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Stromal vascular fraction (SVF) is the primary isolate obtained after enzymatic digestion of adipose tissue that contains various cell types. Its successful application for cell-based construct preparation in an intra-operative setting for clinical bone augmentation and regeneration has been previously reported. However, the performance of SVF-based constructs compared with traditional ex vivo expanded adipose tissue-derived mesenchymal stromal cells (ATMSCs) remains unclear and direct comparative analyses are scarce. Consequently, we here aimed at comparing the in vitro osteogenic differentiation capacity of donor-matched SVF versus ATMSCs as well as their osteoinductive capacity. Human adipose tissue from nine different donors was used to isolate SVF, which was further purified via plastic-adherence to obtain donor-matched ATMSCs. Both cell populations were immunophenotypically characterized for mesenchymal stromal cell, endothelial, and hematopoietic markers after isolation and immunocytochemical staining was used to identify different cell types during prolonged cell culture. Based on normalization using plastic-adherence fraction determination, SVF and ATMSCs were seeded and cultured in osteogenic differentiation medium for 28 days. Further, SVF and ATMSCs were seeded onto devitalized bovine bone granules and subcutaneously implanted into nude mice. After 42 days of implantation, granules were retrieved, histologically processed, and stained with hematoxylin and eosin (HE) to assess ectopic bone formation. The ATMSCs were shown to be a homogenous cell population during cell culture, whereas SVF cultures consisted of multiple cell types. All donor-matched comparisons showed either accelerated or stronger mineralization for SVF cultures in vitro . However, neither SVF nor ATMSCs loaded on devitalized bone granules induced ectopic bone formation on subcutaneous implantation, as opposed to control granules loaded with bone morphogenetic protein-2 (BMP-2), which triggered ectopic bone formation with 100% incidence. Despite the observed lack of osteoinduction, our findings provide important in vitro evidence on the osteogenic superiority of intra-operatively available SVF as compared with donor-matched ATMSCs. Consequently, further studies should focus on optimizing the efficacy of these cell populations for implementation in orthotopic bone fracture or defect treatment.

The large number of animal models used in spinal cord injury (SCI) research complicates the objective selection of the most appropriate model to investigate the efficacy of biomaterial-based therapies. This systematic review aims to identify a list of relevant animal models of SCI by evaluating the confirmation of SCI and animal survival in all published SCI models used in biomaterials research up until April 2021. A search in PubMed and Embase based on "spinal cord injury," "animal models," and "biomaterials" yielded 4606 papers, 393 of which were further evaluated. A total of 404 individual animal experiments were identified based on type of SCI, level of SCI, and the sex, species, and strain of the animals used. Finally, a total of 149 unique animal models were comparatively evaluated, which led to the generation of an evidence-based list of well-documented mid-thoracic rat models of SCI. These models were used most often, clearly confirmed SCI, and had relatively high survival rates, and therefore could serve as a future starting point for studying novel biomaterial-based therapies for SCI. Furthermore, the review discusses (1) the possible risk of bias in SCI animal models, (2) the difficulty in replication of such experiments due to frequent poor reporting of the methods and results, and (3) the clinical relevance of the currently utilized models. Systematic review registration : The study was prospectively registered in PROSPERO, registration number CRD42019141162. Impact statement Studies on biomaterial-based therapies within the field of spinal cord injury (SCI) research show a large inconsistency concerning the selection of animal models. This review goes beyond summarizing the existing gaps between experimental and clinical SCI by systematically evaluating all animal models used within this field. The models identified by this work were used most often, clearly confirmed SCI, and had a relatively high survival rate. This evidence-based list of well-documented animal models will serve as a practical guideline in future research on innovative biomaterial-based therapies for SCI.

