Biomaterial delivery of morphogens to mimic the natural healing cascade in bone

Sequential release of nanoparticle payloads from ultrasonically burstable capsules

In many biomedical contexts ranging from chemotherapy to tissue engineering, it is beneficial to sequentially present bioactive payloads. Explicit control over the timing and dose of these presentations is highly desirable. Here, we present a capsule-based delivery system capable of rapidly releasing multiple payloads in response to ultrasonic signals. In vitro, these alginate capsules exhibited excellent payload retention for up to 1 week when unstimulated and delivered their entire payloads when ultrasonically stimulated for 10-100 s. Shorter exposures (10 s) were required to trigger delivery from capsules embedded in hydrogels placed in a tissue model and did not result in tissue heating or death of encapsulated cells. Different types of capsules were tuned to rupture in response to different ultrasonic stimuli, thus permitting the sequential, on-demand delivery of nanoparticle payloads. As a proof of concept, gold nanoparticles were decorated with bone morphogenetic protein-2 to demonstrate the potential bioactivity of nanoparticle payloads. These nanoparticles were not cytotoxic and induced an osteogenic response in mouse mesenchymal stem cells. This system may enable researchers and physicians to remotely regulate the timing, dose, and sequence of drug delivery on-demand, with a wide range of clinical applications ranging from tissue engineering to cancer treatment.

Effects of rhBMP-2 on Sandblasted and Acid Etched Titanium Implant Surfaces on Bone Regeneration and Osseointegration: Spilt-Mouth Designed Pilot Study

This study was conducted to evaluate effects of rhBMP-2 applied at different concentrations to sandblasted and acid etched (SLA) implants on osseointegration and bone regeneration in a bone defect of beagle dogs as pilot study using split-mouth design. Methods. For experimental groups, SLA implants were coated with different concentrations of rhBMP-2 (0.1, 0.5, and 1 mg/mL). After assessment of surface characteristics and rhBMP-2 releasing profile, the experimental groups and untreated control groups (n = 6 in each group, two animals in each group) were placed in split-mouth designed animal models with buccal open defect. At 8 weeks after implant placement, implant stability quotients (ISQ) values were recorded and vertical bone height (VBH, mm), bone-to-implant contact ratio (BIC, %), and bone volume (BV, %) in the upper 3 mm defect areas were measured. Results. The ISQ values were highest in the 1.0 group. Mean values of VBH (mm), BIC (%), and BV (%) were greater in the 0.5 mg/mL and 1.0 mg/mL groups than those in 0.1 and control groups in buccal defect areas. Conclusion. In the open defect area surrounding the SLA implant, coating with 0.5 and 1.0 mg/mL concentrations of rhBMP-2 was more effective, compared with untreated group, in promoting bone regeneration and osseointegration.

The scope and sequence of growth factor delivery for vascularized bone tissue regeneration

Bone regeneration is a complex process, that in vivo, requires the highly coordinated presentation of biochemical cues to promote the various stages of angiogenesis and osteogenesis. Taking inspiration from the natural healing process, a wide variety of growth factors are currently being released within next generation tissue engineered scaffolds (in a variety of ways) in order to heal non-union fractures and bone defects. This review will focus on the delivery of multiple growth factors to the bone regeneration niche, specifically 1) dual growth factor delivery signaling and crosstalk, 2) the importance of growth factor timing and temporal separation, and 3) the engineering of delivery systems that allow for temporal control over presentation of soluble growth factors. Alternative methods for growth factor presentation, including the use of gene therapy and platelet-rich plasma scaffolds, are also discussed.

From Skeletal Development to Tissue Engineering: Lessons from the Micromass Assay

Damage and degeneration of the skeletal elements due to disease, trauma, and aging lead to a significant health and economical burden. To reduce this burden, skeletal tissue engineering strategies aim to regenerate functional bone and cartilage in the adult body. However, challenges still exist. Such challenges involve the identification of the external cues that determine differentiation, how to control chondrocyte hypertrophy, and how to achieve specific tissue patterns and boundaries. To address these issues, it could be insightful to look at skeletal development, a robust morphogenetic process that takes place during embryonic development and is commonly modeled in vitro by the micromass assay. In this review, we investigate what the tissue engineering field can learn from this assay. By comparing embryonic skeletal precursor cells from different anatomic locations and developmental stages in micromass, the external cues that guide lineage commitment can be identified. The signaling pathways regulating chondrocyte hypertrophy, and the cues required for tissue patterning, can be elucidated by combining the micromass assay with genetic, molecular, and engineering tools. The lessons from the micromass assay are limited by two major differences between developmental and regenerative skeletogenesis: cell type and scale. We highlight an important difference between embryonic and adult skeletal progenitor cells, in that adult progenitors are not able to form mesenchymal condensations spontaneously. Also, the mechanisms of tissue patterning need to be adjusted to the larger tissue engineering constructs. In conclusion, mechanistic insights of skeletal tissue generation gained from the micromass model could lead to improved tissue engineering strategies and constructs.

Physicochemical perspective on "polydopamine" and "poly(catecholamine)" films for their applications in biomaterial coatings

Bioinspired poly(catecholamine) based coatings, mostly "polydopamine," were conceived based on the chemistry used by mussels to adhere strongly to the surface of stones and wood in water and to remain attached to their substrates even under conditions of strong shear stresses. These kinds of films can in turn be easily modified with a plethora of molecules and inorganic (nano)materials. This review shows that poly(catecholamine) based coatings are an ideal film forming method for applications in the field of biomaterials. It is written from a physicochemical and a materials science perspective and discusses optical, chemical, electrochemical, and mechanical properties of polydopamine films. It further demonstrates that a better understanding of the polydopamine film deposition mechanism is warranted to improve the properties of these coatings even further.

