Hydrogels in calcium phosphate moldable and injectable bone substitutes: Sticky excipients or advanced 3-D carriers?

Calcium Orthophosphate (CaPO4) Containing Composites for Biomedical Applications: Formulations, Properties, and Applications

The goal of this review is to present a wide range of hybrid formulations and composites containing calcium orthophosphates (abbreviated as CaPO4) that are suitable for use in biomedical applications and currently on the market. The bioactive, biocompatible, and osteoconductive properties of various CaPO4-based formulations make them valuable in the rapidly developing field of biomedical research, both in vitro and in vivo. Due to the brittleness of CaPO4, it is essential to combine the desired osteologic properties of ceramic CaPO4 with those of other compounds to create novel, multifunctional bone graft biomaterials. Consequently, this analysis offers a thorough overview of the hybrid formulations and CaPO4-based composites that are currently known. To do this, a comprehensive search of the literature on the subject was carried out in all significant databases to extract pertinent papers. There have been many formulations found with different material compositions, production methods, structural and bioactive features, and in vitro and in vivo properties. When these formulations contain additional biofunctional ingredients, such as drugs, proteins, enzymes, or antibacterial agents, they offer improved biomedical applications. Moreover, a lot of these formulations allow cell loading and promote the development of smart formulations based on CaPO4. This evaluation also discusses basic problems and scientific difficulties that call for more investigation and advancements. It also indicates perspectives for the future.

Advances in injectable hydrogel: Design, functional regulation, and biomedical applications

Recently, injectable hydrogels have been considered smart materials and have been widely researched for their use as scaffolds. They resemble the extracellular matrix of native tissue and have the capability for homogeneous mixing with therapeutic agents. It can be implanted into living bodies with minimal invasiveness and usability for irregularly shaped sites. Such unique features make the injectable hydrogels as promising materials in tissue engineering, drug delivery system, and gene/protein delivery. This review article provides a comprehensive overview of the different mechanisms employed in the preparation of injectable hydrogel, as well as a detailed exploration of its applications in the biomedical field. Furthermore, the article highlights the critical importance of developing injectable hydrogels as market‐viable products, highlighting their potential impact in the field of regenerative medicine.

Property and biological effects of the cuttlebone derived calcium phosphate particles, a potential bioactive bone substitute material

Cuttlebone (CB) is a marine waste-derived biomaterial and a rich source of calcium carbonate for the biosynthesis of the calcium phosphate (CaP) particles. The current study aimed to synthesize CB derived biphasic calcium phosphate (CB-BCP) and investigate biological activity of the CB-CaP: hydroxyapatite (CB-HA), beta-tricalcium phosphate (CB-b-TCP) and biphasic 60:40 (w/w) HA/b-TCP (CB-BCP) with the human dental pulp stem cells (hDPSCs). The particles were synthesized using solid state reactions under mild condition and properties of the particles were compared with a commercial BCP as a reference material. Morphology, particle size, physicochemical properties, mineral contents, and the ion released patterns of the particles were examined. Then the particle/cell interaction, cell cytotoxicity and osteogenic property of the particles were investigated in the direct and indirect cell culture models. It was found that an average particles size of the CB-HA was 304.73 ± 4.19 nm, CB-b-TCP, 503.17 ± 23.06 nm and CB-BCP, 1394.67 ± 168.19 nm. The physicochemical characteristics of the CB-CaP were consistent with the HA, b-TCP and BCP. The highest level of calcium (Ca) was found in the mineral contents and the preincubated medium of the CB-BCP and traces of fluoride, magnesium, strontium, and zinc were identified in the CB-CaP. The cell cytotoxicity and osteogenic property of the particles were dose dependent. The particles adhered on cell surface and were internalized into the cell cytoplasm. The CB-BCP and CB-HA indirectly and directly promote osteoblastic differentiations of the hDPSCs in stronger levels than other groups. The CB-BCP and CB-HA were potential bioactive bone substitute materials.

Unmodified Gum Arabic/Chitosan/Nanohydroxyapatite Nanocomposite Hydrogels as Potential Scaffolds for Bone Regeneration

In this work, physical cross-linking was used to create nanocomposite hydrogels composed of unmodified gum arabic (GA), chitosan (Ch), and natural nanohydroxyapatite (nHA), using an acrylic acid (AA) solvent. Different GA/chitosan contents (15%, 25%, and 35% of the used AA) as well as different nHA contents (2, 5, and 10 wt.%), were used and studied. The natural nHA and the fabricated GA/Ch/nHA nanocomposite hydrogels were characterized using different analysis techniques. Using acrylic acid solvent produced novel hydrogels with compressive strength of 15.43–22.20 MPa which is similar to that of natural cortical bone. The addition of natural nHA to the hydrogels resulted in a significant improvement in the compressive strength of the fabricated hydrogels. In vitro studies of water absorption and degradation—and in vivo studies—confirmed that the nanocomposite hydrogels described here are biodegradable, biocompatible, and facilitate apatite formation while immersed in the simulated body fluid (SBF). In light of these findings, the GA/Ch/nHA nanocomposite hydrogels are recommended for preparing bioactive nanoscaffolds for testing in bone regeneration applications.

Rheological properties and decomposition rates of Gellan gum/hyaluronic acid/β-tricalcium phosphate mixtures

The effects of β-tricalcium phosphate (TCP) on the mixture of low acyl gellan gum (LA-GAGR) and hyaluronic acid (HA) were investigated for the rheological properties and decomposition rates. All the tested mixture samples exhibited shear-thinning and typical viscoelastic behaviors. The sample made with 1.0% TCP and 0.30% LA-GAGR had the highest viscosity and loss and storage moduli and displayed gel-like behavior with the highest swelling capacity. The same mixture also exhibited the lowest average cumulative decomposition rate. High concentrations of LA-GAGR and TCP increased the degree of cross-linking of the polysaccharides, and as a result, the mixture was more elastic and less fluidic and decomposed slower. The samples prepared by gradual mixing of LA-GAGR and TCP decomposed slower than the sample prepared by sudden mixing, which indicates the well-dispersed TCP enhanced cross-linking of the polymers. This study demonstrates the possible applicability of natural polysaccharide-based shear-thinning gels for biomedical applications.

