Plug and play: combining materials and technologies to improve bone regenerative strategies

Comparison of bovine serum albumin and chitosan effects on calcium phosphate formation in the presence of silver nanoparticles

The precipitation of calcium phosphates (CaPs) in the presence of more than one type of additive is of interest both from a fundamental point of view and as a possible biomimetic route for the preparation of multicomponent composites in which the activity of the components is preserved. In this study, the effect of bovine serum albumin (BSA) and chitosan (Chi) on the precipitation of CaPs in the presence of silver nanoparticles (AgNPs) stabilized with sodium bis(2-ethylhexyl)sulfosuccinate (AOT-AgNPs), poly(vinylpyrrolidone) (PVP-AgNPs), and citrate (cit-AgNPs) was investigated. In the control system, the precipitation of CaPs occurred in two steps. Amorphous calcium phosphate (ACP) was the first precipitated solid, which transformed into a mixture of calcium-deficient hydroxyapatite (CaDHA) and a smaller amount of octacalcium phosphate (OCP) after 60 min of ageing. Both biomacromolecules inhibited ACP transformation, with Chi being a stronger inhibitor due to its flexible molecular structure. As the concentration of the biomacromolecules increased, the amount of OCP decreased both in the absence and presence of AgNPs. In the presence of cit-AgNPs and two highest BSA concentrations, a change in the composition of the crystalline phase was observed. Calcium hydrogen phosphate dihydrate was formed in the mixture with CaDHA. An effect on the morphology of both the amorphous and crystalline phases was observed. The effect depended on the specific combination of biomacromolecules and differently stabilized AgNP. The results obtained suggest a simple method for fine-tuning the properties of precipitates using different classes of additives. This could be of interest for the biomimetic preparation of multifunctional composites for bone tissue engineering.

Sulfonate Hydrogel-siRNA Conjugate Facilitates Osteogenic Differentiation of Mesenchymal Stem Cells by Controlled Gene Silencing and Activation of BMP Signaling

Hydrogels have been widely used in bone tissue engineering due to their tunable characteristics that allow facile modifications with various biochemical properties to support cell growth and guide proper cell functions. Herein, we report a design of hydrogel-siRNA conjugate that facilitates osteogenesis via gene silencing and activation of bone morphogenetic protein (BMP) signaling. A sulfonate hydrogel is prepared by modifying chitosan with sulfoacetic acid to mimic a natural sulfated polysaccharide and to provide a hydrogel surface that enables BMP binding. Then, siRNA targeting noggin, an endogenous extracellular antagonist of BMP signaling, is covalently conjugated to the sulfonate hydrogel by visible blue light crosslinking. The sulfonate hydrogel-siRNA conjugate is efficient to bind BMPs and also successfully prolongs the release of siRNA for sustained noggin suppression, thereby resulting in significantly increased osteogenic differentiation. Lastly, demineralized bone matrix (DBM) is incorporated into the sulfonate hydrogel-siRNA conjugate, wherein the DBM incorporation induces noggin expression via a negative feedback mechanism that regulates BMP signaling in DBM. However, simultaneous delivery of siRNA downregulates noggin thus facilitating endogenous BMP activity and enhancing the osteogenic efficacy of DBM. These findings support a promising hydrogel RNA silencing platform for bone tissue engineering applications.

Precipitation at Room Temperature as a Fast and Versatile Method for Calcium Phosphate/TiO2 Nanocomposites Synthesis