Towards the design of 3D multiscale instructive tissue engineering constructs: Current approaches and trends

The design of 3D constructs with adequate properties to instruct and guide cells both in vitro and in vivo is one of the major focuses of tissue engineering. Successful tissue regeneration depends on the favorable crosstalk between the supporting structure, the cells and the host tissue so that a balanced matrix production and degradation are achieved. Herein, the major occurring events and players in normal and regenerative tissue are overviewed. These have been inspiring the selection or synthesis of instructive cues to include into the 3D constructs. We further highlight the importance of a multiscale perception of the range of features that can be included on the biomimetic structures. Lastly, we focus on the current and developing tissue-engineering approaches for the preparation of such 3D constructs: top-down, bottom-up and integrative. Bottom-up and integrative approaches present a higher potential for the design of tissue engineering devices with multiscale features and higher biochemical control than top-down strategies, and are the main focus of this review.

The collagen I mimetic peptide DGEA enhances an osteogenic phenotype in mesenchymal stem cells when presented from cell-encapsulating hydrogels

Interactions between cells and the extracellular matrix (ECM) are known to play critical roles in regulating cell phenotype. The identity of ECM ligands presented to mesenchymal stem cells (MSCs) has previously been shown to direct the cell fate commitment of these cells. To enhance osteogenic differentiation of MSCs, alginate hydrogels were prepared that present the DGEA ligand derived from collagen I. When presented from hydrogel surfaces in 2D, the DGEA ligand did not facilitate cell adhesion, while hydrogels presenting the RGD ligand derived from fibronectin did encourage cell adhesion and spreading. However, the osteogenic differentiation of MSCs encapsulated within alginate hydrogels presenting the DGEA ligand was enhanced when compared with unmodified alginate hydrogels and hydrogels presenting the RGD ligand. MSCs cultured in DGEA-presenting gels exhibited increased levels of osteocalcin production and mineral deposition. These data suggest that the presentation of the collagen I-derived DGEA ligand is a feasible approach for selectively inducing an osteogenic phenotype in encapsulated MSCs.

Longitudinal in vivo evaluation of bone regeneration by combined measurement of multi-pinhole SPECT and micro-CT for tissue engineering

Over the last decades, great strides were made in the development of novel implants for the treatment of bone defects. The increasing versatility and complexity of these implant designs request for concurrent advances in means to assess in vivo the course of induced bone formation in preclinical models. Since its discovery, micro-computed tomography (micro-CT) has excelled as powerful high-resolution technique for non-invasive assessment of newly formed bone tissue. However, micro-CT fails to provide spatiotemporal information on biological processes ongoing during bone regeneration. Conversely, due to the versatile applicability and cost-effectiveness, single photon emission computed tomography (SPECT) would be an ideal technique for assessing such biological processes with high sensitivity and for nuclear imaging comparably high resolution (<1 mm). Herein, we employ modular designed poly(ethylene glycol)-based hydrogels that release bone morphogenetic protein to guide the healing of critical sized calvarial bone defects. By combined in vivo longitudinal multi-pinhole SPECT and micro-CT evaluations we determine the spatiotemporal course of bone formation and remodeling within this synthetic hydrogel implant. End point evaluations by high resolution micro-CT and histological evaluation confirm the value of this approach to follow and optimize bone-inducing biomaterials.

Regulation of integrin and growth factor signaling in biomaterials for osteodifferentiation

Stem cells respond to the microenvironment (niche) they are located in. Under natural conditions, the extracellular matrix (ECM) is the essential component the in stem cell niche, in which both integrin ligands and growth factors are important regulators to directly or indirectly modulate the cell behavior. In this review, we summarize the current knowledge about the potential of integrin ligands and growth factors to induce osteogenic differentiation of stem cells, and discuss the signaling pathways that are initiated by both individual and cooperative parameters. The joint effect of integrin ligands and growth factors is highlighted as the multivalent interactions for bone therapy.

Effects of Immobilized BMP-2 and Nanofiber Morphology on In Vitro Osteogenic Differentiation of hMSCs and In Vivo Collagen Assembly of Regenerated Bone

Engineering bone tissue is particularly challenging because of the distinctive structural features of bone within a complex biochemical environment. In the present study, we fabricated poly(L-lactic acid) (PLLA) electrospun nanofibers with random and aligned morphology immobilized with bone morphogenic protein-2 (BMP-2) and investigated how these signals modulate (1) in vitro osteogenic differentiation of human mesenchymal stem cells (hMSCs) and (2) in vivo bone growth rate, mechanical properties, and collagen assembly of newly formed bone. The orientation of adherent cells followed the underlying nanofiber morphology; however, nanofiber alignment did not show any difference in alkaline phosphate activity or in calcium mineralization of hMSCs after 14 days of in vitro culture in osteogenic differentiation media. In vivo bone regeneration was significantly higher in the nanofiber implanted groups (approximately 65-79%) as compared to the defect-only group (11.8 ± 0.2%), while no significant difference in bone regeneration was observed between random and aligned groups. However, nanoindentation studies of regenerated bone revealed Young's modulus and contact hardness with anisotropic feature for aligned group as compared to random group. More importantly, structural analysis of collagen at de novo bone showed the ability of nanofiber morphology to guide collagen deposition. SEM and TEM images revealed regular, highly ordered collagen assemblies on aligned nanofibers as compared to random fibers, which showed irregular, randomly organized collagen deposition. Taken together, we conclude that nanofibers in the presence of osteoinductive signals are a potent tool for bone regeneration, and nanofiber alignment can be used for engineering bone tissues with structurally assembled collagen fibers with defined direction.

