including pro-inflammatory M1, anti-inflammatory M2a, known for facilitating extracellular matrix deposition and proliferation, and M2c, which has recently been associated with tissue remodeling. Using multiple donors, it was found that collagen scaffolds cross-linked with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide (EDC/NHS) promoted the least inflammatory phenotype in primary human macrophages compared with scaffolds cross-linked with formaldehyde or glutaraldehyde. Importantly, gene expression analysis trends were largely conserved between donors, especially TNFa (M1), CCL22 (M2a), and MRC1 (M2a). Then the response of primary and THP1 monocyte-derived macrophages to four commercially available wound matrices were compared-Integra Dermal Regeneration Template (Integra), PriMatrix Dermal Repair Scaffold (PriMatrix), AlloMend Acellular Dermal Matrix (AlloMend), and Oasis Wound Matrix (Oasis). Gene expression trends were different between primary and THP1 monocyte-derived macrophages for all six genes analyzed in this study. Finally, the behavior of primary macrophages cultured onto the wound matrices over time was analyzed. Integra and Oasis caused down-regulation of M2a markers CCL22 and TIMP3. PriMatrix caused up-regulation of TNFa (M1) and CD163 (M2c) and down-regulation of CCL22 and TIMP3 (both M2a). AlloMend caused up-regulation in CD163 (M2c). Lastly, Oasis promoted the largest increase in the combinatorial M1/M2 score, defined as the sum of M1 genes divided by the sum of M2 genes. This preliminary study suggested that biomaterials influenced the wound microenvironment to affect macrophage phenotype.

Microstructure and Mechanical Property of Glutaraldehyde-Treated Porcine Pulmonary Ligament

There is a significant need for fixed biological tissues with desired structural and material constituents for tissue engineering applications. Here, we introduce the lung ligament as a fixed biological material that may have clinical utility for tissue engineering. To characterize the lung tissue for potential clinical applications, we studied glutaraldehyde-treated porcine pulmonary ligament (n = 11) with multiphoton microscopy (MPM) and conducted biaxial planar experiments to characterize the mechanical property of the tissue. The MPM imaging revealed that there are generally two families of collagen fibers distributed in two distinct layers: The first family largely aligns along the longitudinal direction with a mean angle of θ = 10.7 ± 9.3 deg, while the second one exhibits a random distribution with a mean θ = 36.6 ± 27.4. Elastin fibers appear in some intermediate sublayers with a random orientation distribution with a mean θ = 39.6 ± 23 deg. Based on the microstructural observation, a microstructure-based constitutive law was proposed to model the elastic property of the tissue. The material parameters were identified by fitting the model to the biaxial stress-strain data of specimens, and good fitting quality was achieved. The parameter e0 (which denotes the strain beyond which the collagen can withstand tension) of glutaraldehyde-treated tissues demonstrated low variability implying a relatively consistent collagen undulation in different samples, while the stiffness parameters for elastin and collagen fibers showed relatively greater variability. The fixed tissues presented a smaller e0 than that of fresh specimen, confirming that glutaraldehyde crosslinking increases the mechanical strength of collagen-based biomaterials. The present study sheds light on the biomechanics of glutaraldehyde-treated porcine pulmonary ligament that may be a candidate for tissue engineering.

Synergistic influence of collagen I and BMP 2 drives osteogenic differentiation of mesenchymal stem cells: A cell microarray analysis

Cell microarrays are a novel platform for the high throughput discovery of new biomaterials. By re-creating a multitude of cell microenvironments on a single slide, this approach can identify the optimal surface composition to drive a desired cell response. To systematically study the effects of molecular microenvironments on stem cell fate, we designed a cell microarray based on parallel exposure of mesenchymal stem cells (MSCs) to surface-immobilised collagen I (Coll I) and bone morphogenetic protein 2 (BMP 2). This was achieved by means of a reactive coating on a slide surface, enabling the covalent anchoring of Coll I and BMP 2 as microscale spots printed by a robotic contact printer. The surface between the printed protein spots was passivated using poly (ethylene glycol) bisamine 10,000Da (A-PEG). MSCs were then captured and cultured on array spots composed of binary mixtures of Coll I and BMP 2, followed by automated image acquisition and quantitative, multi-parameter analysis of cellular responses. Surface compositions that gave the highest osteogenic differentiation were determined using Runx2 expression and calcium deposition. Quantitative single cell analysis revealed subtle concentration-dependent effects of surface-immobilised proteins on the extent of osteogenic differentiation obscured using conventional analysis. In particular, the synergistic interaction of Coll I and BMP 2 in supporting osteogenic differentiation was confirmed. Our studies demonstrate the value of cell microarray platforms to decipher the combinatorial interactions at play in stem cell niche microenvironments.