Implant coatings are frequently applied to modulate tissue response and delivery of drugs. Copper (Cu)-containing coatings on dental implant abutments have been proposed to improve soft tissue integration and reduce the risk for peri-implant infections. However, precise control over Cu loading and release kinetics remains a major challenge. In this study, we introduced a bottom-up coating deposition method based on nanoparticle assembly to allow for local release of Cu ions from implant surfaces. We first doped mesoporous bioactive glass (MBG) nanoparticles with various amounts of Cu. Subsequently, we suspended these Cu-doped MBG (Cu-MBG), Cu-free MBG nanoparticles, or mixtures thereof in chitosan solution and prepared a series of composite coatings on commercially pure titanium disks as model surfaces for transmucosal components of bone implants through electrophoretic deposition (EPD). By changing the Cu-MBG:MBG ratio of the composite coatings, we controlled the Cu release kinetics without changing other coating properties. Human gingival fibroblasts proliferated on the composite coatings except for coatings with the highest amount of Cu, which inhibited their proliferation. The migration rate of human umbilical vein endothelial cells cultured on the composite coatings was highest on coatings containing equal amounts of Cu-MBG and Cu-free MBG. Antibacterial tests confirmed that Cu-containing coatings reduced the growth of Porphyromonas gingivalis up to fivefold compared with uncoated implants. In conclusion, our data indicate that the EPD method is suitable to deposit nanoparticle-based coatings onto dental implants, which enhance endothelial cell migration and reduce bacterial growth. Impact statement Precise control over the release of therapeutic agents remains a major challenge for implant coatings. In this study, we introduce a simple and cost-effective way to tune the release of angiogenic and antibacterial copper ions using the electrophoretic deposition technique. Due to the flexibility and mild processing conditions of this technique, our method can also be used to incorporate other therapeutic agents onto implant surfaces.

Calcium phosphates (CaPs) and silicate-based bioglasses have been extensively studied since the early 1970s due to their unique capacity to bind to host bone, which led to their clinical translation and commercialization in the 1980s. Since the mid-1990s, researchers have synthesized nanoscale CaP and silicate-based particles of increased specific surface area, chemical reactivity, and solubility, which offer specific advantages compared to their bulk counterparts. This review provides a critical perspective on the history and emerging trends of these two classes of ceramic nanoparticles. Their synthesis and functional properties in terms of particle composition, size, shape, charge, dispersion, and toxicity are discussed as a function of relevant processing parameters. Specifically, emerging trends such as the influence of ion doping and mesoporosity on the biological and pharmaceutical performance of these nanoparticles are reviewed in more detail. Finally, a broad comparative overview is provided on the physicochemical properties and applicability of CaP and silicate-based nanoparticles within the fields of (i) local delivery of therapeutic agents, (ii) functionalization of biomaterial scaffolds or implant coatings, and (iii) bioimaging applications. Impact statement This review provides a critical perspective on the history and emerging trends of the two main classes of bioceramic nanoparticles, that is, calcium phosphate (CaP) and silicate-based nanoparticles. While most reviews in literature focus on either CaP or silicate-based nanoparticles, our review evaluates both classes of bioceramic nanoparticles simultaneously. This combined review offers the opportunity to analyze differences and similarities with respect to the historic development and emerging trends within both fields of bioceramics research.

Bone tissue engineering aims to restore and maintain the function of bone by means of biomaterial-based scaffolds. This review specifically focuses on the use of fibers in biomaterials used for bone tissue engineering as suitable environment for bone tissue repair and regeneration. We present a bioinspired rationale behind the use of fibers in bone tissue engineering and provide an overview of the most common fiber fabrication methods, including solution, melt, and microfluidic spinning. Subsequently, we provide a brief overview of the composition of fibers that are used in bone tissue engineering, including fibers composed of (i) natural polymers (e.g., cellulose, collagen, gelatin, alginate, chitosan, and silk, (ii) synthetic polymers (e.g., polylactic acid [PLA], polycaprolactone, polyglycolic acid [PGA], polyethylene glycol, and polymer blends of PLA and PGA), (iii) ceramic fibers (e.g., aluminium oxide, titanium oxide, and zinc oxide), (iv) metallic fibers (e.g., titanium and its alloys, copper and magnesium), and (v) composite fibers. In addition, we review the most relevant fiber modification strategies that are used to enhance the (bio)functionality of these fibers. Finally, we provide an overview of the applicability of fibers in biomaterials for bone tissue engineering, with a specific focus on mechanical, pharmaceutical, and biological properties of fiber-functionalized biomaterials for bone tissue engineering. Impact statement Natural bone is a complex composite material composed of an extracellular matrix of mineralized fibers containing living cells and bioactive molecules. Consequently, the use of fibers in biomaterial-based scaffolds offers a wide variety of opportunities to replicate the functional performance of bone. This review provides an overview of the use of fibers in biomaterials for bone tissue engineering, thereby contributing to the design of novel fiber-functionalized bone-substituting biomaterials of improved functionality regarding their mechanical, pharmaceutical, and biological properties.