Biomimetic approaches in bone tissue engineering: Integrating biological and physicomechanical strategies

The development of responsive biomaterials capable of demonstrating modulated function in response to dynamic physiological and mechanical changes in vivo remains an important challenge in bone tissue engineering. To achieve long-term repair and good clinical outcomes, biologically responsive approaches that focus on repair and reconstitution of tissue structure and function through drug release, receptor recognition, environmental responsiveness and tuned biodegradability are required. Traditional orthopedic materials lack biomimicry, and mismatches in tissue morphology, or chemical and mechanical properties ultimately accelerate device failure. Multiple stimuli have been proposed as principal contributors or mediators of cell activity and bone tissue formation, including physical (substrate topography, stiffness, shear stress and electrical forces) and biochemical factors (growth factors, genes or proteins). However, optimal solutions to bone regeneration remain elusive. This review will focus on biological and physicomechanical considerations currently being explored in bone tissue engineering.

Spatial regulation of controlled bioactive factor delivery for bone tissue engineering

Limitations of current treatment options for critical size bone defects create a significant clinical need for tissue engineered bone strategies. This review describes how control over the spatiotemporal delivery of growth factors, nucleic acids, and drugs and small molecules may aid in recapitulating signals present in bone development and healing, regenerating interfaces of bone with other connective tissues, and enhancing vascularization of tissue engineered bone. State-of-the-art technologies used to create spatially controlled patterns of bioactive factors on the surfaces of materials, to build up 3D materials with patterns of signal presentation within their bulk, and to pattern bioactive factor delivery after scaffold fabrication are presented, highlighting their applications in bone tissue engineering. As these techniques improve in areas such as spatial resolution and speed of patterning, they will continue to grow in value as model systems for understanding cell responses to spatially regulated bioactive factor signal presentation in vitro, and as strategies to investigate the capacity of the defined spatial arrangement of these signals to drive bone regeneration in vivo.

Strategies for controlled delivery of biologics for cartilage repair

The delivery of biologics is an important component in the treatment of osteoarthritis and the functional restoration of articular cartilage. Numerous factors have been implicated in the cartilage repair process, but the uncontrolled delivery of these factors may not only reduce their full reparative potential but can also cause unwanted morphological effects. It is therefore imperative to consider the type of biologic to be delivered, the method of delivery, and the temporal as well as spatial presentation of the biologic to achieve the desired effect in cartilage repair. Additionally, the delivery of a single factor may not be sufficient in guiding neo-tissue formation, motivating recent research toward the delivery of multiple factors. This review will discuss the roles of various biologics involved in cartilage repair and the different methods of delivery for appropriate healing responses. A number of spatiotemporal strategies will then be emphasized for the controlled delivery of single and multiple bioactive factors in both in vitro and in vivo cartilage tissue engineering applications.

A glass polyalkenoate cement carrier for bone morphogenetic proteins

This work considers a glass polyalkenoate cement (GPC)-based carrier for the effective delivery of bone morphogenetic proteins (BMPs) at an implantation site. A 0.12 CaO-0.04 SrO-0.36 ZnO-0.48 SiO2 based glass and poly(acrylic acid) (PAA, Mw 213,000) were employed for the fabrication of the GPC. The media used for the water source in the GPC reaction was altered to produce a series of GPCs. The GPC liquid media was either 100 % distilled water with additions of albumin at 0, 2, 5 and 8 wt% of the glass content, 100 % formulation buffer (IFB), and 100 % BMP (150 µg rhBMP-2/ml IFB). Rheological properties, compressive strength, ion release profiles and BMP release were evaluated. Working times (Tw) of the formulated GPCs significantly increased with the addition of 2 % albumin and remained constant with further increases in albumin content or IFB solutions. Setting time (Ts) experienced an increase with 2 and 5 % albumin content, but a decrease with 8 % albumin. Changing the liquid source to IFB containing 5 % albumin had no significant effect on Ts compared to the 8 % albumin-containing BT101. Replacing the albumin with IFB/BMP-2 did not significantly affect Tw. However, Ts increased for the BT101_BMP-2 containing GPCs, compared to all other samples. The compressive strength evaluated 1 day post cement mixing was not affected significantly by the incorporation of BMPs, but the ion release did increase from the cements, particularly for Zn and Sr. The GPCs released BMP after the first day, which decreased in content during the following 6 days. This study has proven that BMPs can be immobilized into GPCs and may result in novel materials for clinical applications.

Stromal cells and stem cells in clinical bone regeneration

Stem-cell-mediated bone repair has been used in clinical trials for the regeneration of large craniomaxillofacial defects, to slow the process of bone degeneration in patients with osteonecrosis of the femoral head and for prophylactic treatment of distal tibial fractures. Successful regenerative outcomes in these investigations have provided a solid foundation for wider use of stromal cells in skeletal repair therapy. However, employing stromal cells to facilitate or enhance bone repair is far from being adopted into clinical practice. Scientific, technical, practical and regulatory obstacles prevent the widespread therapeutic use of stromal cells. Ironically, one of the major challenges lies in the limited understanding of the mechanisms via which transplanted cells mediate regeneration. Animal models have been used to provide insight, but these models largely fail to reproduce the nuances of human diseases and bone defects. Consequently, the development of targeted approaches to optimize cell-mediated outcomes is difficult. In this Review, we highlight the successes and challenges reported in several clinical trials that involved the use of bone-marrow-derived mesenchymal or adipose-tissue-derived stromal cells. We identify several obstacles blocking the mainstream use of stromal cells to enhance skeletal repair and highlight technological innovations or areas in which novel techniques might be particularly fruitful in continuing to advance the field of skeletal regenerative medicine.

