The unexplored role of alkali and alkaline earth elements (ALAEs) on the structure, processing, and biological effects of bioactive glasses

Bioactive glass has been employed in several medical applications since its inception in 1969. The compositions of these materials have been investigated extensively with emphasis on glass network formers, therapeutic transition metals, and glass network modifiers. Through these experiments, several commercial and experimental compositions have been developed with varying chemical durability, induced physiological responses, and hydroxyapatite forming abilities. In many of these studies, the concentrations of each alkali and alkaline earth element have been altered to monitor changes in structure and biological response. This review aims to discuss the impact of each alkali and alkaline earth element on the structure, processing, and biological effects of bioactive glass. We explore critical questions regarding these elements from both a glass science and biological perspective. Should elements with little biological impact be included? Are alkali free bioactive glasses more promising for greater biological responses? Does this mixed alkali effect show increased degradation rates and should it be employed for optimized dissolution? Each of these questions along with others are evaluated comprehensively and discussed in the final section where guidance for compositional design is provided.

Ecofriendly and scalable production of bioglass using an organic calcium source enhanced bioactivity for tissue repair

The eco-friendly and scalable production of bioglass remains a challenging but attractive strategy for advancing its widespread biomedical applications. Although the sol-gel method has been considered a valuable approach for bioglass production, the application of calcium nitrate as a calcium source markedly limits its industrialization owing to environmental pollution, high administration costs, and numerous calcium-rich regions in the as-prepared bioglass. Therefore, organic Ca has been proposed as an alternative to inorganic Ca. In the current study, bioglass was successfully prepared using a novel calcium source (calcium glycerol) and was named regeneration silicon (RegeSi). The biocompatibity of bioglass was examined by performing the methyl thiazolyl tetrazolium (MTT) assay using L929 fibroblasts. The biological and tissue repair properties of RegeSi were better than those of bioglass prepared with calcium nitrate using the sol-gel or traditional melting methods. The applicability of RegeSi was validated using suitable wound healing and dental restoration models. Notably, RegeSi ensured closure of a deep wound (1.6 cm diameter, 2 mm depth) within 11 d. Moreover, RegeSi facilitated tooth repair with a blocking rate of 97.1%. More importantly, large-scale production of RegeSi was achieved at low cost, high bioactivity, and using environmental technology, reaching a capacity of 100 kg/batch.

3D printed hybrid scaffolds for bone regeneration using calcium methoxyethoxide as a calcium source

Introduction: Hybrids consist of inorganic and organic co-networks that are indistinguishable above the nanoscale, which can lead to unprecedented combinations of properties, such as high toughness and controlled degradation. Methods: We present 3D printed bioactive hybrid scaffolds for bone regeneration, produced by incorporating calcium into our "Bouncy Bioglass", using calcium methoxyethoxide (CME) as the calcium precursor. SiO2-CaOCME/PTHF/PCL-diCOOH hybrid "inks" for additive manufacturing (Direct Ink Writing) were optimised for synergy of mechanical properties and open interconnected pore channels. Results and Discussion: Adding calcium improved printability. Changing calcium content (5, 10, 20, 30, and 40 mol.%) of the SiO2-CaOCME/PTHF/PCL-diCOOH hybrids affected printability and mechanical properties of the lattice-like scaffolds. Hybrids containing 30 mol.% calcium in the inorganic network (70S30CCME-CL) printed with 500 µm channels and 100 µm strut size achieved the highest strength (0.90 ± 0.23 MPa) and modulus of toughness (0.22 ± 0.04 MPa). These values were higher than Ca-free SiO2/PTHF/PCL-diCOOH hybrids (0.36 ± 0.14 MPa strength and 0.06 ± 0.01 MPa toughness modulus). Over a period of 90 days of immersion in simulated body fluid (SBF), the 70S30CCME-CL hybrids also kept a stable strain to failure (~30 %) and formed hydroxycarbonate apatite within three days. The extracts released by the 70S30CCME-CL hybrids in growth medium did not cause cytotoxic effects on human bone marrow stromal cells over 24 h of culture.