The constantly growing need for advanced bone regeneration materials has motivated the development of calcium phosphates (CaPs) composites with a different metal or metal-oxide nanomaterials and their economical and environmentally friendly production. Here, two procedures for the synthesis of CaPs composites with TiO₂ nanoplates (TiNPl) and nanowires (TiNWs) were tested, with the immersion of TiO₂ nanomaterials (TiNMs) in corrected simulated body fluid (c-SBF) and precipitation of CaP in the presence of TiNMs. The materials obtained were analyzed by powder X-ray diffraction, spectroscopic and microscopic techniques, Brunauer–Emmett–Teller surface area analysis, thermogravimetric analysis, dynamic and electrophoretic light scattering, and their hemocompatibility and ability to induce reactive oxygen species were evaluated. After 28 days of immersion in c-SBF, no significant CaP coating was formed on TiNMs. However, the composites with calcium-deficient apatite (CaDHA) were obtained after one hour in the spontaneous precipitation system. In the absence of TiNMs, CaDHA was also formed, indicating that control of the CaP phase formed can be accomplished by fine-tuning conditions in the precipitation system. Although the morphology and size of crystalline domains of CaDHA obtained on the different nanomaterials differed, no significant difference was detected in their local structure. Composites showed low reactive oxygen species (ROS) production and did not induce hemolysis. The results obtained indicate that precipitation is a suitable and fast method for the preparation of CaPs/TiNMs nanocomposites which shows great potential for biomedical applications.

Two-Photon Polymerized Poly(2-Ethyl-2-Oxazoline) Hydrogel 3D Microstructures with Tunable Mechanical Properties for Tissue Engineering

The main task of tissue engineering (TE) is to reproduce, replicate, and mimic all kinds of tissues in the human body. Nowadays, it has been proven useful in TE to mimic the natural extracellular matrix (ECM) by an artificial ECM (scaffold) based on synthetic or natural biomaterials to regenerate the physiological tissue/organ architecture and function. Hydrogels have gained interest in the TE community because of their ability to absorb water similar to physiological tissues, thus mechanically simulating the ECM. In this work, we present a novel hydrogel platform based on poly(2-ethyl-2-oxazoline)s, which can be processed to 3D microstructures via two-photon polymerization (2PP) with tunable mechanical properties using monomers and crosslinker with different degrees of polymerization (DP) for future applications in TE. The ideal parameters (laser power and writing speed) for optimal polymerization via 2PP were obtained using a specially developed evaluation method in which the obtained structures were binarized and compared to the computer-aided design (CAD) model. This evaluation was performed for each composition. We found that it was possible to tune the mechanical properties not only by application of different laser parameters but also by mixing poly(2-ethyl-2-oxazoline)s with different chain lengths and variation of the crosslink density. In addition, the swelling behavior of different fabricated hydrogels were investigated. To gain more insight into the viscoelastic behavior of different fabricated materials, stress relaxation tests via nanoindentation experiments were performed. These new hydrogels can be processed to 3D microstructures with high structural integrity using optimal laser parameter settings, opening a wide range of application properties in TE for this material platform.

Oxygen and nutrient delivery in tissue engineering: Approaches to graft vascularization

The field of tissue engineering is making great strides in developing replacement tissue grafts for clinical use, marked by the rapid development of novel biomaterials, their improved integration with cells, better-directed growth and differentiation of cells, and improved three-dimensional tissue mass culturing. One major obstacle that remains, however, is the lack of graft vascularization, which in turn renders many grafts to fail upon clinical application. With that, graft vascularization has turned into one of the holy grails of tissue engineering, and for the majority of tissues, it will be imperative to achieve adequate vascularization if tissue graft implantation is to succeed. Many different approaches have been developed to induce or augment graft vascularization, both in vitro and in vivo. In this review, we highlight the importance of vascularization in tissue engineering and outline various approaches inspired by both biology and engineering to achieve and augment graft vascularization.

Biomimetic Designer Scaffolds Made of D,L-Lactide-ɛ-Caprolactone Polymers by 2-Photon Polymerization

IMPACT STATEMENT

In tissue engineering (TE), the establishment of cell targeting materials, which mimic the conditions of the physiological extracellular matrix (ECM), seems to be a mission impossible without advanced materials and fabrication techniques. With this in mind we established a toolbox based on (D,L)-lactide-ɛ-caprolactone methacrylate (LCM) copolymers in combination with a nano-micromaskless lithography technique, the two-photon polymerization (2-PP) to mimic the hierarchical structured and complex milieu of the natural ECM. To demonstrate the versatility of this toolbox, we choose two completely different application scenarios in bone and tumor TE to show the high potential of this concept in therapeutic and diagnostic application.