In vitro and in vivo evaluation of the inflammatory potential of various nanoporous hydroxyapatite biomaterials

Aim: To discriminate the most important physicochemical parameters for bone reconstruction, the inflammatory potential of seven nanoporous hydroxyapatite powders synthesized by hard or soft templating was evaluated both in vitro and in vivo. Materials & methods: After physical and chemical characterization of the powders, we studied the production of inflammatory mediators by human primary monocytes after 4 and 24 h in contact with powders, and the host response after 2 weeks implantation in a mouse critical size defect model. Results: In vitro results highlighted increases in the secretion of TNF-α, IL-1, -8, -10 and proMMP-2 and -9 and decreases in the secretion of IL-6 only for powders prepared by hard templating. In vivo observations confirmed an extensive inflammatory tissue reaction and a strong resorption for the most inflammatory powder in vitro. Conclusion: These findings highlight that the most critical physicochemical parameters for these nanoporous hydroxyapatite are, the crystallinity that controls dissolution potential, the specific surface area and the size and shape of crystallites.

Multifunctional Self-Assembled Films for Rapid Hemostat and Sustained Anti-infective Delivery

Uncontrolled bleeding and infection are the major causes of death and morbidity from traumatic wounds during military conflicts, disasters, and accidents. Because immediate treatment is critical to survival, it is desirable to have a lightweight and rapidly applicable bandage-one capable of delivering a hemostat that can quickly resolve bleeding while addressing infection over short and longer time frames. It is challenging to design thin film coatings capable of multidrug release, particularly when the drugs are quite different in nature (biologic versus small molecule, charged versus neutral) and the desired release profiles are different for each drug. Herein we have adopted a layer-by-layer film assembly technique to create a linear combination of two independently functional films capable of rapidly releasing thrombin within minutes while sustaining vancomycin elution for more than 24 h. By conjugating vancomycin to a hydrolytically degradable polyacid, poly(β-L-malic acid), we were able to create a robust thin film with loading and release kinetics that remain unaffected by the additional deposition of a thrombin-based film, demonstrating the possibility for future multitherapeutic films with independently tunable release kinetics.

A mild one-pot process for synthesising hydroxyapatite/biomolecule bone scaffolds for sustained and controlled antibiotic release

The release of active molecules or the control of nosocomial infections for improved osteoinduction is ideally addressed by a bone substitute material. For this purpose, the feasibility of a mild one-pot process is probed for incorporating directly active proteins and antibiotics in a hydroxyapatite (HAp) based scaffold. The effect of two serum model proteins, bovine serum albumin (BSA) and fibrinogen (FIB), on the microstructure, on selected mechanical properties as well as on degradation behaviour and on protein release are investigated. By protein incorporation, the porosity can be adjusted between 54 and 70% especially due to the foaming ability of BSA. The addition of 5 wt% FIB doubles the biaxial flexural strength up to 6 MPa in comparison to samples without proteins (3 MPa). Protein release experiments show that a rapid release takes place within the first days (between around 3% for FIB and 38% for BSA). As a possible application for osteomyelitis treatment, vancomycin and gentamicin were subsequently added instead of proteins to study their release behaviour and their antibacterial activity, respectively. A controlled antibiotic release was observed for a period of 18 d. By varying the protein type, mixture and quantity, the mechanical strength porosity as well as the protein release and calcium solubility can be controlled. Our studies underpin the suitability of this mild one-pot process as a promising simple-to-use platform for controlled local drug release and bone treatment.

Nanostructured platforms for the sustained and local delivery of antibiotics in the treatment of osteomyelitis

This article provides a critical view of the current state of the development of nanoparticulate and other solid-state carriers for the local delivery of antibiotics in the treatment of osteomyelitis. Mentioned are the downsides of traditional means for treating bone infection, which involve systemic administration of antibiotics and surgical debridement, along with the rather imperfect local delivery options currently available in the clinic. Envisaged are more sophisticated carriers for the local and sustained delivery of antimicrobials, including bioresorbable polymeric, collagenous, liquid crystalline, and bioglass- and nanotube-based carriers, as well as those composed of calcium phosphate, the mineral component of bone and teeth. A special emphasis is placed on composite multifunctional antibiotic carriers of a nanoparticulate nature and on their ability to induce osteogenesis of hard tissues demineralized due to disease. An ideal carrier of this type would prevent the long-term, repetitive, and systemic administration of antibiotics and either minimize or completely eliminate the need for surgical debridement of necrotic tissue. Potential problems faced by even hypothetically "perfect" antibiotic delivery vehicles are mentioned too, including (i) intracellular bacterial colonies involved in recurrent, chronic osteomyelitis; (ii) the need for mechanical and release properties to be adjusted to the area of surgical placement; (iii) different environments in which in vitro and in vivo testings are carried out; (iv) unpredictable synergies between drug delivery system components; and (v) experimental sensitivity issues entailing the increasing subtlety of the design of nanoplatforms for the controlled delivery of therapeutics.