Three-Dimensional Printing of a Hybrid Bioceramic and Biopolymer Porous Scaffold for Promoting Bone Regeneration Potential

In this study, we proposed a three-dimensional (3D) printed porous (termed as 3DPP) scaffold composed of bioceramic (beta-tricalcium phosphate (β-TCP)) and thermoreversible biopolymer (pluronic F-127 (PF127)) that may provide bone tissue ingrowth and loading support for bone defect treatment. The investigated scaffolds were printed in three different ranges of pore sizes for comparison (3DPP-1: 150−200 μm, 3DPP-2: 250−300 μm, and 3DPP-3: 300−350 μm). The material properties and biocompatibility of the 3DPP scaffolds were characterized using scanning electron microscopy, X-ray diffractometry, contact angle goniometry, compression testing, and cell viability assay. In addition, micro-computed tomography was applied to investigate bone regeneration behavior of the 3DPP scaffolds in the mini-pig model. Analytical results showed that the 3DPP scaffolds exhibited well-defined porosity, excellent microstructural interconnectivity, and acceptable wettability (θ < 90°). Among all groups, the 3DPP-1 possessed a significantly highest compressive force 273 ± 20.8 Kgf (* p < 0.05). In vitro experiment results also revealed good cell viability and cell attachment behavior in all 3DPP scaffolds. Furthermore, the 3DPP-3 scaffold showed a significantly higher percentage of bone formation volume than the 3DPP-1 scaffold at week 8 (* p < 0.05) and week 12 (* p < 0.05). Hence, the 3DPP scaffold composed of β-TCP and F-127 is a promising candidate to promote bone tissue ingrowth into the porous scaffold with decent biocompatibility. This scaffold particularly fabricated with a pore size of around 350 μm (i.e., 3DPP-3 scaffold) can provide proper loading support and promote bone regeneration in bone defects when applied in dental and orthopedic fields.

Injectable silanized hyaluronic acid hydrogel/biphasic calcium phosphate granule composites with improved handling and biodegradability promote bone regeneration in rabbits

Biphasic calcium phosphate (BCP) granules are osteoconductive biomaterials used in clinics to favor bone reconstruction. Yet, poor cohesivity, injectability and mechanical properties restrain their use as bone fillers. In this study, we incorporated BCP granules into in situ forming silanized hyaluronic acid (Si-HA) and hydroxypropylmethylcellulose (Si-HPMC) hydrogels. Hydrogel composites were shown to be easily injectable (F < 30 N), with fast hardening properties (<5 min), and similar mechanical properties (E∼ 60 kPa). In vivo, both hydrogels were well tolerated by the host, but showed different biodegradability with Si-HA gels being partially degraded after 21d, while Si-HPMC gels remained stable. Both composites were easily injected into critical size rabbit defects and remained cohesive. After 4 weeks, Si-HPMC/BCP led to poor bone healing due to a lack of degradation. Conversely, Si-HA/BCP composites were fully degraded and beneficially influenced bone regeneration by increasing the space available for bone ingrowth, and by accelerating BCP granules turnover. Our study demonstrates that the degradation rate is key to control bone regeneration and that Si-HA/BCP composites are promising biomaterials to regenerate bone defects.

Regeneration of critical-sized defects, in a goat model, using a dextrin-based hydrogel associated with granular synthetic bone substitute

The development of injectable bone substitutes (IBS) have obtained great importance in the bone regeneration field, as a strategy to reach hardly accessible defects using minimally invasive techniques and able to fit to irregular topographies. In this scenario, the association of injectable hydrogels and bone graft granules is emerging as a well-established trend. Particularly, in situ forming hydrogels have arisen as a new IBS generation. An in situ forming and injectable dextrin-based hydrogel (HG) was developed, aiming to act as a carrier of granular bone substitutes and bioactive agents. In this work, the HG was associated to a granular bone substitute (Bonelike^®) and implanted in goat critical-sized calvarial defects (14 mm) for 3, 6 and 12 weeks. The results showed that HG improved the handling properties of the Bonelike^® granules and did not affect its osteoconductive features, neither impairing the bone regeneration process. Human multipotent mesenchymal stromal cells from the umbilical cord, extracellular matrix hydrolysates and the pro-angiogenic peptide LLKKK18 were also combined with the IBS. These bioactive agents did not enhance the new bone formation significantly under the conditions tested, according to micro-computed tomography and histological analysis.

Synthesis and characterization of biphasic calcium phosphate laden thiolated hyaluronic acid hydrogel based scaffold: physical and in-vitro biocompatibility evaluations

The present study focused on the combination of biphasic calcium phosphate (BCP) nanoparticles into the modified hyaluronic acid based injectable hydrogels for bone tissue engineering. Self-cross-linked thiolated hyaluronic acid (HA-HS) injectable hydrogels loaded with biphasic calcium phosphate (BCP) nanoparticles were prepared by disulfide cross-linking to mimic the extracellular matrix as a potential material for bone treatment. Varying concentration of HA-HS ranging between 1 and 5w/v% was tested to optimize the optimum concentration and were further modified with varying BCP concentrations for final optimization. Physico-chemical characterizations of the prepared hydrogel such as SEM, EDS, FT-IR, and XRD confirmed that the BCP has effectively loaded and distributed homogeneously in the HA-HS hydrogel. The results showed that the 3% (w/v) HA-HS hydrogel exhibits the appropriate properties for injectable hydrogel system such as gelation times, swelling rate and in vitro degradation behavior among all tested concentrations. Cell viability and cell proliferation using osteoblast cells (MC3T3-E1) demonstrated that the BCP laden modified hydrogel are biocompatible in vitro. In light of the encouraging results obtained, BCP laden HA-HS hydrogels might offer the potential to be used as injectable hydrogel in bone tissue engineering.

From injectable to 3D printed hydrogels in maxillofacial tissue engineering: A review

Introduction

This review aims at describing different types of hydrogels in context to their composition, fabrication techniques and other specific features along with an insight into the latest advancements including smart hydrogels, 3D printed, programmable, shape memory and self-healing hydrogels for their applicability as scaffold in maxillofacial bone and cartilage tissue regeneration.

Methods

Electronic database searches were undertaken on PubMed, Ovid, Medline, Embase, ProQuest and science direct for English language literature, published for application of hydrogels in maxillofacial bone and cartilage tissue engineering. The search items used in this article were hydrogel, bone and cartilage tissue engineering, maxillofacial, clinical trials. Reviews and in vitro studies were excluded.

Results

Search for injectable hydrogel showed 4955 articles, when restricted to bone tissue engineering results were reduced to 463 and for cartilage engineering to 335; when we limited it to maxillofacial bone and cartilage tissue engineering, search results showed 49 articles to which 9 additional articles were included from references, after exclusion of in-vitro studies and duplicates 16 articles were obtained for our study. Similarly, for 3D printed hydrogels, result showed 1126 articles, which got restricted to 19 when searched for maxillofacial bone and cartilage engineering, then 2 additional articles were included directly from references, and finally after exclusion of the invitro studies and duplicates, a total of 5 articles were obtained.

Conclusion

Modifications in hydrogel can improve the mechanical properties, biocompatibility and unique chemistries for its use in bone and cartilage tissue engineering for future research.