Osteoclasts are multinucleated cells derived from the hematopoietic monocyte/macrophage lineage that possess the unique capacity to resorb bone. Due to the crucial role of osteoclasts in maintaining bone homeostasis and pathologies, this cell type is pivotal in multiple research areas dedicated to bone physiology in health and disease. Although numerous methods for generation of human osteoclasts are already available, those rely either on cell labeling-based purification or an intermediate adhesion step after which cells are directly differentiated toward osteoclasts. While the former requires additional reagents and equipment, the latter harbors the risk of variable osteoclast formation due to varying numbers of osteoclast precursors available for different donors. In this study, we report a facile and reliable three-step method for the generation of human osteoclasts from blood-derived precursor cells. Monocytes were obtained after adhering peripheral blood-derived mononuclear cells to plastic substrates followed by macrophage induction and proliferation resulting in a homogeneous population of osteoclast precursors. Finally, macrophages were seeded into suitable culture vessels and differentiated toward osteoclasts. Osteoclastogenesis was monitored longitudinally using nondestructive techniques, while the functionality of mature osteoclasts was confirmed after 14 days of culture by analysis of functional (e.g., elevated tartrate-resistant acid phosphatase [TRAP]-activity, resorption) and morphological (e.g., presence of TRAP, actin ring, and integrin β 3 ) characteristics. Furthermore, we propose to use combinatory staining of three morphological osteoclast markers, rather than previously reported staining of a single or maximal two markers, to clearly distinguish osteoclasts from undifferentiated mononuclear cells. Impact statement Research related to bone biology requires a standardized and reliable method for in vitro generation of human osteoclasts. We here describe such a procedure which avoids shortcomings of previously published protocols. Further, we report on nondestructive methods to qualitatively and quantitatively monitor osteoclastogenesis longitudinally, and on analysis of osteoclast generation and functionality after 14 days. Specifically, we recommend assessment of morphological human osteoclast characteristics using combinatory staining of three markers to confirm successful osteoclast generation.

Herein, we present a method to release chemotherapeutic platinum-bisphosphonate (Pt-BP) complexes from apatitic calcium phosphate cements (CPCs). Pt-BP-loaded hydroxyapatite nanoparticles (HA NPs) were added at different ratios to the powder phase of the cements, which contained poly(D,L-lactic-co-glycolic acid) (PLGA) microspheres as porogens to accelerate their degradation. In vitro release kinetics of Pt-BP complexes revealed that the release rate of Pt species can be tuned by varying the amount of drug-loaded HA NPs as well as modifying the chemical structure of the Pt-BP complex to tailor its affinity with HA NPs. In addition, the incorporation of PLGA microspheres into the CPCs increased the degradation rate of the materials without affecting the release rate of Pt species. Finally, the antiproliferative activity of the free Pt-BP complexes and Pt-BP-loaded CPCs was evaluated using both human osteosarcoma cancer cells (MG-63) and human bone marrow-derived mesenchymal stromal cells (h-BMMSCs). This study demonstrated that both free Pt-BP complexes and the releasates from the CPCs were antiproliferative in a dose-dependent manner. Moreover, their antiproliferative activity was higher on MG-63 cells compared to h-BMMSC primary cells. In summary, it was shown that injectable CPCs can be rendered chemotherapeutically active by incorporation of HA NPs loaded with HA-binding Pt-BP complexes.