Beneficial Effect of Covalently Grafted α-MSH on Endothelial Release of Inflammatory Mediators for Applications in Implantable Devices

Intravascular devices for continuous glucose monitoring are promising tools for the follow up and treatment of diabetic patients. Limiting the inflammatory response to the implanted devices in order to achieve better biocompatibility is a critical challenge. Herein we report on the production and the characterization of gold surfaces covalently derivatized with the peptide α-alpha-melanocyte stimulating hormone (α-MSH), with a quantifiable surface density. In vitro study demonstrated that the tethered α-MSH is able to decrease the expression of an inflammatory cytokine produced by endothelial cells.

Chimeric Aptamer-Gelatin Hydrogels as an Extracellular Matrix Mimic for Loading Cells and Growth Factors

It is important to synthesize materials to recapitulate critical functions of biological systems for a variety of applications such as tissue engineering and regenerative medicine. The purpose of this study was to synthesize a chimeric hydrogel as a promising extracellular matrix (ECM) mimic using gelatin, a nucleic acid aptamer, and polyethylene glycol. This hydrogel had a macroporous structure that was highly permeable for fast molecular transport. Despite its high permeability, it could strongly sequester and sustainably release growth factors with high bioactivity. Notably, growth factors retained in the hydrogel could maintain ∼ 50% bioactivity during a 14-day release test. It also provided cells with effective binding sites, which led to high efficiency of cell loading into the macroporous hydrogel matrix. When cells and growth factors were coloaded into the chimeric hydrogel, living cells could still be observed by day 14 in a static serum-reduced culture condition. Thus, this chimeric aptamer-gelatin hydrogel constitutes a promising biomolecular ECM mimic for loading cells and growth factors.

Collagen Membranes Adsorb the Transforming Growth Factor-β Receptor I Kinase-Dependent Activity of Enamel Matrix Derivative

BACKGROUND

Enamel matrix derivative (EMD) and collagen membranes (CMs) are simultaneously applied in regenerative periodontal surgery. The aim of this study is to evaluate the ability of two CMs and a collagen matrix to adsorb the activity intrinsic to EMD that provokes transforming growth factor (TGF)-β signaling in oral fibroblasts.

METHODS

Three commercially available collagen products were exposed to EMD or recombinant TGF-β1, followed by vigorous washing. Oral fibroblasts were either seeded directly onto collagen products or were incubated with the respective supernatant. Expression of TGF-β target genes interleukin (IL)-11 and proteoglycan 4 (PRG4) was evaluated by real time polymerase chain reaction. Proteomic analysis was used to study the fraction of EMD proteins binding to collagen.

RESULTS

EMD or TGF-β1 provoked a significant increase of IL-11 and PRG4 expression of oral fibroblasts when seeded onto collagen products and when incubated with the respective supernatant. Gene expression was blocked by the TGF-β receptor I kinase inhibitor SB431542. Amelogenin bound most abundantly to gelatin-coated culture dishes. However, incubation of palatal fibroblasts with recombinant amelogenin did not alter expression of IL-11 and PRG4.

CONCLUSION

These in vitro findings suggest that collagen products adsorb a TGF-β receptor I kinase-dependent activity of EMD and make it available for potential target cells.

Recombinant Human Bone Morphogenetic Protein 9 (rhBMP9) Induced Osteoblastic Behavior on a Collagen Membrane Compared With rhBMP2

BACKGROUND

Bone morphogenetic protein 9 (BMP9) has previously been characterized as one of the most osteogenic growth factors of the BMP family. To the best of the authors' knowledge, previous experiments have only used adenovirus transfection (gene therapy). With the recent development of recombinant human BMP9 (rhBMP9), the present study investigates the osteopromotive potential of BMP9 versus rhBMP2 when loaded onto collagen membranes.