Photoencapsulated-BMP2 in visible light-cured thiol-acrylate hydrogels for craniofacial bone tissue engineering

Aim: The study aimed to examine the impact of crosslinking BMP2 in biodegradable visible light-cured thiol-acrylate hydrogels. Materials & methods: BMP2 was photoencapsulated in 10 wt% PEG-diacrylate hydrogels with or without immortalized mouse bone marrow stromal cells (BMSC). Results & conclusion: Photoencapsulated-BMSC with BMP2 (BMBMP2) showed a significantly (p < 0.05) increased level in metabolic activity, by 54.61%, compared with photoencapsulated-BMSC at day 3. Furthermore, BMBMP2 groups showed significantly increased levels in ALP activity compared with BMSC at days, 1, 3, 7 (p < 0.01) and 10 (p < 0.05). This study shows promising results photoencapsulating BMP2 in thiol-acrylate hydrogels for craniofacial bone tissue engineering applications.

Complexation of Injectable Biphasic Calcium Phosphate with Phosphoserine-Presenting Dendrons with Enhanced Osteoregenerative Properties

Injectable biphasic calcium phosphates have been proposed as a solution in the treatment of a range of clinical applications including as fillers in the augmentation of osteoporotic bone. To date, various biodegradable natural or synthetic organics have been used as a polymer component of bone materials to increase their cohesiveness. Herein, a novel bone material was developed combining osteoconductive biphasic calcium phosphate (BCP) nanoparticles with phosphoserine-tethered generation 3 poly(epsilon-lysine) dendron (G3-K PS), a class of hyperbranched peptides previously shown to induce biomineralization and stem cell osteogenic differentiation. Strontium was also incorporated into the BCP nanocrystals (SrBCP) to prevent bone resorption. Within 24 h, an antiwashout behavior was observed in G3-K PS-integrated pure BCP group (BCPG3). Moreover, both in vitro tests by relevant cell phenotypes and an in vivo tissue regeneration study by an osteoporotic animal bone implantation showed that the integration of G3-K PS would downregulate Cxcl9 gene and protein expressions, thus enhancing bone regeneration measured as bone mineral density, new bone volume ratio, and trabecular microarchitectural parameters. However, no synergistic effect was found when Sr was incorporated into the BCPG3 bone pastes. Notably, results indicated a concomitant reduction of bone regeneration potential assessed as reduced Runx2 and PINP expression when bone resorptive RANKL and CTX-I levels were reduced by Sr supplementation. Altogether, the results suggest the potential of injectable BCPG3 bone materials in the treatment of osteoporotic bone defects.

Photo-crosslinked alginate nano-hydroxyapatite paste for bone tissue engineering

In this study, methacrylation of alginate was carried out by reacting sodium alginate with methacrylic anhydride in the presence of sodium hydroxide. Separately synthesized nano-hydroxyapatite (nano-HAp) powder was surface functionalized using mercaptopropionic acid and ethylene glycol methacrylate phosphate (EGMP) in the presence of azobisisobutyronitrile benzene as a free radical initiator in a nitrogen atmosphere. Methacrylated alginate solution was mixed with the required amount of surface-functionalized HAp nanoparticles in the presence of 0.05% Irgacure 2959 as a photoinitiator and was placed at the centre of a 8 kW UV light source (265 nm) to prepare photo-crosslinked bone paste. X-ray diffraction analysis indicated that surface functionalization did not alter phase purity of HAp nanopowder in the prepared paste. The graft polymerization of EGMP on the surface of HAp was confirmed by the presence of the 1732 cm^-1 band, which belongs to C=-O stretching of EGMP, in addition to the characteristic peaks of nano-HAp and alginate in the composite paste. The storage and loss moduli of all the prepared pastes increased non-linearly with time up to 100 s, demonstrating their pseudo plastic behaviour. The rate of release of bone morphogenetic protein 2 (BMP-2) was significantly faster in the first few days, and the release curve gradually levelled off prior to slowing down up to 22 d. Mesenchymal stem cell adhesion studies revealed that cells could attach to the paste material and stretch over the surface of the material after 14 d of incubation. MTT assay showed that prepared paste materials were conducive to attachment and proliferation of mesenchymal stem cells. Immunocytochemical analysis revealed that the addition of surface-functionalized nano-HAp and BMP-2 to alginate hydrogel enhanced the osteogenic potential of the prepared paste. The results indicate that the newly developed photo-crosslinked paste may be physically and biologically suitable for application as a bone filler.

Natural hydrogels R&D process: technical and regulatory aspects for industrial implementation

Since hydrogel therapies have been introduced into clinic treatment procedures, the biomedical industry has to face the technology transfer and the scale-up of the processes. This will be key in the roadmap of the new technology implementation. Transfer technology and scale-up are already known for some applications but other applications, such as 3D printing, are still challenging. Decellularized tissues offer a lot of advantages when compared to other natural gels, for example they display enhanced biological properties, due to their ability to preserve natural molecules. For this reason, even though their use as a source for bioinks represents a challenge for the scale-up process, it is very important to consider the advantages that originate with overcoming this challenge. Therefore, many aspects that influence the scaling of the industrial process should be considered, like the addition of drugs or cells to the hydrogel, also, the gelling process is important to determine the chemical and physical parameters that must be controlled in order to guarantee a successful process. Legal aspects are also crucial when carrying out the scale-up of the process since they determine the industrial implementation success from the regulatory point of view. In this context, the new law Regulation (EU) 2017/745 on biomedical devices will be considered. This review summarizes the different aspects, including the legal ones, that should be considered when scaling up hydrogels of natural origin, in order to balance these different aspects and to optimize the costs in terms of raw materials and engine.

Effects of alginate/chondroitin sulfate-based hydrogels on bone defects healing

Repairing bone defects remains challenging in orthopedics. Here, strontium (Sr) alginate hydrogels containing chondroitin sulfate (CS) were fabricated for enhancing bone defects repair. The effects of CS incorporation ratio on the morphology, structure, thermal stability, water uptake and mechanical performance of Sr-CS/alginate hydrogels were also evaluated. Increasing CS incorporation ratio, Sr-CS/alginate hydrogels exhibit decreasing mechanical properties and lower water retention capacity. In vitro results suggest that Sr-CS/alginate hydrogels with higher CS ratio facilitate the proliferation of osteoblasts. Additionally, the osteogenic genes expressions were investigated by real-time quantitative polymerase chain reaction (RT-qPCR). The results reveal that Sr-CS/alginate hydrogels should have positive effects on modulating the osteogenic factors. Moreover, by employing repair femoral cylindrical defects rabbit model, the efficiency of as-fabricated Sr-CS/alginate hydrogels in bone regeneration was evaluated. The animal study suggests that Sr-CS/alginate hydrogel could significantly facilitate bone defects repair and therefore should potentially be useful for osteochondral tissue engineering.