Advancing Frontiers in Bone Bioprinting

Three-dimensional (3D) bioprinting of cell-laden biomaterials is used to fabricate constructs that can mimic the structure of native tissues. The main techniques used for 3D bioprinting include microextrusion, inkjet, and laser-assisted bioprinting. Bioinks used for bone bioprinting include hydrogels loaded with bioactive ceramics, cells, and growth factors. In this review, a critical overview of the recent literature on various types of bioinks used for bone bioprinting is presented. Major challenges, such as the vascularity, clinically relevant size, and mechanical properties of 3D printed structures, that need to be addressed to successfully use the technology in clinical settings, are discussed. Emerging approaches to solve these problems are reviewed, and future strategies to design customized 3D printed structures are proposed.

Cell-instructive high-resolution micropatterned polylactic acid surfaces

Micro and nanoscale topographical structuring of biomaterial surfaces has been a valuable tool for influencing cell behavior, including cell attachment, proliferation and differentiation. However, most fabrication techniques for surface patterning of implantable biomaterials suffer from a limited resolution, not allowing controlled generation of sub-cellular three-dimensional features. Here, a direct laser lithography technique based on two-photon absorption was used to construct several patterns varying in size between 500 nm and 15 μm. Through replication via an intermediate mold, the patterns were transferred into polylactic acid (PLA), a widely used biomedical polymer, while retaining the original geometry. An osteoblast-like cell line, MG-63 was used for characterizing the morphological response to the topographical patterns. The results indicated that semi-continuous (dashed) lines, with a height of 1 μm were able to induce cell elongation in the direction of the lines. However, when dashes with a height of 0.5 μm were combined with perpendicularly crossing continuous lines (rails) with a height of 8 μm, the contact guidance effect of the dashes was lost and elongation of the cells was observed in the direction of the larger features. A second pattern, consisting of different arrays of pillars showed that, depending on the pillar height, the cells were either able to spread over the pattern or were confined between the pattern features. These differences in the ability of cells to spread further resulted in the formation of tension forces through stress fibers and displacement of vimentin. The method for high-resolution micropatterning of PLA as presented here can also be applied to other biomedical polymers, making it useful both for fundamental studies and for designing new biomaterials with improved functionality.

Biofabrication and Bone Tissue Regeneration: Cell Source, Approaches, and Challenges

The growing occurrence of bone disorders and the increase in aging population have resulted in the need for more effective therapies to meet this request. Bone tissue engineering strategies, by combining biomaterials, cells, and signaling factors, are seen as alternatives to conventional bone grafts for repairing or rebuilding bone defects. Indeed, skeletal tissue engineering has not yet achieved full translation into clinical practice because of several challenges. Bone biofabrication by additive manufacturing techniques may represent a possible solution, with its intrinsic capability for accuracy, reproducibility, and customization of scaffolds as well as cell and signaling molecule delivery. This review examines the existing research in bone biofabrication and the appropriate cells and factors selection for successful bone regeneration as well as limitations affecting these approaches. Challenges that need to be tackled with the highest priority are the obtainment of appropriate vascularized scaffolds with an accurate spatiotemporal biochemical and mechanical stimuli release, in order to improve osseointegration as well as osteogenesis.

Simple and economic elaboration of high purity CaCO3 particles for bone graft applications using a spray pyrolysis technique

CaCO₃ particles obtained using spray pyrolysis possess all the requirements to constitute promising multi-purpose materials for bone graft applications.

In vitro colonization of stratified bioactive scaffolds by pre-osteoblast cells

Mesoporous bioactive glass-polycaprolactone (MBG-PCL) scaffolds have been prepared by robocasting, a layer by layer rapid prototyping method, by stacking of individual strati. Each stratus was independently analyzed during the cell culture tests with MC3T3-E1 preosteblast-like cells. The presence of MBG stimulates the colonization of the scaffolds by increasing the cell proliferation and differentiation. MBG-PCL composites not only enhanced pre-osteoblast functions but also allowed cell movement along its surface, reaching the upper stratus faster than in pure PCL scaffolds. The cells behavior on each individual stratus revealed that the scaffolds colonization depends on the chemical stimuli supplied by the MBG dissolution and surface changes associated to the apatite-like formation during the bioactive process. Finally, scanning electron and fluorescence microscopy revealed that the kinetic of cell migration strongly depends on the architectural features of the scaffolds, in such a way that layers interconnections are used as migration routes to reach the farther scaffolds locations from the initial cells source.