Bioactive glass-based organic/inorganic hybrids: an analysis of the current trends in polymer design and selection

Bioactive glass-based organic/inorganic hybrids are a family of materials holding great promise in the biomedical field. Developed from bioactive glasses following recent advances in sol-gel and polymer chemistry, they can overcome many limitations of traditional composites typically used in bone repair and orthopedics. Thanks to their unique molecular structure, hybrids are often characterized by synergistic properties that go beyond a mere combination of their two components; it is possible to synthesize materials with a wide variety of mechanical and biological properties. The polymeric component, in particular, can be tailored to prepare tough, load-bearing materials, or rubber-like elastomers. It can also be a key factor in the determination of a wide range of interesting biological properties. In addition, polymers can also be used within hybrids as carriers for therapeutic ions (although this is normally the role of silica). This review offers a brief look into the history of hybrids, from the discovery of bioactive glasses to the latest developments, with a particular emphasis on polymer design and chemistry. First the benefits and limitations of hybrids will be discussed and compared with those of alternative approaches (for instance, nanocomposites). Then, key advances in the field will be presented focusing on the polymeric component: its chemistry, its physicochemical and biological advantages, its drawbacks, and selected applications. Comprehensive tables summarizing all the polymers used to date to fabricate sol-gel hybrids for biomedical applications are also provided, to offer a handbook of all the available candidates for hybrid synthesis. In addition to the current trends, open challenges and possible avenues of future development are proposed.

Gelatin and Bioactive Glass Composites for Tissue Engineering: A Review

Nano-/micron-sized bioactive glass (BG) particles are attractive candidates for both soft and hard tissue engineering. They can chemically bond to the host tissues, enhance new tissue formation, activate cell proliferation, stimulate the genetic expression of proteins, and trigger unique anti-bacterial, anti-inflammatory, and anti-cancer functionalities. Recently, composites based on biopolymers and BG particles have been developed with various state-of-the-art techniques for tissue engineering. Gelatin, a semi-synthetic biopolymer, has attracted the attention of researchers because it is derived from the most abundant protein in the body, viz., collagen. It is a polymer that can be dissolved in water and processed to acquire different configurations, such as hydrogels, fibers, films, and scaffolds. Searching "bioactive glass gelatin" in the tile on Scopus renders 80 highly relevant articles published in the last ~10 years, which signifies the importance of such composites. First, this review addresses the basic concepts of soft and hard tissue engineering, including the healing mechanisms and limitations ahead. Then, current knowledge on gelatin/BG composites including composition, processing and properties is summarized and discussed both for soft and hard tissue applications. This review explores physical, chemical and mechanical features and ion-release effects of such composites concerning osteogenic and angiogenic responses in vivo and in vitro. Additionally, recent developments of BG/gelatin composites using 3D/4D printing for tissue engineering are presented. Finally, the perspectives and current challenges in developing desirable composites for the regeneration of different tissues are outlined.

Osteogenic Effect of Fisetin Doping in Bioactive Glass/Poly(caprolactone) Hybrid Scaffolds

Treating large bone defects or fragile patients may require enhancing the bone regeneration rate to overcome a weak contribution from the body. This work investigates the osteogenic potential of nutrient fisetin, a flavonoid found in fruits and vegetables, as a doping agent inside the structure of a SiO₂-CaO bioactive glass-poly(caprolactone) (BG-PCL) hybrid scaffold. Embedded in the full mass of the BG-PCL hybrid during one-pot synthesis, we demonstrate fisetin to be delivered sustainably; the release follows a first-order kinetics with active fisetin concentration being delivered for more than 1 month (36 days). The biological effect of BG-PCL-fisetin-doped scaffolds (BG-PCL-Fis) has been highlighted by in vitro and in vivo studies. A positive impact is demonstrated on the adhesion and the differentiation of rat primary osteoblasts, without an adverse cytotoxic effect. Implantation in critical-size mouse calvaria defects shows bone remodeling characteristics and remarkable enhancement of bone regeneration for fisetin-doped scaffolds, with the regenerated bone volume being twofold that of nondoped scaffolds and fourfold that of a commercial trabecular bovine bone substitute. Such highly bioactive materials could stand as competitive alternative strategies involving biomaterials loaded with growth factors, the use of the latter being the subject of growing concerns.