Calcium Phosphate Nanoparticles for Therapeutic Applications in Bone Regeneration

Bone injuries and diseases constitute a burden both socially and economically, as the consequences of a lack of effective treatments affect both the patients' quality of life and the costs on the health systems. This impended need has led the research community's efforts to establish efficacious bone tissue engineering solutions. There has been a recent focus on the use of biomaterial-based nanoparticles for the delivery of therapeutic factors. Among the biomaterials being considered to date, calcium phosphates have emerged as one of the most promising materials for bone repair applications due to their osteoconductivity, osteoinductivity and their ability to be resorbed in the body. Calcium phosphate nanoparticles have received particular attention as non-viral vectors for gene therapy, as factors such as plasmid DNAs, microRNAs (miRNA) and silencing RNA (siRNAs) can be easily incorporated on their surface. Calcium phosphate nanoparticles loaded with therapeutic factors have also been delivered to the site of bone injury using scaffolds and hydrogels. This review provides an extensive overview of the current state-of-the-art relating to the design and synthesis of calcium phosphate nanoparticles as carriers for therapeutic factors, the mechanisms of therapeutic factors' loading and release, and their application in bone tissue engineering.

Scaffolding Strategies for Tissue Engineering and Regenerative Medicine Applications

During the past two decades, tissue engineering and the regenerative medicine field have invested in the regeneration and reconstruction of pathologically altered tissues, such as cartilage, bone, skin, heart valves, nerves and tendons, and many others. The 3D structured scaffolds and hydrogels alone or combined with bioactive molecules or genes and cells are able to guide the development of functional engineered tissues, and provide mechanical support during in vivo implantation. Naturally derived and synthetic polymers, bioresorbable inorganic materials, and respective hybrids, and decellularized tissue have been considered as scaffolding biomaterials, owing to their boosted structural, mechanical, and biological properties. A diversity of biomaterials, current treatment strategies, and emergent technologies used for 3D scaffolds and hydrogel processing, and the tissue-specific considerations for scaffolding for Tissue engineering (TE) purposes are herein highlighted and discussed in depth. The newest procedures focusing on the 3D behavior and multi-cellular interactions of native tissues for further use for in vitro model processing are also outlined. Completed and ongoing preclinical research trials for TE applications using scaffolds and hydrogels, challenges, and future prospects of research in the regenerative medicine field are also presented.

Advanced biomaterials for repairing and reconstruction of mandibular defects

Mandibles are the largest and strongest bone in the human face and are often severely compromised by mandibular defects, compromising the quality of life of patients. Mandibular defects may result from trauma, inflammatory disease and benign or malignant tumours. The reconstruction of mandibular defect has been a research hotspot in oral and maxillofacial surgery. Although the principles and techniques of mandibular reconstruction have made great progress in recent years, the development of biomedical materials is still facing technical bottleneck, and new materials directly affect technological breakthroughs in this field. This paper reviews the current status of research and application of various biomaterials in mandibular defects and systematically elaborates different allogeneic biomaterial-based approaches. It is expected that various biomaterials, in combination with new technologies such as digital navigation and 3D printing, could be tuned to build new types of scaffold with more precise structure and components, addressing needs of surgery and post-reconstruction. With the illustration and systematization of different solutions, aims to inspire the development of reconstruction biomaterials.

A carboxymethyl cellulose bone graft carrier delays early bone healing in an ovine model

A limitation in the use of calcium phosphate (CaP) is that in its raw form, it comprises blocks or granules, which are limited in their utility for orthopedic surgery and a number of commercial bone grafts are supplied within an aqueous based carboxymethyl cellulose (CMC) putty. Our hypothesis was that CMC combined with a porous silicate-substituted CaP (SiCaP) scaffold would have no negative effect on bone formation after implantation in an ovine femoral condyle. Defects were either (a) empty or filled with (b) SiCaP granules, (c) CMC-SiCaP Putty or (d) a SiCaP press-fit dry block. Scaffolds were identical in composition and remained in vivo for 4, 8, and 12 weeks. Bone apposition rates, bone area, percentage of bone-implant contact and graft area were quantified. At 4 and 8 weeks, significantly more new bone and percentage of bone-implant contact was measured within granules when compared with both putty and block scaffolds. At 12 weeks, significantly increased bone was measured for the granules when compared with blocks and no significant difference was found when the granules and putty scaffolds were compared. Results showed the disadvantageous effect that CMC may have on early bone growth and that granules increased new bone formation when compared with a press-fit block composed of the same material.

In vivo systemic toxicity assessment of an oxidized dextrin-based hydrogel and its effectiveness as a carrier and stabilizer of granular synthetic bone substitutes

The worldwide incidence of bone disorders is raising, mainly due to aging population. The lack of effective treatments is pushing the development of synthetic bone substitutes (SBSs). Most ceramic-based SBSs commercially available display limited handling properties. Attempting to solve these issues and achieve wider acceptance by the clinicians, granular ceramics have been associated with hydrogels (HGs) to produce injectable/moldable SBSs. Dextrin, a low-molecular-weight carbohydrate, was used to develop a fully resorbable and injectable HG. It was first oxidized with sodium periodate and then cross-linked with adipic acid dihydrazide. The in vivo biocompatibility and safety of the dextrin-based HG was assessed by subacute systemic toxicity and skin sensitization tests, using rodent models. The results showed that the HG did not induce any systemic toxic effect, skin reaction, or genotoxicity, neither impaired the bone repair/regeneration process. Then, the HG was successfully combined with granular bone substitute, registered as Bonelike (250-500 μm) to obtain a moldable/injectable SBS, which was implanted in tibial fractures in goats for 3 and 6 weeks. The obtained results showed that HG allowed the stabilization of the granules into the defect, ensuring effective handling, and molding properties of the formulation, as well as an efficient cohesion of the granules. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1678-1689, 2019.

Flow Behavior Prior to Crosslinking: The Need for Precursor Rheology for Placement of Hydrogels in Medical Applications and for 3D Bioprinting

Hydrogels - water swollen cross-linked networks - have demonstrated considerable promise in tissue engineering and regenerative medicine applications. However, ambiguity over which rheological properties are needed to characterize these gels before crosslinking still exists. Most hydrogel research focuses on the performance of the hydrogel construct after implantation, but for clinical practice, and for related applications such as bioinks for 3D bioprinting, the behavior of the pre-gelled state is also critical. Therefore, the goal of this review is to emphasize the need for better rheological characterization of hydrogel precursor formulations, and standardized testing for surgical placement or 3D bioprinting. In particular, we consider engineering paste or putty precursor solutions (i.e., suspensions with a yield stress), and distinguish between these differences to ease the path to clinical translation. The connection between rheology and surgical application as well as how the use of paste and putty nomenclature can help to qualitatively identify material properties are explained. Quantitative rheological properties for defining materials as either pastes or putties are proposed to enable easier adoption to current methods. Specifically, the three-parameter Herschel-Bulkley model is proposed as a suitable model to correlate experimental data and provide a basis for meaningful comparison between different materials. This model combines a yield stress, the critical parameter distinguishing solutions from pastes (100-2000 Pa) and from putties (>2000 Pa), with power law fluid behavior once the yield stress is exceeded. Overall, successful implementation of paste or putty handling properties to the hydrogel precursor may minimize the surgeon-technology learning time and ultimately ease incorporation into current practice. Furthermore, improved understanding and reporting of rheological properties will lead to better theoretical explanations of how materials affect rheological performances, to better predict and design the next generation of biomaterials.