Local drug delivery for enhancing fracture healing in osteoporotic bone

Fragility fractures can cause significant morbidity and mortality in patients with osteoporosis and inflict a considerable medical and socioeconomic burden. Moreover, treatment of an osteoporotic fracture is challenging due to the decreased strength of the surrounding bone and suboptimal healing capacity, predisposing both to fixation failure and non-union. Whereas a systemic osteoporosis treatment acts slowly, local release of osteogenic agents in osteoporotic fracture would act rapidly to increase bone strength and quality, as well as to reduce the bone healing period and prevent development of a problematic non-union. The identification of agents with potential to stimulate bone formation and improve implant fixation strength in osteoporotic bone has raised hope for the fast augmentation of osteoporotic fractures. Stimulation of bone formation by local delivery of growth factors is an approach already in clinical use for the treatment of non-unions, and could be utilized for osteoporotic fractures as well. Small molecules have also gained ground as stable and inexpensive compounds to enhance bone formation and tackle osteoporosis. The aim of this paper is to present the state of the art on local drug delivery in osteoporotic fractures. Advantages, disadvantages and underlying molecular mechanisms of different active species for local bone healing in osteoporotic bone are discussed. This review also identifies promising new candidate molecules and innovative approaches for the local drug delivery in osteoporotic bone.

Endochondral ossification for enhancing bone regeneration: converging native extracellular matrix biomaterials and developmental engineering in vivo

Autologous bone grafting (ABG) remains entrenched as the gold standard of treatment in bone regenerative surgery. Consequently, many marginally successful bone tissue engineering strategies have focused on mimicking portions of ABG's "ideal" osteoconductive, osteoinductive, and osteogenic composition resembling the late reparative stage extracellular matrix (ECM) in bone fracture repair, also known as the "hard" or "bony" callus. An alternative, less common approach that has emerged in the last decade harnesses endochondral (EC) ossification through developmental engineering principles, which acknowledges that the molecular and cellular mechanisms involved in developmental skeletogenesis, specifically EC ossification, are closely paralleled during native bone healing. EC ossification naturally occurs during the majority of bone fractures and, thus, can potentially be utilized to enhance bone regeneration for nearly any orthopedic indication, especially in avascular critical-sized defects where hypoxic conditions favor initial chondrogenesis instead of direct intramembranous ossification. The body's native EC ossification response, however, is not capable of regenerating critical-sized defects without intervention. We propose that an underexplored potential exists to regenerate bone through the native EC ossification response by utilizing strategies which mimic the initial inflammatory or fibrocartilaginous ECM (i.e., "pro-" or "soft" callus) observed in the early reparative stage of bone fracture repair. To date, the majority of strategies utilizing this approach rely on clinically burdensome in vitro cell expansion protocols. This review will focus on the confluence of two evolving areas, (1) native ECM biomaterials and (2) developmental engineering, which will attempt to overcome the technical, business, and regulatory challenges that persist in the area of bone regeneration. Significant attention will be given to native "raw" materials and ECM-based designs that provide necessary osteo- and chondro-conductive and inductive features for enhancing EC ossification. In addition, critical perspectives on existing stem cell-based therapeutic strategies will be discussed with a focus on their use as an extension of the acellular ECM-based designs for specific clinical indications. Within this framework, a novel realm of unexplored design strategies for bone tissue engineering will be introduced into the collective consciousness of the regenerative medicine field.

Release and bioactivity of bone morphogenetic protein-2 are affected by scaffold binding techniques in vitro and in vivo

A low dose of 1μg rhBMP-2 was immobilised by four different functionalising techniques on recently developed poly(l-lactide)-co-(ε-caprolactone) [(poly(LLA-co-CL)] scaffolds. It was either (i) physisorbed on unmodified scaffolds [PHY], (ii) physisorbed onto scaffolds modified with nanodiamond particles [nDP-PHY], (iii) covalently linked onto nDPs that were used to modify the scaffolds [nDP-COV] or (iv) encapsulated in microspheres distributed on the scaffolds [MICS]. Release kinetics of BMP-2 from the different scaffolds was quantified using targeted mass spectrometry for up to 70days. PHY scaffolds had an initial burst of release while MICS showed a gradual and sustained increase in release. In contrast, NDP-PHY and nDP-COV scaffolds showed no significant release, although nDP-PHY scaffolds maintained bioactivity of BMP-2. Human mesenchymal stem cells cultured in vitro showed upregulated BMP-2 and osteocalcin gene expression at both week 1 and week 3 in the MICS and nDP-PHY scaffold groups. These groups also demonstrated the highest BMP-2 extracellular protein levels as assessed by ELISA, and mineralization confirmed by Alizarin red. Cells grown on the PHY scaffolds in vitro expressed collagen type 1 alpha 2 early but the scaffold could not sustain rhBMP-2 release to express mineralization. After 4weeks post-implantation using a rat mandible critical-sized defect model, micro-CT and Masson trichrome results showed accelerated bone regeneration in the PHY, nDP-PHY and MICS groups. The results demonstrate that PHY scaffolds may not be desirable for clinical use, since similar osteogenic potential was not seen under both in vitro and in vivo conditions, in contrast to nDP-PHY and MICS groups, where continuous low doses of BMP-2 induced satisfactory bone regeneration in both conditions. The nDP-PHY scaffolds used here in critical-sized bone defects for the first time appear to have promise compared to growth factors adsorbed onto a polymer alone and the short distance effect prevents adverse systemic side effects.