Synthesis of an organic-inorganic hybrid with short organic molecular chains by sol-gel chemistry

An oligomer with short organic molecular chains was successfully synthesized with KH550 and KH560. This oligomer was combined with tetraethyl orthosilicate and calcium chloride to prepare an organic-inorganic hybrid biological material (OI-BM) by sol-gel chemistry. The hybrid was fully characterized by a series of instrumental characterizations including nuclear magnetic resonance spectrometry, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray energy spectroscopy and inductively coupled plasma spectrometry. OI-BM presented elastic deformation under compression. The elastic modulus and ultimate stress of OI-BM were 0.4 ± 0.1 GPa and 23.0 ± 4.0 MPa, respectively, lower than those of 45S5 bioactive glass (45S5-BG), whereas the strain at failure and modulus of toughness of OI-BM was about 4.5 times and 4 times higher. The hybrid is easy to form due to the improved mechanical property, suggesting excellent machining properties. The hybrid OI-BM produced hydroxyapatite in 1 h in simulated body fluid due to its excellent bioactivity. CCK-8 assay further demonstrated the desirable cytocompatibility of the hybrid. Thus, the hybrid can be a potential material for satisfying the mechanical property requirement of an implant.

Sustained Calcium(II)-Release to Impart Bioactivity in Hybrid Glass Scaffolds for Bone Tissue Engineering

In this study, we integrated different calcium sources into sol-gel hybrid glass scaffolds with the aim of producing implants with long-lasting calcium release while maintaining mechanical strength of the implant. Calcium(II)-release was used to introduce bioactivity to the material and eventually support implant integration into a bone tissue defect. Tetraethyl orthosilicate (TEOS) derived silica sols were cross-linked with an ethoxysilylated 4-armed macromer, pentaerythritol ethoxylate and processed into macroporous scaffolds with defined pore structure by indirect rapid prototyping. Triethyl phosphate (TEP) was shown to function as silica sol solvent. In a first approach, we investigated the integration of 1 to 10% CaCl2 in order to test the hypothesis that small CaCl2 amounts can be physically entrapped and slowly released from hybrid glass scaffolds. With 5 and 10% CaCl2 we observed an extensive burst release, whereas slightly improved release profiles were found for lower Calcium(II) contents. In contrast, introduction of melt-derived bioactive 45S5 glass microparticles (BG-MP) into the hybrid glass scaffolds as another Calcium(II) source led to an approximately linear release of Calcium(II) in Tris(hydroxymethyl)aminomethane (TRIS) buffer over 12 weeks. pH increase caused by BG-MP could be controlled by their amount integrated into the scaffolds. Compression strength remained unchanged compared to scaffolds without BG-MP. In cell culture medium as well as in simulated body fluid, we observed a rapid formation of a carbonated hydroxyapatite layer on BG-MP containing scaffolds. However, this mineral layer consumed the released Calcium(II) ions and prevented an additional increase in Calcium(II) concentration in the cell culture medium. Cell culture studies on the different scaffolds with osteoblast-like SaOS-2 cells as well as bone marrow derived mesenchymal stem cells (hMSC) did not show any advantages concerning osteogenic differentiation due to the integration of BG-MP into the scaffolds. Nonetheless, via the formation of a hydroxyapatite layer and the ability to control the pH increase, we speculate that implant integration in vivo and bone regeneration may benefit from this concept.

A Review of Bioactive Glass/Natural Polymer Composites: State of the Art

Collagen, gelatin, silk fibroin, hyaluronic acid, chitosan, alginate, and cellulose are biocompatible and non-cytotoxic, being attractive natural polymers for medical devices for both soft and hard tissues. However, such natural polymers have low bioactivity and poor mechanical properties, which limit their applications. To tackle these drawbacks, collagen, gelatin, silk fibroin, hyaluronic acid, chitosan, alginate, and cellulose can be combined with bioactive glass (BG) nanoparticles and microparticles to produce composites. The incorporation of BGs improves the mechanical properties of the final system as well as its bioactivity and regenerative potential. Indeed, several studies have demonstrated that polymer/BG composites may improve angiogenesis, neo-vascularization, cells adhesion, and proliferation. This review presents the state of the art and future perspectives of collagen, gelatin, silk fibroin, hyaluronic acid, chitosan, alginate, and cellulose matrices combined with BG particles to develop composites such as scaffolds, injectable fillers, membranes, hydrogels, and coatings. Emphasis is devoted to the biological potentialities of these hybrid systems, which look rather promising toward a wide spectrum of applications.