Osteogenic Effects of VEGF-Overexpressed Human Adipose-Derived Stem Cells with Whitlockite Reinforced Cryogel for Bone Regeneration

Bone is a vascularized tissue that is comprised of collagen fibers and calcium phosphate crystals such as hydroxyapatite (HAp) and whitlockite (WH). HAp and WH are known to elicit bone regeneration by stimulating osteoblast activities and osteogenic commitment of stem cells. In addition, vascular endothelial growth factor (VEGF) is shown to promote osteogenesis and angiogenesis which is considered as an essential process in bone repair by providing nutrients. In this study, VEGF-secreting human adipose-derived stem cells (VEGF-ADSCs) are developed by transducing ADSCs with VEGF-encoded lentivirus. Additionally, WH-reinforced gelatin/heparin cryogels (WH-C) are fabricated by loading WH into gelatin/heparin cryogels. VEGF-ADSC secrete tenfold more VEGF than ADSC and show increased VEGF secretion with cell growth. Also, incorporation of WH into cryogels provides a mineralized environment with ions secreted from WH. When the VEGF-ADSCs are seeded on WH-C, sustained release of VEGF is observed due to the specific affinity of VEGF to heparin. Finally, the synergistic effect of VEGF-ADSC and WH on osteogenesis is successfully confirmed by alkaline phosphatase and real-time polymerase chain reaction analysis. In vivo bone formation is demonstrated via implantation of VEGF-ADSC seeded WH-C into mouse calvarial bone defect model, resulted in enhanced bone development with the highest bone volume/total volume.

Osteogenic, anti-osteoclastogenic and immunomodulatory properties of a strontium-releasing hybrid scaffold for bone repair

Strontium (Sr) is known to stimulate osteogenesis, while inhibiting osteoclastogenesis, thus encouraging research on its application as a therapeutic agent for bone repair/regeneration. It has been suggested that it may possess immunomodulatory properties, which might act synergistically in bone repair/regeneration processes. To further explore this hypothesis we have designed a Sr-hybrid system composed of an in situ forming Sr-crosslinked RGD-alginate hydrogel reinforced with Sr-doped hydroxyapatite (HAp) microspheres and studied its in vitro osteoinductive behaviour and in vivo inflammatory response. The Sr-hybrid scaffold acts as a dual Sr²⁺ delivery system, showing a cumulative Sr²⁺ release of ca. 0.3 mM after 15 days. In vitro studies using Sr²⁺concentrations within this range (0 to 3 mM Sr²⁺) confirmed its ability to induce osteogenic differentiation of mesenchymal stem/stromal cells (MSC), as well as to reduce osteoclastogenesis and osteoclasts (OC) functionality. In comparison with a similar Sr-free system, the Sr-hybrid system stimulated osteogenic differentiation of MSC, while inhibiting the formation of OC. Implantation in an in vivo model of inflammation, revealed an increase in F4/80⁺/CD206⁺ cells, highlighting its ability to modulate the inflammatory response as a pro-resolution mediator, through M2 macrophage polarization. Therefore, the Sr-hybrid system is potentially an appealing biomaterial for future clinical applications.

Cartilage Tissue Engineering Using Stem Cells and Bioprinting Technology-Barriers to Clinical Translation

There is no long-term treatment strategy for young and active patients with cartilage defects. Early and effective joint preserving treatments in these patients are crucial in preventing the development of osteoarthritis. Tissue engineering over the past few decades has presented hope in overcoming the issues involved with current treatment strategies. Novel advances in 3D bioprinting technology have promoted more focus on efficient delivery of engineered tissue constructs. There have been promising in-vitro studies and several animal studies looking at 3D bioprinting of engineered cartilage tissue. However, to date there are still no human clinical trials using 3D printed engineered cartilage tissue. This review begins with discussion surrounding the difficulties with articular cartilage repair and the limitations of current clinical management options which have led to research in cartilage tissue engineering. Next, the major barriers in each of the 4 components of cartilage tissue engineering; cells, scaffolds, chemical, and physical stimulation will be reviewed. Strategies that may overcome these barriers will be discussed. Finally, we will discuss the barriers surrounding intraoperative delivery of engineered tissue constructs and possible solutions.

Multifunctional smart hydrogels: potential in tissue engineering and cancer therapy

In recent years, clinical applications have been proposed for various hydrogel products. Hydrogels can be derived from animal tissues, plant extracts and/or adipose tissue extracellular matrices; each type of hydrogel presents significantly different functional properties and may be used for many different applications, including medical therapies, environmental pollution treatments, and industrial materials. Due to complicated preparation techniques and the complexities associated with the selection of suitable materials, the applications of many host-guest supramolecular polymeric hydrogels are limited. Thus, improvements in the design and construction of smart materials are highly desirable in order to increase the lifetimes of functional materials. Here, we summarize different functional hydrogels and their varied preparation methods and source materials. The multifunctional properties of hydrogels, particularly their unique ability to adapt to certain environmental stimuli, are chiefly based on the incorporation of smart materials. Smart materials may be temperature sensitive, pH sensitive, pH/temperature dual sensitive, photoresponsive or salt responsive and may be used for hydrogel wound repair, hydrogel bone repair, hydrogel drug delivery, cancer therapy, and so on. This review focuses on the recent development of smart hydrogels for tissue engineering applications and describes some of the latest advances in using smart materials to create hydrogels for cancer therapy.

Formulation and viscoelasticity of mineralised hydrogels for use in bone-cartilage interfacial reconstruction

Articular cartilage is a viscoelastic tissue whose structural integrity is important in maintaining joint health. To restore the functionality of osteoarthritic joints it is vital that regenerative strategies mimic the dynamic loading response of cartilage and bone. Here, a rotating simplex model was employed to optimise the composition of agarose and gellan hydrogel constructs structured with hydroxyapatite (HA) with the aim of obtaining composites mechanically comparable to human cartilage in terms of their ability to dissipate energy. Addition of ceramic particles was found to reinforce both matrices up to a critical concentration (< 3w/v%). Beyond this, larger agglomerates were formed, as evidenced by micro computed tomography data, which acted as stress risers and reduced the ability of composites to dissipate energy demonstrated by a reduction in tan δ values. A maximum compressive modulus of 450.7±24.9 kPa was achieved with a composition of 5.8w/v% agarose and 0.5w/v% HA. Interestingly, when loaded dynamically (1-20Hz) this optimised formulation did not exhibit the highest complex modulus instead a sample with a higher concentration of mineral was identified (5.8w/v% agarose and 25w/v% HA). Thus, demonstrating the importance of examining the mechanical behaviour of biomaterials under conditions representative of physiological environments. While the complex moduli of the optimised gellan (1.0 ± 0.2MPa at 1Hz) and agarose (1.7 ± 0.2MPa at 1Hz) constructs did not match the complex moduli of healthy human cartilage samples (26.3 ± 6.5MPa at 1Hz), similar tan δ values were observed between 1 and 5Hz. This is promising since these frequencies represent the typical heel strike time of the general population. In summary, this study demonstrates the importance of considering more than just the strength of biomaterials since tissues like cartilage play a more complex role.