In vitro degradation rates of calcium phosphate bioceramics are investigated using a large variation of soaking protocols that do not all match the dynamic conditions of the perfused physiological environment. Therefore, we studied the effect of stirring and fluid perfusion on the in vitro degradation rate of apatitic calcium phosphate cements (CPC) containing poly(lactic-co-glycolic acid) (PLGA) microspheres. The composites were soaked in phosphate-buffered saline up to 6 weeks under unstirred, stirred, or perfused conditions followed by analysis of mass loss, compression strength, porosity, crystal phase composition, and morphology of the cement composites. The results showed that fluid perfusion reduced the decrease in pH and corresponding degradation rates, while nonperfused soaking conditions (i.e., stirred and unstirred conditions) resulted into more extensive acidification, the rate of which increased with stirring. After 2 weeks, the formation of a secondary brushite phase was observed for cement composites soaked under nonperfused (i.e., stirred and unstirred) conditions, whereas this phase was not detected in cements soaked under perfused conditions. The degradation rate of cement composites decreased in the order unstirred>stirred>perfused, as evidenced by quantification of mass loss, compression strength, and pore morphology. To summarize, we have demonstrated that soaking conditions strongly affected the in vitro degradation process of CPCs. As a consequence, it can be concluded that the experimental design of current in vitro degradation studies does not allow for correlation to (pre-)clinical studies.

Calcium phosphate cements (CPCs) are frequently used as synthetic bone graft materials in view of their excellent osteocompatibility and clinical handling behavior. Hydroxyapatite-forming CPCs, however, degrade at very low rates, thereby limiting complete bone regeneration. The current study has investigated whether degradation of apatite-forming cements can be tuned by incorporating acid-producing slow-resorbing poly(D,L-lactic-co-glycolic) acid (PLGA) porogens, fast-resorbing glucono-delta-lactone (GDL) porogens, or mixtures thereof. The physicochemical, mechanical, and degradation characteristics of these CPC formulations were systematically analyzed upon soaking in phosphate-buffered saline (PBS). In parallel, various CPC formulations were implanted intramuscularly and orthotopically on top of the transverse process of goats followed by analysis of the soft tissue response and bone ingrowth after 12 weeks. In vitro degradation of GDL was almost completed after 2 weeks, as evidenced by characterization of the release of gluconic acid, while PLGA-containing CPCs released glycolic acid throughout the entire study (12 weeks), resulting in a decrease in compression strength of CPC. Extensive in vitro degradation of the CPC matrix was observed upon simultaneous incorporation of 30% PLGA-10% GDL. Histomorphometrical evaluation of the intramuscularly implanted samples revealed that all CPCs exhibited degradation, accompanied by an increase in capsule thickness. In the in vivo goat transverse process model, incorporation of 43% PLGA, 30% PLGA-5% GDL, and 30% PLGA-10% GDL in CPC significantly increased bone formation and resulted in higher bone height compared with both 10% GDL and 20% GDL-containing CPC samples.

The current study focused on the development of a rapid, straightforward quantification method based on the use of enzymatic decomposition of urea using urease to assess the mineralization capacity of a wide range of biomaterials for bone regeneration. Urea-containing mineralizing solutions (MSs) (containing: Na2HPO4, CaCl2, and NaCl at 37°C and pH 6.0) were used in the mineralization experiments. Urease was added to these solutions to induce enzymatic decomposition of urea resulting in increased pH and deposition of calcium phosphate. By optimizing the ionic and urease concentrations in these MSs, it was shown that the proposed system could mineralize titanium substrates with six different pretreatments, as opposed to normal simulated body fluid that mineralized only two of them. It was possible to rank the mineralization capacity of these substrates by measuring the amount of calcium deposited. Furthermore, the ranking of (i) various polymeric substrates and (ii) hydrogels with and without functionalization with calcium-binding bisphosphonate groups was also possible. These results confirm that the proposed testing system has a broad applicability in the field of biomaterials due to its inherent versatility and discriminative power.