METHODS

ST2 stromal bone marrow cells were seeded onto: 1) control; 2) low-dose rhBMP2 (10 ng/mL); 3) high-dose rhBMP2 (100 ng/mL); 4) low-dose rhBMP9 (10 ng/mL); and 5) high-dose rhBMP9 (100 ng/mL) porcine collagen membranes. The following parameters were compared among groups: 1) cell adhesion (at 8 hours); 2) cell proliferation (at 1, 3, and 5 days); 3) real-time polymerase chain reaction for genes encoding runt-related transcription factor 2; 4) alkaline phosphatase (ALP); 5) bone sialoprotein ([BSP] at 3 and 14 days); and 6) alizarin red staining (at 14 days).

RESULTS

rhBMP2 and rhBMP9 demonstrated little effect on cell attachment and proliferation; however, pronounced increases were observed in osteoblast differentiation. All groups significantly induced ALP messenger RNA (mRNA) levels at 3 days and BSP levels at 14 days; however, high-dose rhBMP9 showed significantly higher values compared with all other groups for ALP levels (five-fold increase at 3 days and two-fold increase at 14 days). Alizarin red staining further revealed both concentrations of rhBMP9 induced up to three-fold more staining compared with rhBMP2.

CONCLUSIONS

Results indicate that the combination of collagen membranes with rhBMP9 induced significantly higher ALP mRNA expression and alizarin red staining compared with rhBMP2. These findings suggest that rhBMP9 may be a suitable growth factor for future regenerative procedures in bone biology.

A review on carrier systems for bone morphogenetic protein-2

Bone morphogenetic protein-2 (BMP-2) has unique bone regeneration property. The powerful osteoinductive nature makes it considered as second line of therapy in nonunion bone defect. A large number of carriers and delivery systems made up of different materials have been investigated for controlled and sustained release of BMP-2. The delivery systems are in the form of hydrogel, microsphere, nanoparticles, and fibers. The carriers used for the delivery are made up of metals, ceramics, polymers, and composites. Implantation of these protein-loaded carrier leads to cell adhesion, degradation which eventually releases the drug/protein at site specific. But, problems like ectopic growth, lesser protein delivery, inactivation of the protein are reported in the available carrier systems. Therefore, it is need of an hour to modify the available carrier systems as well as explore other biomaterials with desired properties. In this review, all the reported carrier systems made of metals, ceramics, polymers, composites are evaluated in terms of their processing conditions, loading capacity and release pattern of BMP-2. Along with these biomaterials, the attempts of protein modification by adding some functional group to BMP-2 or extracting functional peptides from the protein to achieve the desired effect, is also evaluated. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 904-925, 2017.

Self-assembly of dual drug-delivery coating for synergistic bone regeneration

Bone regeneration for the treatment of bone diseases represents a major clinical need.

3D printed structures for delivery of biomolecules and cells: tissue repair and regeneration

This paper reviews the current approaches to using 3D printed structures to deliver bioactive factors (e.g., cells and biomolecules) for tissue repair and regeneration.

Biomaterial strategies for controlling stem cell fate via morphogen sequestration

This review explores the role of protein sequestration in the stem cell niche and how it has inspired the design of biomaterials that exploit natural protein sequestration to influence stem cell fate.

Poly-ε-caprolactone Coated and Functionalized Porous Titanium and Magnesium Implants for Enhancing Angiogenesis in Critically Sized Bone Defects