Functionalization of phosphocalcic bioceramics for bone repair applications

This work is devoted to the processing of bone morphogenetic protein (BMP-2) functionalized silicate substituted hydroxyapatite (SiHA) ceramic spheres. The motivation behind it is to develop injectable hydrogel/bioceramic composites for bone reconstruction applications. SiHA microspheres were shaped by spray drying and thoroughly characterized. The silicate substitution was used to provide preferred chemical sites at the ceramic surface for the covalent immobilization of BMP-2. In order to control the density and the release of the immobilized BMP-2, its grafting was performed via ethoxysilanes and polyethylene glycols. A method based on Kaiser's test was used to quantify the free amino groups of grafted organosilanes available at the ceramic surface for BMP-2 immobilization. The SiHA surface modification was investigated by means of X-ray photoelectron spectroscopy, Fourier transformed infrared spectroscopy and thermogravimetry coupled with mass spectrometry. The BMP-2 bioactivity was assessed, in vitro, by measuring the luciferase expression of a stably transfected C3H10 cell line (C3H10-BRE/Luc cells). The results provided evidence that the BMP-2 grafted onto SiHA spheres remained bioactive.

Three-dimensional macroporous materials for tissue engineering of craniofacial bone

Repair of critical-size defects caused by trauma, removal of a tumour, or congenital abnormalities is a challenge in the craniomaxillofacial region because of the limitations associated with treatment. We have reviewed research papers and updated information relevant to the various types of macroporous scaffolds. We have included papers on several biomaterials and their use in various craniofacial defects such as mandibular, calvarial, and others, as well as the latest technological developments such as 3-dimensional printed scaffolds. We selected all papers about scaffolds, stem cells, and growth factors for review. Initial selection was by review of titles and abstracts, and the full texts of potentially suitable articles were then assessed. Methods of tissue engineering for repair of critical-size defects in the craniofacial bones seem to be viable options for surgical treatment in the future. Macroporous scaffolds with interconnected pores are of great value in regeneration of bone in the craniofacial region. In recent years, various natural or synthetic materials, or both, have been developed, on which macroporous scaffolds can be based. In this review we present a review on the various types of three-dimensional macroporous scaffolds that have been developed in recent years, and evaluate their potential for regeneration of craniofacial bone.

Electrochemical preparation and characterization of PNIPAM-HAp scaffolds for bone tissue engineering

In the last decade, a variety of methods for fabrication of three-dimensional biomimetic scaffolds based on hydrogels have been developed for tissue engineering. However, many methods require the use of catalysts which compromises the biocompatibility of the scaffolds. The electrochemical polymerization (ECP) of acrylic monomers has received an increased attention in recent years due to its versatility in the production of highly biocompatible coatings for the electrodes used in medical devices. The main aim of this work was the use of ECP as scaffold fabrication technique to produce highly porous poly(N-isopropylacrylamide) (PNIPAM)/hydroxyapatite (HAp) composite for bone tissue regeneration. The prepared PNIPAM-HAp porous scaffolds were characterized by SEM, FTIR, water swelling, porosity measurements and X-ray diffraction (XRD) techniques. FTIR indicates that ECP promotes a successful conversion of NIPAM to PNIPAM. The water swelling and porosity were shown to be controlled by the HAp content in PNIPAM-HAp scaffolds. The PNIPAM-HAp scaffolds exhibited no cytotoxicity to MG63 cells, showing that ECP are potentially useful for the production of PNIPAM-HAp scaffolds. To address the osteomyelitis, a significant complication in orthopedic surgeries, PNIPAM-HAp scaffolds were loaded with the antibiotic oxacillin. The oxacillin release and the bacterial killing activity of the released oxacillin from PNIPAM-HAp against S. aureus and P. aeruginosa were demonstrated. These observations demonstrate that ECP are promising technique for the production of non-toxic, biocompatible PNIPAM-HAp scaffolds for tissue engineering.

Injectable hybrid system for strontium local delivery promotes bone regeneration in a rat critical-sized defect model

Strontium (Sr) has been described as having beneficial influence in bone strength and architecture. However, negative systemic effects have been reported on oral administration of Sr ranelate, leading to strict restrictions in clinical application. We hypothesized that local delivery of Sr improves osteogenesis without eliciting detrimental side effects. Therefore, the in vivo response to an injectable Sr-hybrid system composed of RGD-alginate hydrogel cross-linked in situ with Sr and reinforced with Sr-doped hydroxyapatite microspheres, was investigated. The system was injected in a critical-sized bone defect model and compared to a similar Sr-free material. Micro-CT results show a trend towards higher new bone formed in Sr-hybrid group and major histological differences were observed between groups. Higher cell invasion was detected at the center of the defect of Sr-hybrid group after 15 days with earlier bone formation. Higher material degradation with increase of collagen fibers and bone formation in the center of the defect after 60 days was observed as opposed to bone formation restricted to the periphery of the defect in the control. These histological findings support the evidence of an improved response with the Sr enriched material. Importantly, no alterations were observed in the Sr levels in systemic organs or serum.

Functional calcium phosphate composites in nanomedicine

Calcium phosphate (CaP) materials have many peculiar and intriguing properties. In nature, CaP is found in nanostructured form embedded in a soft proteic matrix as the main mineral component of bones and teeth. The extraordinary stoichiometric flexibility, the different stabilities exhibited by its different forms as a function of pH and the highly dynamic nature of its surface ions, render CaP one of the most versatile materials for nanomedicine. This review summarizes some of the guidelines so far emerged for the synthesis of CaP composites in aqueous media that endow the material with tailored crystallinity, morphology, size, and functional properties. First, we introduce very briefly the areas of application of CaP within the nanomedicine field. Then through some selected examples, we review some synthetic routes where the presence of functional units (small templating molecules like surfactants, or oligomers and polymers) assists the synthesis and at the same time impart the functionality or the responsiveness desired for the end-application of the material. Finally, we illustrate two examples from our laboratory, where CaP is decorated by biologically active polymers or prepared within a thermo- and magneto-responsive hydrogel, respectively.