Mimicking the nanostructure of bone and understanding the interactions between the nanoscale inorganic and organic components of the extracellular bone matrix are crucial for the design of biomaterials with structural properties and a functionality similar to the natural bone tissue. Generally, these interactions involve anionic and/or cationic functional groups as present in the organic matrix, which exhibit a strong affinity for either calcium or phosphate ions from the mineral phase of bone. This study reviews the interactions between the mineral and organic extracellular matrix components in bone tissue as a source of inspiration for the design of novel nanocomposites. After providing a brief description of the various structural levels of bone and its main constituents, a concise overview is presented on the process of bone mineralization as well as the interactions between calcium phosphate (CaP) nanocrystals and the organic matrix of bone tissue. Bioinspired synthetic approaches for obtaining nanocomposites are subsequently addressed, with specific focus on chemical groups that have affinity for CaPs or are involved in stimulating and controlling mineral formation, that is, anionic functional groups, including carboxyl, phosphate, sulfate, hydroxyl, and catechol groups.

Injectable calcium phosphate cements (CPC) are frequently used for filling of bone defects due to their excellent osteocompatibility. Their poor degradability, however, limits complete regeneration of bone defects. Organic additives that produce acid by-products are particularly attractive to create macroporosity in situ since CPC degrade by acid dissolution. The aim of the current study was to investigate whether glucono-delta-lactone (GDL) can be used as acid-producing microparticles for incorporation into CPC without compromising its osteocompatibility. Characterization studies confirmed that CPCs containing either low or high amounts of GDL were injectable and self-setting, while a considerable amount of porosity was formed already within 1 day of incubation in phosphate buffered saline due to dissolution of GDL. Histomorphometrical evaluation after 2 weeks of implantation revealed that CPC containing 10% of GDL degraded faster and was replaced by more bone tissue than CPCs containing either Poly (lactic-co-glycolic acid) (PLGA) or gelatin microspheres. Summarizing, the current study showed that CPCs containing appropriate amounts of GDL display accelerated degradation and new bone formation compared with CPCs containing microparticles made of conventional polymers such as PLGA or gelatin.

Porous titanium scaffolds are a promising class of biomaterials for grafting large bone defects, because titanium provides sufficient mechanical support, whereas its porous structure allows bone ingrowth resulting in good osseointegration. To reinforce porous titanium scaffolds with biological cues that enhance and continue bone regeneration, scaffolds can be incorporated with bioactive gels for time- and dose-controlled delivery of multiple growth factors (GFs). In this study, critical femoral bone defects in rats were grafted with porous titanium scaffolds incorporated with nanostructured colloidal gelatin gels. Gels were loaded with bone morphogenetic protein-2 (BMP-2, 3 μg), fibroblast growth factor-2 (FGF-2, 0.6 μg), BMP-2, and FGF-2 (BMP-2/FGF-2, ratio 5:1) or were left unloaded. GF delivery was controlled by fine tuning the crosslinking density of oppositely charged nanospheres. Grafted femurs were evaluated using in vivo and ex vivo micro-CT, histology, and three-point bending tests. All porous titanium scaffolds containing GF-loaded gels accelerated and enhanced bone regeneration: BMP-2 gels gave an early increase (0-4 weeks), and FGF-2 gels gave a late increase (8-12 weeks). Interestingly, stimulatory effects of 0.6 μg FGF-2 were similar to a fivefold higher dose of BMP-2 (3 μg). BMP-2/FGF-2 gels gave more bone outside the porous titanium scaffolds than gels with only BMP-2 or FGF-2, resulted in bridging of most defects and showed superior bone-implant integrity in three-point bending tests. In conclusion, incorporation of nanostructured colloidal gelatin gels capable of time- and dose-controlled delivery of BMP-2 and FGF-2 in porous titanium scaffolds is a promising strategy to enhance and continue bone regeneration of large bone defects.