Ordered and kinetically discrete sequential protein release from biodegradable thin films

Multidrug regimens can sometimes treat recalcitrant diseases when single-drug therapies fail. Recapitulating complex multidrug administration from controlled release films for localized delivery remains challenging because their release kinetics are frequently intertwined, and an initial burst release of each drug is usually uncontrollable. Kinetic control over protein release is demonstrated by cross-linking layer-by-layer films during the assembly process. We used biodegradable and naturally derived components and relied on copper-free click chemistry for bioorthogonal covalent cross-links throughout the film that entrap but do not modify the embedded protein. We found that this strategy restricted the interdiffusion of protein while maintaining its activity. By depositing a barrier layer and a second protein-containing layer atop this construct, we generated well-defined sequential protein release with minimal overlap that follows their spatial distribution within the film.

Strategies for osteochondral repair: Focus on scaffolds

Interest in osteochondral repair has been increasing with the growing number of sports-related injuries, accident traumas, and congenital diseases and disorders. Although therapeutic interventions are entering an advanced stage, current surgical procedures are still in their infancy. Unlike other tissues, the osteochondral zone shows a high level of gradient and interfacial tissue organization between bone and cartilage, and thus has unique characteristics related to the ability to resist mechanical compression and restoration. Among the possible therapies, tissue engineering of osteochondral tissues has shown considerable promise where multiple approaches of utilizing cells, scaffolds, and signaling molecules have been pursued. This review focuses particularly on the importance of scaffold design and its role in the success of osteochondral tissue engineering. Biphasic and gradient composition with proper pore configurations are the basic design consideration for scaffolds. Surface modification is an essential technique to improve the scaffold function associated with cell regulation or delivery of signaling molecules. The use of functional scaffolds with a controllable delivery strategy of multiple signaling molecules is also considered a promising therapeutic approach. In this review, we updated the recent advances in scaffolding approaches for osteochondral tissue engineering.

Amphiphilic degradable polymers for immobilization and sustained delivery of sphingosine 1-phosphate

Controlled delivery of the angiogenic factor sphingosine 1-phosphate (S1P) represents a promising strategy for promoting vascularization during tissue repair and regeneration. In this study, we developed an amphiphilic biodegradable polymer platform for the stable encapsulation and sustained release of S1P. Mimicking the interaction between amphiphilic S1P and its binding proteins, a series of polymers with hydrophilic poly(ethylene glycol) core and lipophilic flanking segments of polylactide and/or poly(alkylated lactide) with different alkyl chain lengths were synthesized. These polymers were electrospun into fibrous meshes, and loaded with S1P in generally high loading efficiencies (>90%). Sustained S1P release from these scaffolds could be tuned by adjusting the alkyl chain length, blockiness and lipophilic block length, achieving 35-55% and 45-80% accumulative releases in the first 8h and by 7 days, respectively. Furthermore, using endothelial cell tube formation assay and chicken chorioallantoic membrane assay, we showed that the different S1P loading doses and release kinetics translated into distinct pro-angiogenic outcomes. These results suggest that these amphiphilic polymers are effective delivery vehicles for S1P and may be explored as tissue engineering scaffolds where the delivery of lipophilic or amphiphilic bioactive factors is desired.

Improvements in biomaterial matrices for neural precursor cell transplantation

Progress is being made in developing neuroprotective strategies for traumatic brain injuries; however, there will never be a therapy that will fully preserve neurons that are injured from moderate to severe head injuries. Therefore, to restore neurological function, regenerative strategies will be required. Given the limited regenerative capacity of the resident neural precursors of the CNS, many investigators have evaluated the regenerative potential of transplanted precursors. Unfortunately, these precursors do not thrive when engrafted without a biomaterial scaffold. In this article we review the types of natural and synthetic materials that are being used in brain tissue engineering applications for traumatic brain injury and stroke. We also analyze modifications of the scaffolds including immobilizing drugs, growth factors and extracellular matrix molecules to improve CNS regeneration and functional recovery. We conclude with a discussion of some of the challenges that remain to be solved towards repairing and regenerating the brain.

Improvements in biomaterial matrices for neural precursor cell transplantation

Progress is being made in developing neuroprotective strategies for traumatic brain injuries; however, there will never be a therapy that will fully preserve neurons that are injured from moderate to severe head injuries. Therefore, to restore neurological function, regenerative strategies will be required. Given the limited regenerative capacity of the resident neural precursors of the CNS, many investigators have evaluated the regenerative potential of transplanted precursors. Unfortunately, these precursors do not thrive when engrafted without a biomaterial scaffold. In this article we review the types of natural and synthetic materials that are being used in brain tissue engineering applications for traumatic brain injury and stroke. We also analyze modifications of the scaffolds including immobilizing drugs, growth factors and extracellular matrix molecules to improve CNS regeneration and functional recovery. We conclude with a discussion of some of the challenges that remain to be solved towards repairing and regenerating the brain.

Multilayered Inorganic Microparticles for Tunable Dual Growth Factor Delivery

There is an increasing need to control the type, quantity, and timing of growth factors released during tissue healing. Sophisticated delivery systems offering the ability to deliver multiple growth factors with independently tunable kinetics are highly desirable. Here, a multilayered, mineral coated micro-particle (MCMs) platform that can serve as an adaptable dual growth factor delivery system is developed. Bone morphogenetic protein-2 (BMP-2) and vascular endothelial growth factor (VEGF) are bound to the mineral coatings with high binding efficiencies of up to 80%. BMP-2 is firstly bound onto a 1st mineral coating layer; then VEGF is bound onto a 2nd mineral coating layer. The release of BMP-2 is sustained over a period of 50 days while the release of VEGF is a typical two-phase release with rapid release in the first 14 days and more sustained release for the following 36 days. Notably, the release behaviors of both growth factors can be independently tailored by changing the intrinsic properties of the mineral coatings. Furthermore, the release of BMP-2 can be tuned by changing the thickness of the 2nd layer. This injectable microparticle based delivery platform with tunable growth factor release has immense potential for applications in tissue engineering and regenerative medicine.