Augmenting in vitro osteogenesis of a glycine-arginine-glycine-aspartic-conjugated oxidized alginate-gelatin-biphasic calcium phosphate hydrogel composite and in vivo bone biogenesis through stem cell delivery

A functionally modified peptide-conjugated hydrogel system was fabricated with oxidized alginate/gelatin loaded with biphasic calcium phosphate to improve its biocompatibility and functionality. Sodium alginate was treated by controlled oxidation to transform the cis-diol group into an aldehyde group in a controlled manner, which was then conjugated to the amine terminus of glycine-arginine-glycine-aspartic. Oxidized alginate glycine-arginine-glycine-aspartic was then combined with gelatin-loaded biphasic calcium phosphate to form a hydrogel of composite oxidized alginate/gelatin/biphasic calcium phosphate that displayed enhanced human adipose stem cell adhesion, spreading and differentiation. 1H nuclear magnetic resonance and electron spectroscopy for chemical analysis confirmed that the glycine-arginine-glycine-aspartic was successfully grafted to the oxidized alginate. Co-delivery of glycine-arginine-glycine-aspartic and human adipose stem cell in a hydrogel matrix was studied with the results indicating that hydrogel incorporated modified with glycine-arginine-glycine-aspartic and seeded with human adipose stem cell enhanced osteogenesis in vitro and bone formation in vivo.

Microstructured β-Tricalcium Phosphate Putty Versus Autologous Bone for Repair of Alveolar Clefts in a Goat Model

For the first time it was demonstrated that an osteoinductive calcium phosphate-based putty is effective in the restoration of complex maxillofacial defects. In these defects, adequate mechanical confinement by multiple bony walls and osteoconduction from multiple surfaces are usually lacking. This study compares the efficacy of a microstructured beta-tricalcium phosphate (β-TCP) putty with autologous bone for the repair of alveolar cleft defects. A total of 10 Dutch milk goats were operated on in a split-mouth study design in which two-wall bony alveolar clefts were created and successively repaired with autologous bone (the gold standard) at one side and β-TCP putty at the other. After 24 weeks of implantation, histomorphometric and micro-computer tomography analyses proved that the β-TCP putty group showed equal bone quality and volume to clefts reconstructed with autologous bone. In addition, surgical handling of the putty is superior to the use of calcium phosphates in a granular form. Therefore, the results of this study open a clear trajectory for the clinical use of β-TCP putty in the reconstruction of the alveolar cleft and other challenging two-wall bony defects.

An Injectable Hydrogel as Bone Graft Material with Added Antimicrobial Properties

Currently, the technique which provides the best chances for a successful bone graft, is the use of bone tissue from the same patient receiving it (autograft); the main limitations are the limited availability and the risks involved in removing living bone tissue, for example, explant site pain and morbidity. Allografts and xenografts may overcome these limitations; however, they increase the risk of rejection. For all these reasons the development of an artificial bone graft material is particularly important and hydrogels are a promising alternative for bone regeneration. Gels were prepared using 1,4-butanediol diacrylate as crosslinker and alpha tricalciumphosphate; ZnCl₂ and SrCl₂ were added to the aqueous phase. MTT results demonstrated that the addition of strontium had a beneficial effect on the osteoblast cells density on hydrogels, and zinc instead did not increase osteoblast proliferation. The amount of calcium produced by the osteoblast cells quantified through the Alizarin Red protocol revealed that both strontium and zinc positively influenced the formation of calcium; furthermore, their effect was synergistic. Rheology properties were used to mechanically characterize the hydrogels and especially the influence of crosslinker's concentration on them, showing the hydrogels presented had extremely good mechanical properties. Furthermore, the antimicrobial activity of strontium and zinc in the hydrogels against methicillin-resistant Staphylococcus aureus and Staphylococcus epidermidis was determined.

Characterization of alginate-brushite in-situ hydrogel composites

In the present study alginate-brushite composite hydrogels were in-situ synthetized and characterized with respect to preparation parameters. Specifically, the influence of initial pH value and initial concentration of phosphate precursor on the in-situ fabrication of the composite hydrogel were taken into account. The composite hydrogels were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric (TGA, DTG) and differential thermal analysis (DTA). Finally, the cell viability tests were carried out (MTT) over the incubation time period of 3, 7, and 14days. The results revealed that the formation and the crystalline stability of brushite were highly dependent on the initial pH value. It was shown that as the pH reached to the value of 6, characteristics peaks of brushite appeared in the FTIR spectra. Besides, the XRD and thermal analysis results were in a good accordance with those of FTIR. In addition, the SEM images demonstrated that the plate like brushite was formed inside the alginate matrix. Also, a considerable impact of pH variation on the biocompatibility of samples was noticed so that the majority of samples especially those prepared in the acidic conditions were toxic.

Composite Hydrogels for Bone Regeneration

Over the past few decades, bone related disorders have constantly increased. Among all pathological conditions, osteoporosis is one of the most common and often leads to bone fractures. This is a massive burden and it affects an estimated 3 million people only in the UK. Furthermore, as the population ages, numbers are due to increase. In this context, novel biomaterials for bone fracture regeneration are constantly under development. Typically, these materials aim at favoring optimal bone integration in the scaffold, up to complete bone regeneration; this approach to regenerative medicine is also known as tissue engineering (TE). Hydrogels are among the most promising biomaterials in TE applications: they are very flexible materials that allow a number of different properties to be targeted for different applications, through appropriate chemical modifications. The present review will focus on the strategies that have been developed for formulating hydrogels with ideal properties for bone regeneration applications. In particular, aspects related to the improvement of hydrogels' mechanical competence, controlled delivery of drugs and growth factors are treated in detail. It is hoped that this review can provide an exhaustive compendium of the main aspects in hydrogel related research and, therefore, stimulate future biomaterial development and applications.

Calcium phosphate/thermoresponsive hyaluronan hydrogel composite delivering hydrophilic and hydrophobic drugs

BACKGROUND/OBJECTIVE

Advanced synthetic biomaterials that are able to reduce or replace the need for autologous bone transplantation are still a major clinical need in orthopaedics, dentistry, and trauma. Key requirements for improved bone substitutes are optimal handling properties, ability to fill defects of irregular shape, and capacity for delivering osteoinductive stimuli.

MATERIALS AND METHODS

In this study, we targeted these requirements by preparing a new composite of β-tricalcium phosphate (TCP) and a thermoresponsive hyaluronan (HA) hydrogel. Dissolution properties of the composite as a function of the particle size and polymeric phase molecular weight and concentration were analysed to identify the best compositions.

RESULTS

Owing to its amphiphilic character, the composite was able to provide controlled release of both recombinant human bone morphogenetic protein-2 and dexamethasone, selected as models for a biologic and a small hydrophobic molecule, respectively.

CONCLUSION

The TCP-thermoresponsive HA hydrogel composite developed in this work can be used for preparing synthetic bone substitutes in the form of injectable or mouldable pastes and can be supplemented with small hydrophobic molecules or biologics for improved osteoinductivity.

Rheological properties, biocompatibility and in vivo performance of new hydrogel-based bone fillers

Three different heterologous substitutes for bone regeneration, manufactured with equine-derived cortical powder, cancellous chips and demineralized bone matrix granules, were compared in vitro and in vivo.