STATEMENT OF SIGNIFICANCE

This manuscript provides new insights on cell behavior in bioceramic/polymer macroporous scaffolds prepared by rapid prototyping methods. The experiments proposed in this work have allowed the evaluation of cell behavior within the different levels of the scaffolds, i.e. from the initials source of cells towards the farther scaffold locations. We could demonstrate that the in vitro cell colonization is encouraged by the presence of a highly bioactive mesoporous glass (MBG). This bioceramic enhances the cell migration towards upper strati through the dissolution of chemical signals and the changes occurred on the scaffolds surface during the bioactive process. In addition the MBG promotes preosteblastic proliferation and differentiation respect to scaffolds made of pure polycaprolactone. Finally, this study reveals the significance of the architectural design to accelerate the cell colonization. These experiments put light on the factors that should be taken into account to accelerate the regeneration processes under in vivo conditions.

Independent effects of the chemical and microstructural surface properties of polymer/ceramic composites on proliferation and osteogenic differentiation of human MSCs

Within the general aim of finding affordable and sustainable regenerative solutions for damaged and diseased tissues and organs, significant efforts have been invested in developing synthetic alternatives to natural bone grafts, such as autografts. Calcium phosphate (CaP) ceramics are among widely used synthetic bone graft substitutes, but their mechanical properties and bone regenerative capacity are still outperformed by their natural counterparts. In order to improve the existing synthetic bone graft substitutes, it is imperative to understand the effects of their individual properties on a biological response, and to find a way to combine the desired properties into new, improved functional biomaterials. To this end, we studied the independent effects of the chemical composition and surface microstructure of a poly(lactic acid)/hydroxyapatite (PLA/HA) composite material on the proliferation and osteogenic differentiation of clinically relevant bone marrow-derived human mesenchymal stromal cells (hMSCs). While the molecular weight of the polymer and presence/absence of the ceramic phase were used as the chemical variables, a soft embossing technique was used to pattern the surfaces of all materials with either pits or pillars with identical microscale dimensions. The results indicated that, while cell morphology was affected by both the presence and availability of HA and by the surface microstructure, the effect of the latter parameter on cell proliferation was negligible. The osteogenic differentiation of hMSCs, and in particular the expression of bone morphogenetic protein 2 (BMP-2) and osteopontin (OP) were significantly enhanced when cells were cultured on the composite based on low-molecular-weight PLA, as compared to the high-molecular-weight PLA-based composite and the two pure polymers. The OP expression on the low-molecular-weight PLA-based composite was further enhanced when the surface was patterned with pits. Taken together, within this experimental set up, the individual effect of the chemistry, and in particular of the presence of CaP, was more pronounced than the individual effect of the surface microstructure, although their combined effects were, in some cases, synergistic. The approach presented here opens new routes to study the interactions of biomaterials with the biological environment in greater depths, which can serve as a starting point for developing biomaterials with improved bioactivity.

STATEMENT OF SIGNIFICANCE

The aim of the this study was to obtain insight into independent effects of the chemical composition and surface microstructure of a poly(lactic acid)/hydroxyapatite (PLA/HA) composite material on the morphology, proliferation and osteogenic differentiation of clinically relevant bone marrow-derived human mesenchymal stromal cells (hMSCs). While the need for synthetic alternatives for natural bone in bone regenerative strategies is rapidly increasing, the clinical performance of synthetic biomaterials needs to be further improved. To do this successfully, we believe that a better understanding of the relationship between a property of a material and a biological response is imperative. This study is a step forward in this direction, and we are therefore convinced that it will be of interest to the readers of Acta Biomaterialia.

Physicochemical properties and biocompatibility of PZL/PLGA/bioglass composite scaffolds for bone tissue engineering

Foamy poly(N_ε-Cbz-l-lysine)/poly(lactic-co-glycolic acid)/bioglass composite scaffolds had appropriate physicochemical properties, good biomineralization ability, excellent cytocompatibility and histocompatibility, and desirable osteogenic ability.