Poly(lactic-co-glycolic acid) (PLGA) is the most often used synthetic polymer within the field of bone regeneration owing to its biocompatibility and biodegradability. As a consequence, a large number of medical devices comprising PLGA have been approved for clinical use in humans by the American Food and Drug Administration. As compared with the homopolymers of lactic acid poly(lactic acid) and poly(glycolic acid), the co-polymer PLGA is much more versatile with regard to the control over degradation rate. As a material for bone regeneration, the use of PLGA has been extensively studied for application and is included as either scaffolds, coatings, fibers, or micro- and nanospheres to meet various clinical requirements.

Immobilization of biomolecules onto implant surfaces is highly relevant in many areas of biomaterial research. Recently, a 2-step immobilization procedure was developed for the facile conjugation of biomolecules onto various surfaces using self-polymerization of dopamine into polydopamine. In the current study, a 1-step polydopamine-based approach was applied for alkaline phosphatase (ALP) and bone morphogenetic protein-2 (BMP-2) immobilization, and compared to the conventional 2-step polydopamine-based immobilization and plain adsorption. To this end, ALP and BMP-2 were immobilized onto titanium and polytetrafluoroethylene (PTFE) substrates. The absolute quantity and biological activity of immobilized ALP were assessed quantitatively to compare the three types of immobilization. Plain adsorption of both ALP and BMP-2 was inferior to both polydopamine-based immobilization approaches. ALP was successfully immobilized onto titanium and PTFE surfaces via the 1-step approach, and the immobilized ALP retained its enzymatic activity. Using the 1-step approach, the amount of immobilized ALP was increased twofold to threefold compared to the conventional 2-step immobilization process. In contrast, more BMP-2 was immobilized using the conventional 2-step immobilization approach. Retention of ALP and BMP-2 was measured over a period of 4 weeks and was found to be similar for the 1-step and 2-step methods and far superior to the retention of adsorbed biomolecules due to the formation of covalent linkages between catechol moieties and immobilized proteins. The biological behavior of ALP and BMP-2 coatings immobilized using polydopamine (1- and 2-step) as well as adsorption was assessed by culturing rat bone marrow cells, which revealed that the cell responses to the various experimental groups were not statistically different. In conclusion, the 1-step polydopamine-based immobilization method was shown to be more efficient for immobilization of ALP, whereas the conventional 2-step method was shown to be more efficient for attachment of BMP-2 onto implant surfaces.

Preprosthetic surgery has become a routine procedure to obtain sufficient bone quantity and quality for dental implant installation in patients with an initial inadequate bone volume. Although autologous bone onlay or inlay grafting is still the preferred bone augmentation technique, a broad range of synthetic bone substitutes have been developed, for example, calcium phosphate cement (CPC). The introduction of porosity within CPC can be used to increase CPC degradation and bone ingrowth. Therefore, three different strategies to obtain porous CPCs were evaluated in this preclinical study. Instantaneously porous CPC (CPC-IP) was compared with delayed porous CPC in vitro and in vivo. CPC-IP was obtained by the creation of CO₂ bubbles during setting, whereas delayed porous CPC was obtained after the degradation of incorporated poly(lactic-co-glycolic acid) (PLGA) microspheres. As an additional aspect, delayed porous CPC was created by the incorporation of either hollow or dense degradable PLGA microspheres (CPC-hPLGA and CPC-dPLGA). All CPC compositions showed appropriate clinical handling properties and an interconnected porous structure with a final porosity above 70% (v/v). In vitro degradation studies showed the gradual formation of pores and further CPC-matrix dissolution for CPCs containing PLGA microspheres (dPLGA microspheres > hPLGA microspheres). For in vivo evaluation of the CPCs, an augmentation model was used, allowing a CPC injection into a rigidly immobilized Teflon ring on the rat skull. Histological evaluation after 12 weeks of implantation showed bone formation using all three CPCs. Bone apposition reached volumetric amounts of up to 10% of the augmentation area and a maximum augmentation height of ∼1 mm. CPC-IP showed significantly more bone formation and resulted in a superior bone apposition height compared with both CPCs containing PLGA microspheres. No differences in biological performance were observed between the CPCs containing hPLGA and those containing dPLGA microspheres. Further research is necessary to enhance the bone appositional speed and amount of CPCs for bone augmentation procedures before them being used in a potential clinical setting.