Hydrogels That Allow and Facilitate Bone Repair, Remodeling, and Regeneration

Bone defects can originate from a variety of causes, including trauma, cancer, congenital deformity, and surgical reconstruction. Success of the current "gold standard" treatment (i.e., autologous bone grafts) is greatly influenced by insufficient or inappropriate bone stock. There is thus a critical need for the development of new, engineered materials for bone repair. This review describes the use of natural and synthetic hydrogels as scaffolds for bone tissue engineering. We discuss many of the advantages that hydrogels offer as bone repair materials, including their potential for osteoconductivity, biodegradability, controlled growth factor release, and cell encapsulation. We also discuss the use of hydrogels in composite devices with metals, ceramics, or polymers. These composites are useful because of the low mechanical moduli of hydrogels. Finally, the potential for thermosetting and photo-cross-linked hydrogels as three-dimensionally (3D) printed, patient-specific devices is highlighted. Three-dimensional printing enables controlled spatial distribution of scaffold materials, cells, and growth factors. Hydrogels, especially natural hydrogels present in bone matrix, have great potential to augment existing bone tissue engineering devices for the treatment of critical size bone defects.

3D Bioprinting of Cartilage for Orthopedic Surgeons: Reading between the Lines

Chondral and osteochondral lesions represent one of the most challenging and frustrating scenarios for the orthopedic surgeon and for the patient. The lack of therapeutic strategies capable to reconstitute the function and structure of hyaline cartilage and to halt the progression toward osteoarthritis has brought clinicians and scientists together, to investigate the potential role of tissue engineering as a viable alternative to current treatment modalities. In particular, the role of bioprinting is emerging as an innovative technology that allows for the creation of organized 3D tissue constructs via a "layer-by-layer" deposition process. This process also has the capability to combine cells and biomaterials in an ordered and predetermined way. Here, we review the recent advances in cartilage bioprinting and we identify the current challenges and the directions for future developments in cartilage regeneration.

Calcium Orthophosphate-Containing Biocomposites and Hybrid Biomaterials for Biomedical Applications

The state-of-the-art on calcium orthophosphate (CaPO4)-containing biocomposites and hybrid biomaterials suitable for biomedical applications is presented. Since these types of biomaterials offer many significant and exciting possibilities for hard tissue regeneration, this subject belongs to a rapidly expanding area of biomedical research. Through the successful combinations of the desired properties of matrix materials with those of fillers (in such systems, CaPO4 might play either role), innovative bone graft biomaterials can be designed. Various types of CaPO4-based biocomposites and hybrid biomaterials those are either already in use or being investigated for biomedical applications are extensively discussed. Many different formulations in terms of the material constituents, fabrication technologies, structural and bioactive properties, as well as both in vitro and in vivo characteristics have been already proposed. Among the others, the nano-structurally controlled biocomposites, those containing nanodimensional compounds, biomimetically fabricated formulations with collagen, chitin and/or gelatin, as well as various functionally graded structures seem to be the most promising candidates for clinical applications. The specific advantages of using CaPO4-based biocomposites and hybrid biomaterials in the selected applications are highlighted. As the way from a laboratory to a hospital is a long one and the prospective biomedical candidates have to meet many different necessities, the critical issues and scientific challenges that require further research and development are also examined.

The effect of an alginate carrier on bone formation in a hydroxyapatite scaffold

This study investigated the osteoconductive properties of a porous hydroxyapatite (HA) scaffold manufactured using a novel technique similar to the bread-making process, alone and in combination with an alginate polysaccharide fiber gel (HA/APFG putty) and autologous bone marrow aspirate (BMA). The hypothesis was that the HA/APFG putty would be as osteoconductive as granular HA and that the presence of BMA would further enhance bone formation in an ovine femoral condyle critical defect model. Thirty-six defects were created and either (1) porous HA granules, (2) HA/APFG putty, or (3) HA/APFG putty + BMA were implanted. After retrieval at 6 and 12 weeks, image analysis techniques were used to quantify bone apposition rates, new bone area, bone-HA scaffold contact, and implant resorption. At 6 weeks postsurgery, significantly lower bone apposition rates were observed in the HA/APFG putty group when compared to the HA (p = 0.014) and HA/APFG putty + BMA (p = 0.014) groups. At 12 weeks, significantly increased amounts of new bone formation were measured within the HA scaffold (33.56 ± 3.53%) when compared to both the HA/APFG putty (16.69 ± 2.7%; p = 0.043) and the defects containing HA/APFG putty + BMA (19.31 ± 3.8%; p = 0.043). The use of an APFG gel as a carrier for injectable CaP bone substitute materials delayed bone formation in this model compared to HA granules alone which enhanced bone formation especially within the interconnected smaller pores. Our results also showed that the addition of autologous BMA did not further enhance its osteoconductive properties. Further study is required to optimize the degradation rate of this APFG binding agent before using as a directly injectable material for repair of bone defect. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1328-1335, 2016.

Supramolecular hydrogels as drug delivery systems

Drug delivery from a hydrogel carrier implanted under the kidney capsule is an innovative way to induce kidney tissue regeneration and/or prevent kidney inflammation or fibrosis. We report here on the development of supramolecular hydrogels for this application. Chain-extended hydrogelators containing hydrogen bonding units in the main chain, and bifunctional hydrogelators end-functionalized with hydrogen bonding moieties, were made. The influence of these hydrogels on the renal cortex when implanted under the kidney capsule was studied. The overall tissue response to these hydrogels was found to be mild, and minimal damage to the cortex was observed, using the infiltration of macrophages, formation of myofibroblasts, and the deposition of collagen III as relevant read-out parameters. Differences in tissue response to these hydrogels could be related to the different physico-chemical properties of the three hydrogels.

Development of Injectable Citrate-Based Bioadhesive Bone Implants

Injectable bone implants have been widely used in bone tissue repairs including the treatment of comminuted bone fractures (CBF). However, most injectable bone implants are not suitable for the treatment of CBF due to their weak tissue adhesion strengths and minimal osteoinduction. Citrate has been recently reported to promote bone formation through enhanced bioceramic integration and osteoinductivity. Herein, a novel injectable citrate-based mussel-inspired bioadhesive hydroxyapatite (iCMBA/HA) bone substitute was developed for CBF treatment. iCMBA/HA can be set within 2-4 minutes and the as-prepared (wet) iCMBA/HA possess low swelling ratios, compressive mechanical strengths of up to 3.2±0.27 MPa, complete degradation in 30 days, suitable biocompatibility, and osteoinductivity. This is also the first time to demonstrate that citrate supplementation in osteogenic medium and citrate released from iCMBA/HA degradation can promote the mineralization of osteoblastic committed human mesenchymal stem cells (hMSCs). In vivo evaluation of iCMBA/HA in a rabbit comminuted radial fracture model showed significantly increased bone formation with markedly enhanced three-point bending strength compared to the negative control. Neovascularization and bone ingrowth as well as highly organized bone formation were also observed showing the potential of iCMBA/HA in treating CBF.

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.