Sustained localized presentation of RNA interfering molecules from in situ forming hydrogels to guide stem cell osteogenic differentiation

To date, RNA interfering molecules have been used to differentiate stem cells on two-dimensional (2D) substrates that do not mimic three-dimensional (3D) microenvironments in the body. Here, in situ forming poly(ethylene glycol) (PEG) hydrogels were engineered for controlled, localized and sustained delivery of RNA interfering molecules to differentiate stem cells encapsulated within the 3D polymer network. RNA interfering molecules were released from the hydrogels in a sustained and controlled manner over the course of 3-6 weeks, and exhibited high bioactivity. Importantly, it was demonstrated that the delivery of siRNA and/or miRNA from the hydrogel constructs enhanced the osteogenic differentiation of encapsulated stem cells. Prolonged delivery of siRNA and/or miRNA from this polymeric scaffold permitted extended regulation of cell behavior, unlike traditional siRNA experiments performed in vitro. This approach presents a powerful new methodology for controlling cell fate, and is promising for multiple applications in tissue engineering and regenerative medicine.

Human mesenchymal stem cell spheroids in fibrin hydrogels exhibit improved cell survival and potential for bone healing

Mesenchymal stem cells (MSCs) have great therapeutic potential for the repair of nonhealing bone defects, because of their proliferative capacity, multilineage potential, trophic factor secretion and lack of immunogenicity. However, a major challenge to the translation of cell-based therapies into clinical practice is ensuring their survival and function upon implantation into the defect site. We hypothesize that forming MSCs into more physiologic three-dimensional spheroids, rather than employing dissociated cells from two-dimensional monolayer culture, will enhance their survival when exposed to a harsh microenvironment but maintain their osteogenic potential. MSC spheroids were formed by using the hanging drop method with increasing cell numbers. Compared with larger spheroids, the smallest spheroids, which contained 15,000 cells, exhibited increased metabolic activity, reduced apoptosis and the most uniform distribution of proliferating cells. Spheroids were then entrapped in fibrin gels and cultured in serum-free medium and 1 % oxygen. Compared with identical numbers of dissociated MSCs in fibrin gels, spheroids exhibited significantly reduced apoptosis and secreted up to 100-fold more vascular endothelial growth factor. Moreover, fibrin gels containing spheroids and those containing an equivalent number of dissociated cells exhibited similar expression levels of early and late markers of osteogenic differentiation. Thus, MSC spheroids exhibit greater resistance to apoptosis and enhanced proangiogenic potential while maintaining similar osteogenic potential to dissociated MSCs entrapped in a clinically relevant biomaterial, supporting the use of MSC spheroids in cell-based approaches to bone repair.

PEG-maleimide hydrogels for protein and cell delivery in regenerative medicine

Protein- and cell-based therapies represent highly promising strategies for regenerative medicine, immunotherapy, and oncology. However, these therapies are significantly limited by delivery considerations, particularly in terms of protein stability and dosing kinetics as well as cell survival, engraftment, and function. Hydrogels represent versatile and robust delivery vehicles for proteins and cells due to their high water content that retains protein biological activity, high cytocompatibility and minimal adverse host reactions, flexibility and tunability in terms of chemistry, structure, and polymerization format, ability to incorporate various biomolecules to convey biofunctionality, and opportunity for minimally invasive delivery as injectable carriers. This review highlights recent progress in the engineering of poly(ethylene glycol) hydrogels cross-linked using maleimide reactive groups for protein and cell delivery.

Presentation of BMP-2 mimicking peptides in 3D hydrogels directs cell fate commitment in osteoblasts and mesenchymal stem cells

Many strategies for controlling the fate of transplanted stem cells rely on the concurrent delivery of soluble growth factors that have the potential to produce undesirable secondary effects in surrounding tissue. Such off target effects could be eliminated by locally presenting growth factor peptide mimics from biomaterial scaffolds to control stem cell fate. Peptide mimics of bone morphogenetic protein 2 (BMP-2) were synthesized by solid phase Fmoc-peptide synthesis and covalently bound to alginate hydrogels via either carbodiimide or sulfhydryl-based coupling strategies. Successful peptide conjugation was confirmed by (1)H NMR spectroscopy and quantified by fluorescently labeling the peptides. Peptides derived from the knuckle epitope of BMP-2, presented from both 2D surfaces and 3D alginate hydrogels, were shown to increase alkaline phosphatase activity in clonally derived murine osteoblasts. Furthermore, when presented in 3D hydrogels, these peptides were shown to initiate Smad signaling, upregulate osteopontin production, and increase mineral deposition with clonally derived murine mesenchymal stem cells. These data suggest that these peptide-conjugated hydrogels may be effective alternatives to local BMP-2 release in directly and spatially eliciting osteogenesis from transplanted or host osteoprogenitors in the future.

Mimicking the nanostructure of bone matrix to regenerate bone

Key features of bone tissue structure and composition are capable of directing cellular behavior towards the generation of new bone tissue. Bone tissue, as well as materials derived from bone, have a long and successful history of use as bone grafting materials. Recent developments in design and processing of synthetic scaffolding systems has allowed the replication of the bone's desirable biological activity in easy to fabricate polymeric materials with nano-scale features exposed on the surface. The biological response to these new tissue-engineering scaffold materials oftentimes exceeds that seen on scaffolds produced using biological materials.