During the last decade, the use of micro- and nanospheres as functional components for bone tissue regeneration has drawn increasing interest. Scaffolds comprising micro- and nanospheres display several advantages compared with traditional monolithic scaffolds that are related to (i) an improved control over sustained delivery of therapeutic agents, signaling biomolecules and even pluripotent stem cells, (ii) the introduction of spheres as stimulus-sensitive delivery vehicles for triggered release, (iii) the use of spheres to introduce porosity and/or improve the mechanical properties of bulk scaffolds by acting as porogen or reinforcement phase, (iv) the use of spheres as compartmentalized microreactors for dedicated biochemical processes, (v) the use of spheres as cell delivery vehicle, and, finally, (vi) the possibility of preparing injectable and/or moldable formulations to be applied by using minimally invasive surgery. This article focuses on recent developments with regard to the use of micro- and nanospheres for bone regeneration by categorizing micro-/nanospheres by material class (polymers, ceramics, and composites) as well as summarizing the main strategies that employ these spheres to improve the functionality of scaffolds for bone tissue engineering.

The chemical resemblance of calcium phosphate (CaP) cements and the mineral phase of bone is a problem in distinguishing CaP cement from bone tissue by means of common, noninvasive techniques (e.g., X-ray imaging and microcomputed tomography [μCT]). In this study, the feasibility of using tantalumpentoxide (Ta(2)O(5)) powder as radiopacifier in CaP cements was analyzed. A distal femoral condyle model in male adult Wistar rats was used. After 6 weeks of implantation time, the results were analyzed by means of μCT and histology. Unambiguous distinction of CaP cement from native bone tissue and volumetric measurements of the materials appeared to be possible by means of μCT scanning. Furthermore, there was no evidence of either inflammation or fibrous tissue around the implant materials or at the bone-material interface. In conclusion, the addition of Ta(2)O(5) as a radiopacifying additive to CaP cements allows discrimination between bone substitute and surrounding bone tissue. Consequently, Ta(2)O(5) represents an effective and biocompatible additive in CaP cements for in vivo monitoring purposes., (© Mary Ann Liebert, Inc.)

Hydrogels are an important class of highly hydrated polymers that are widely investigated for potential use in soft tissue engineering. Generally, however, hydrogels lack the ability to mineralize, preventing the formation of chemical bonds with hard tissues such as bone. A recent trend in tissue engineering involves the development of hydrogels that possess the capacity to mineralize. The strategy that has attracted most interest has been the incorporation of inorganic phases such as calcium phosphate ceramics and bioglasses into hydrogel matrices. These inorganic particles act as nucleation sites that enable further mineralization, thus improving the mechanical properties of the composite material. A second route to create nucleation sites for calcification of hydrogels involves the use of features from the physiological mineralization process. Examples of these biomimetic mineralization strategies include (1) soaking of hydrogels in solutions that are saturated with respect to calcium phosphate, (2) incorporation of enzymes that catalyze deposition of bone mineral, and (3) incorporation of synthetic analogues to matrix vesicles that are the initial sites of biomineralization. Functionalization of the polymeric hydrogel backbone with negatively charged groups is a third mechanism to promote mineralization in otherwise inert hydrogels. This review summarizes the main strategies that have been developed in the past decade to calcify hydrogel matrices and render these hydrogels suitable for applications in bone regeneration.