Bone Repair and Regenerative Biomaterials: Towards Recapitulating the Microenvironment

Biomaterials and tissue engineering scaffolds play a central role to repair bone defects. Although ceramic derivatives have been historically used to repair bone, hybrid materials have emerged as viable alternatives. The rationale for hybrid bone biomaterials is to recapitulate the native bone composition to which these materials are intended to replace. In addition to the mechanical and dimensional stability, bone repair scaffolds are needed to provide suitable microenvironments for cells. Therefore, scaffolds serve more than a mere structural template suggesting a need for better and interactive biomaterials. In this review article, we aim to provide a summary of the current materials used in bone tissue engineering. Due to the ever-increasing scientific publications on this topic, this review cannot be exhaustive; however, we attempted to provide readers with the latest advance without being redundant. Furthermore, every attempt is made to ensure that seminal works and significant research findings are included, with minimal bias. After a concise review of crystalline calcium phosphates and non-crystalline bioactive glasses, the remaining sections of the manuscript are focused on organic-inorganic hybrid materials.

The synergistic effects of graphene-contained 3D-printed calcium silicate/poly-ε-caprolactone scaffolds promote FGFR-induced osteogenic/angiogenic differentiation of mesenchymal stem cells

Graphene-contained calcium silicate (CS)/polycaprolactone (PCL) scaffold (GCP) provides an alternative solution that can bring several bone formation properties, such as osteoinductive. This study finds out the optimal percentage of graphene additive to calcium silicate and polycaprolactone mixture for excellent in vitro and in vivo bone-regeneration ability, in addition, this scaffold could fabricate by 3D printing technology and demonstrates distinct mechanical, degradation, and biological behavior. With controlled structure and porosity by 3D printing, osteogenesis and proliferation capabilities of Wharton's Jelly derived mesenchymal stem cells (WJMSCs) were significantly enhanced when cultured on 3D printed GCP scaffolds. In this study, it was also discovered that fibroblast growth factor receptor (FGFR) plays an active role in modulating differentiation behavior of WJMSCs cultured on GCP scaffolds. The validation has been proved by analyzed the decreased cell proliferation, osteogenic-related protein (ALP and OC), and angiogenic-related protein (VEGF and vWF) with FGFR knockdown on all experimental groups. Moreover, this study infers that the GCP scaffold could induce the effects of proliferation, differentiation and related protein expression on WJMSCs through FGFR pathway. In summary, this research indicated the 3D-printed GCP scaffolds own the dual bioactivities to reach the osteogenesis and vascularization for bone regeneration.

Optimized Bioactive Glass: the Quest for the Bony Graft

Technological advances have provided surgeons with a wide range of biomaterials. Yet improvements are still to be made, especially for large bone defect treatment. Biomaterial scaffolds represent a promising alternative to autologous bone grafts but in spite of the numerous studies carried out on this subject, no biomaterial scaffold is yet completely satisfying. Bioactive glass (BAG) presents many qualifying characteristics but they are brittle and their combination with a plastic polymer appears essential to overcome this drawback. Recent advances have allowed the synthesis of organic-inorganic hybrid scaffolds combining the osteogenic properties of BAG and the plastic characteristics of polymers. Such biomaterials can now be obtained at room temperature allowing organic doping of the glass/polymer network for a homogeneous delivery of the doping agent. Despite these new avenues, further studies are required to highlight the biological properties of these materials and particularly their behavior once implanted in vivo. This review focuses on BAG with a particular interest in their combination with polymers to form organic-inorganic hybrids for the design of innovative graft strategies.