Advances in regenerative orthopedics

Orthopedic injuries are common and a source of much misery and economic stress. Several relevant tissues, such as cartilage, meniscus, and intra-articular ligaments, do not heal. And even bone, which normally regenerates spontaneously, can fail to mend. The regeneration of orthopedic tissues requires 4 key components: cells, morphogenetic signals, scaffolds, and an appropriate mechanical environment. Although differentiated cells from the tissue in question can be used, most cellular research focuses on the use of mesenchymal stem cells. These can be retrieved from many different tissues, and one unresolved question is the degree to which the origin of the cells matters. Embryonic and induced pluripotent stem cells are also under investigation. Morphogenetic signals are most frequently supplied by individual recombinant growth factors or native mixtures provided by, for example, platelet-rich plasma; mesenchymal stem cells are also a rich source of trophic factors. Obstacles to the sustained delivery of individual growth factors can be addressed by gene transfer or smart scaffolds, but we still lack detailed, necessary information on which delivery profiles are needed. Scaffolds may be based on natural products, synthetic materials, or devitalized extracellular matrix. Strategies to combine these components to regenerate tissue can follow traditional tissue engineering practices, but these are costly, cumbersome, and not well suited to treating large numbers of individuals. More expeditious approaches make full use of intrinsic biological processes in vivo to avoid the need for ex vivo expansion of autologous cells and multiple procedures. Clinical translation remains a bottleneck.

Enhancement of bone regeneration by dual release of a macrophage recruitment agent and platelet-rich plasma from gelatin hydrogels

Macrophages play an important role in regulating inflammatory responses and tissue regeneration. In the present study, their effect on bone remodeling is investigated by the simultaneous application of a macrophage recruiting agent, SEW2871 of a sphingosine-1 phosphate agonist, and platelet-rich plasma (PRP). The non-water soluble SEW2871 was solubilized in water through micelles formation with l-lactic acid grafted gelatin, and the resulting micelles with PRP were incorporated into gelatin hydrogels. Mixed SEW2871-micelles and PRP were released from gelatin hydrogels in a controlled fashion both in vitro and in vivo. In vitro migration assay revealed that the presence of PRP synergistically promoted SEW2871-induced macrophages migration. When applied to a bone defect of rats, the hydrogels incorporating mixed SEW2871-micelles and PRP recruited a higher number of macrophages than those hydrogels incorporating either SEW2871-micelles or PRP. The hydrogels incorporating mixed SEW2871-micelles and PRP enhanced the level of tumor necrosis factor (TNF)-α of pro-inflammatory cytokine, 3 days after application, while pro-inflammatory responses coupled with a significant increase in the expression level of osteoprotegerin (OPG) and interleukin (IL)-10 and transforming growth factor (TGF)-β1 of anti-inflammatory cytokine were observed 10 days postoperatively. The hydrogels incorporating mixed SEW2871-micelles and PRP promoted bone regeneration to a significant great extent compared with those incorporating PBS and either SEW2871-micelles or PRP. It is concluded that macrophages recruitment contributed to PRP-induced bone regeneration.

Recent developments of functional scaffolds for craniomaxillofacial bone tissue engineering applications

Autogenous bone grafting remains a gold standard for the reconstruction critical-sized bone defects in the craniomaxillofacial region. Nevertheless, this graft procedure has several disadvantages such as restricted availability, donor-site morbidity, and limitations in regard to fully restoring the complicated three-dimensional structures in the craniomaxillofacial bone. The ultimate goal of craniomaxillofacial bone reconstruction is the regeneration of the physiological bone that simultaneously fulfills both morphological and functional restorations. Developments of tissue engineering in the last two decades have brought such a goal closer to reality. In bone tissue engineering, the scaffolds are fundamental, elemental and mesenchymal stem cells/osteoprogenitor cells and bioactive factors. A variety of scaffolds have been developed and used as spacemakers, biodegradable bone substitutes for transplanting to the new bone, matrices of drug delivery system, or supporting structures enhancing adhesion, proliferation, and matrix production of seeded cells according to the circumstances of the bone defects. However, scaffolds to be clinically completely satisfied have not been developed yet. Development of more functional scaffolds is required to be applied widely to cranio-maxillofacial bone defects. This paper reviews recent trends of scaffolds for crania-maxillofacial bone tissue engineering, including our studies.

Chitosan stabilizes platelet growth factors and modulates stem cell differentiation toward tissue regeneration

The idea of using chitosan as a functional delivery aid to support simultaneously PRP, stem cells and growth factors (GF) is associated with the intention to use morphogenic biomaterials to modulate the natural healing sequence in bone and other tissues. For example, chitosan-chondroitin sulfate loaded with platelet lysate was included in a poly(D,L-lactate) foam that was then seeded with human adipose-derived stem cells and cultured in vitro under osteogenic stimulus: the platelet lysate provided to the bone tissue the most suitable assortment of GF which induces the osteogenic differentiation of the mesenchymal stem cells. PDGF, FGF, IGF and TGF-β were protagonists in the repair of callus fractures. The release of GF from the composites of chitosan-PRP and either nano-hydroxyapatite or tricalcium phosphate was highly beneficial for enhancing MSC proliferation and differentiation, thus qualifying chitosan as an excellent vehicle. A number of biochemical characteristics of chitosan exert synergism with stem cells in the regeneration of soft tissues.