For healing of critically sized bone defects, biocompatible and angiogenesis supporting implants are favorable. Murine osteoblasts showed equal proliferation behavior on the polymers poly-ε-caprolactone (PCL) and poly-(3-hydroxybutyrate)/poly-(4-hydroxybutyrate) (P(3HB)/P(4HB)). As vitality was significantly better for PCL, it was chosen as a suitable coating material for further experiments. Titanium implants with 600 µm pore size were evaluated and found to be a good implant material for bone, as primary osteoblasts showed a vitality and proliferation onto the implants comparable to well bottom (WB). Pure porous titanium implants and PCL coated porous titanium implants were compared using Live Cell Imaging (LCI) with Green fluorescent protein (GFP)-osteoblasts. Cell count and cell covered area did not differ between the implants after seven days. To improve ingrowth of blood vessels into porous implants, proangiogenic factors like Vascular Endothelial Growth Factor (VEGF) and High Mobility Group Box 1 (HMGB1) were incorporated into PCL coated, porous titanium and magnesium implants. An angiogenesis assay was performed to establish an in vitro method for evaluating the impact of metallic implants on angiogenesis to reduce and refine animal experiments in future. Incorporated concentrations of proangiogenic factors were probably too low, as they did not lead to any effect. Magnesium implants did not yield evaluable results, as they led to pH increase and subsequent cell death.

Bone-tissue engineering: complex tunable structural and biological responses to injury, drug delivery, and cell-based therapies

Bone loss and failure of proper bone healing continues to be a significant medical condition in need of solutions that can be implemented successfully both in human and veterinary medicine. This is particularly true when large segmental defects are present, the bone has failed to return to normal form or function, or the healing process is extremely prolonged. Given the inherent complexity of bone tissue - its unique structural, mechanical, and compositional properties, as well as its ability to support various cells - it is difficult to find ideal candidate materials that could be used as the foundation for tissue regeneration from technological platforms. Recently, important developments have been made in the implementation of complex structures built both at the macro- and the nano-level that have been shown to positively impact bone formation and to have the ability to deliver active biological molecules (drugs, growth factors, proteins, cells) for controlled tissue regeneration and the prevention of infection. These materials are diverse, ranging from polymers to ceramics and various composites. This review presents developments in this area with a focus on the role of scaffold structure and chemistry on the biologic processes that influence bone physiology and regeneration.

Tuning cellular responses to BMP-2 with material surfaces

Bone morphogenetic protein 2 (BMP-2) has been known for decades as a strong osteoinductive factor and for clinical applications is combined solely with collagen as carrier material. The growing concerns regarding side effects and the importance of BMP-2 in several developmental and physiological processes have raised the need to improve the design of materials by controlling BMP-2 presentation. Inspired by the natural cell environment, new material surfaces have been engineered and tailored to provide both physical and chemical cues that regulate BMP-2 activity. Here we describe surfaces designed to present BMP-2 to cells in a spatially and temporally controlled manner. This is achieved by trapping BMP-2 using physicochemical interactions, either covalently grafted or combined with other extracellular matrix components. In the near future, we anticipate that material science and biology will integrate and further develop tools for in vitro studies and potentially bring some of them toward in vivo applications.

An in vivo model to assess magnesium alloys and their biological effect on human bone marrow stromal cells

Magnesium (Mg) alloys have many unique qualities which make them ideal candidates for bone fixation devices, including biocompatibility and degradation in vivo. Despite a rise in Mg alloy production and research, there remains no standardized system to assess their degradation or biological effect on human stem cells in vivo. In this study, we developed a novel in vivo model to assess Mg alloys for craniofacial and orthopedic applications. Our model consists of a collagen sponge seeded with human bone marrow stromal cells (hBMSCs) around a central Mg alloy rod. These scaffolds were implanted subcutaneously in mice and analyzed after eight weeks. Alloy degradation and biological effect were determined by microcomputed tomography (microCT), histological staining, and immunohistochemistry (IHC). MicroCT showed greater volume loss for pure Mg compared to AZ31 after eight weeks in vivo. Histological analysis showed that hBMSCs were retained around the Mg implants after 8 weeks. Furthermore, immunohistochemistry showed the expression of dentin matrix protein 1 and osteopontin around both pure Mg and AZ31 with implanted hBMSCs. In addition, histological sections showed a thin mineral layer around all degrading alloys at the alloy-tissue interface. In conclusion, our data show that degrading pure Mg and AZ31 implants are cytocompatible and do not inhibit the osteogenic property of hBMSCs in vivo. These results demonstrate that this model can be used to efficiently assess the biological effect of corroding Mg alloys in vivo. Importantly, this model may be modified to accommodate additional cell types and clinical applications.