Polycaprolactone / bioactive glass hybrid scaffolds for bone regeneration

Bioactive glasses (BG) bond to bone and stimulate bone regeneration, but they are brittle. Inorganicorganic hybrids appear as promising bone substitutes since they associate the bone mineral forming ability of BG with the toughness of polymers. Hybrids comprised of polycaprolactone (PCL) and SiO2-CaO BG were produced by sol-gel chemistry and processed into porous scaffolds with controlled pore and interconnection sizes. The obtained scaffolds are highly flexible, meaning that PCL effectively introduces toughness. Apatite formation is observed within 24 hours of immersion in simulated body fluid (SBF) and is not limited to the surface as the entire hybrid progressively changes into bone-like minerals. The degradation rate is suitable for bone regeneration with a 13.2% weight loss after 8 weeks of immersion. Primary osteoblasts cultured in scaffolds demonstrate that the samples are not cytotoxic and provide good cell adhesion. The in vivo study confirms the bioactivity, biocompatibility and suitable degradation rate of the hybrid. A physiological bone made of trabeculae and bone marrow regenerates. The structure and kinetic of bone regeneration was similar to the implanted commercial standard based on bovine bone, demonstrating that this new synthetic PCL-BG hybrid could perform as well as animal-derived bone substitutes.

Improved cell adhesion and osteogenesis using a PLTGA (poly l-lactide, 1,3-trimethylene carbonate, and glycolide) terpolymer by gelatin-assisted hydroxyapatite immobilization for bone regeneration

Schematic illustration of the procedures for preparing the GEL/HAP-coated PLTGA film, and representative images of the improved cellular behaviors.

Biodegradable and adjustable sol-gel glass based hybrid scaffolds from multi-armed oligomeric building blocks

Biodegradability is a crucial characteristic to improve the clinical potential of sol-gel-derived glass materials. To this end, a set of degradable organic/inorganic class II hybrids from a tetraethoxysilane(TEOS)-derived silica sol and oligovalent cross-linker oligomers containing oligo(d,l-lactide) domains was developed and characterized. A series of 18 oligomers (Mn: 1100-3200Da) with different degrees of ethoxylation and varying length of oligoester units was established and chemical composition was determined. Applicability of an established indirect rapid prototyping method enabled fabrication of a total of 85 different hybrid scaffold formulations from 3-isocyanatopropyltriethoxysilane-functionalized macromers. In vitro degradation was analyzed over 12months and a continuous linear weight loss (0.2-0.5wt%/d) combined with only moderate material swelling was detected which was controlled by oligo(lactide) content and matrix hydrophilicity. Compressive strength (2-30MPa) and compressive modulus (44-716MPa) were determined and total content, oligo(ethylene oxide) content, oligo(lactide) content and molecular weight of the oligomeric cross-linkers as well as material porosity were identified as the main factors determining hybrid mechanics. Cytocompatibility was assessed by cell culture experiments with human adipose tissue-derived stem cells (hASC). Cell migration into the entire scaffold pore network was indicated and continuous proliferation over 14days was found. ALP activity linearly increased over 2weeks indicating osteogenic differentiation. The presented glass-based hybrid concept with precisely adjustable material properties holds promise for regenerative purposes.

STATEMENT OF SIGNIFICANCE

Adaption of degradation kinetics toward physiological relevance is still an unmet challenge of (bio-)glass engineering. We therefore present a glass-derived hybrid material with adjustable degradation. A flexible design concept based on degradable multi-armed oligomers was combined with an established indirect rapid prototyping method to produce a systematic set of porous sol-gel-derived class II hybrid scaffolds. Mechanical properties in the range of cancellous bone were narrowly controlled by hybrid composition. The oligoester introduction resulted in significantly increased compressive moduli. Cytocompatible hybrids degraded in physiologically relevant time frames and a promising linear and controllable weight loss profile was found. To our knowledge, our degradation study represents the most extensive long-term investigation of sol-gel-derived class II hybrids. Due to the broad adjustability of material properties, our concept offers potential for engineering of biodegradable hybrid materials for versatile applications.