STATEMENT OF SIGNIFICANCE

Magnesium (Mg) alloys have been investigated as ideal candidates for bone fixation devices due to high biocompatibility and degradation in vivo, and there is a growing need of establishing an efficient in vivo material screening system. In this study, we assessed degradation rate and biological effect of Mg alloys by transplanting Mg alloy rod with human bone marrow stromal cells seeded on collagen sponge subcutaneously in mice. After 8 weeks, samples were analyzed by microcomputed tomography and histological staining. Our data show that degrading Mg alloys are cytocompatible and do not inhibit the osteogenic property of hBMSCs in vivo. These results demonstrate that this model can be used to efficiently assess the biological effect of corroding Mg alloys in vivo.

Chitosan-Coated Collagen Membranes Promote Chondrocyte Adhesion, Growth, and Interleukin-6 Secretion

Designing scaffolds made from natural polymers may be highly attractive for tissue engineering strategies. We sought to produce and characterize chitosan-coated collagen membranes and to assess their efficacy in promoting chondrocyte adhesion, growth, and cytokine secretion. Porous collagen membranes were placed in chitosan solutions then crosslinked with glutaraldehyde vapor. Fourier transform infrared (FTIR) analyses showed elevated absorption at 1655 cm-1 of the carbon-nitrogen (N=C) bonds formed by the reaction between the (NH₂) of the chitosan and the (C=O) of the glutaraldehyde. A significant peak in the amide II region revealed a significant deacetylation of the chitosan. Scanning electron microscopy (SEM) images of the chitosan-coated membranes exhibited surface variations, with pore size ranging from 20 to 50 µm. X-ray photoelectron spectroscopy (XPS) revealed a decreased C-C groups and an increased C-N/C-O groups due to the reaction between the carbon from the collagen and the NH2 from the chitosan. Increased rigidity of these membranes was also observed when comparing the chitosan-coated and uncoated membranes at dried conditions. However, under wet conditions, the chitosan coated collagen membranes showed lower rigidity as compared to dried conditions. Of great interest, the glutaraldehyde-crosslinked chitosan-coated collagen membranes promoted chondrocyte adhesion, growth, and interleukin (IL)-6 secretion. Overall results confirm the feasibility of using designed chitosan-coated collagen membranes in future applications, such as cartilage repair.

Improvement of the fracture toughness of hydroxyapatite (HAp) by incorporation of carboxyl functionalized single walled carbon nanotubes (CfSWCNTs) and nylon

The potential of improving the fracture toughness of synthetic hydroxyapatite (HAp) by incorporating carboxyl functionalized single walled carbon nanotubes (CfSWCNTs) and polymerized ε-caprolactam (nylon) was studied. A series of HAp samples with CfSWCNTs concentrations varying from 0 to 1.5 wt.%, without, and with nylon addition was prepared. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) were used to characterize the samples. The three point bending test was applied to measure the fracture toughness of the composites. A reproducible value of 3.6±0.3 MPa.√m was found for samples containing 1 wt.% CfSWCNTs and nylon. This value is in the range of the cortical bone fracture toughness. Increase of the CfSWCNTs content results to decrease of the fracture toughness, and formation of secondary phases.

Nanostructured hydroxyapatite surfaces-mediated adsorption alters recognition of BMP receptor IA and bioactivity of bone morphogenetic protein-2

Highly efficient loading of bone morphogenetic protein-2 (BMP-2) onto carriers with desirable performance is still a major challenge in the field of bone regeneration. Till now, the nanoscaled surface-induced changes of the structure and bioactivity of BMP-2 remains poorly understood. Here, the effect of nanoscaled surface on the adsorption and bioactivity of BMP-2 was investigated with a series of hydroxyapatite surfaces (HAPs): HAP crystal-coated surface (HAP), HAP crystal-coated polished surface (HAP-Pol), and sintered HAP crystal-coated surface (HAP-Sin). The adsorption dynamics of recombinant human BMP-2 (rhBMP-2) and the accessibility of the binding epitopes of adsorbed rhBMP-2 for BMP receptors (BMPRs) were examined by a quartz crystal microbalance with dissipation. Moreover, the bioactivity of adsorbed rhBMP-2 and the BMP-induced Smad signaling were investigated with C2C12 model cells. A noticeably high mass-uptake of rhBMP-2 and enhanced recognition of BMPR-IA to adsorbed rhBMP-2 were found on the HAP-Pol surface. For the rhBMP-2-adsorbed HAPs, both ALP activity and Smad signaling increased in the order of HAP-Sin<HAP<HAP-Pol. Furthermore, hybrid molecular dynamics and steered molecular dynamics simulations validated that BMP-2 tightly anchored on the HAP-Pol surface with a relative loosened conformation, but the HAP-Sin surface induced a compact conformation of BMP-2. In conclusion, the nanostructured HAPs can modulate the way of adsorption of rhBMP-2, and thus the recognition of BMPR-IA and the bioactivity of rhBMP-2. These findings can provide insightful suggestions for the future design and fabrication of rhBMP-2-based scaffolds/implants.

STATEMENT OF SIGNIFICANCE

This study provides strong evidences that nanoscaled HAPs yield extraordinary influence on the adsorption behaviors and bioactivity of rhBMP-2. It has been found that the surface roughness and crystallinity played a crucial role in governing the way of rhBMP-2 binding to HAPs, and thus the conformation, recognition of BMPR-IA and bioactivity of adsorbed rhBMP-2. It is also for the first time to correlate numerical modeling and experimental results of the bioactivity of rhBMP-2 on nanostructured HAPs. This work can pave an avenue for the wider uses of rhBMP-2 in clinical applications and arouse broad interests among researchers in the fields of nano-biotechnology, biomaterials and bone tissue engineering.

Fibrous proteins: At the crossroads of genetic engineering and biotechnological applications

Fibrous proteins, such as silk, elastin and collagen are finding broad impact in biomaterial systems for a range of biomedical and industrial applications. Some of the key advantages of biosynthetic fibrous proteins compared to synthetic polymers include the tailorability of sequence, protein size, degradation pattern, and mechanical properties. Recombinant DNA production and precise control over genetic sequence of these proteins allows expansion and fine tuning of material properties to meet the needs for specific applications. We review current approaches in the design, cloning, and expression of fibrous proteins, with a focus on strategies utilized to meet the challenges of repetitive fibrous protein production. We discuss recent advances in understanding the fundamental basis of structure-function relationships and the designs that foster fibrous protein self-assembly towards predictable architectures and properties for a range of applications. We highlight the potential of functionalization through genetic engineering to design fibrous protein systems for biotechnological and biomedical applications.

Synergistic Effects of SDF-1α and BMP-2 Delivery from Proteolytically Degradable Hyaluronic Acid Hydrogels for Bone Repair

In order to achieve bone repair, bone morphogenetic protein-2 (BMP-2) is typically delivered in non-physiological doses and can result in significant adverse side effects. To reduce the amount of BMP-2 necessary for bone formation, we delivered a known chemokine (stromal cell derived factor-1α, SDF-1α) in combination with BMP-2 using proteolytically degradable hydrogels. A critical-sized calvarial defect was used to determine the effect of biomolecule delivery on bone formation in vivo. The treatment group with combined SDF-1α and BMP-2 hydrogel delivery showed significantly higher bone formation when compared to hydrogels loaded with the same BMP-2 or SDF-1α concentrations alone, suggesting the combined delivery of both biomolecules synergistically improves osteogenesis.

Biodegradable Materials for Bone Repair and Tissue Engineering Applications

This review discusses and summarizes the recent developments and advances in the use of biodegradable materials for bone repair purposes. The choice between using degradable and non-degradable devices for orthopedic and maxillofacial applications must be carefully weighed. Traditional biodegradable devices for osteosynthesis have been successful in low or mild load bearing applications. However, continuing research and recent developments in the field of material science has resulted in development of biomaterials with improved strength and mechanical properties. For this purpose, biodegradable materials, including polymers, ceramics and magnesium alloys have attracted much attention for osteologic repair and applications. The next generation of biodegradable materials would benefit from recent knowledge gained regarding cell material interactions, with better control of interfacing between the material and the surrounding bone tissue. The next generations of biodegradable materials for bone repair and regeneration applications require better control of interfacing between the material and the surrounding bone tissue. Also, the mechanical properties and degradation/resorption profiles of these materials require further improvement to broaden their use and achieve better clinical results.