Bisphosphonate-Functionalized Scaffolds for Enhanced Bone Regeneration

The local sustained release of bioactive substances are attracting increasing attention in bone tissue engineering, which is beneficial to bone tissue formation and helps to improve the bone ingrowth ability of a scaffold. Bisphosphonates (BPs), as a representative kind of osteoclast inhibitors, are proven to possess excellent osteogenic induction capability. Accordingly, various physical and chemical strategies are developed to functionalize bone tissue scaffolds with BPs to achieve controlled release profiles. Compared with traditional treatment modalities, local release of BPs from these composite scaffolds will contribute to continuous bone integration without the risk of many complications. This review explores the molecular mechanisms of BPs on bone metabolism and analyzes the appropriate concentrations of BPs that promote bone regeneration. The advanced BP loading strategies, implant modification technologies, and BP-loaded composite scaffolds based on different matrices are summarized. Finally, the latest advances and the future development of BP-modified scaffolds for enhanced bone regeneration are discussed. This article provides leading-edge design strategies of the BP-functionalized bone engineering scaffolds for improved bone repairability.

Modular Strategies to Build Cell-Free and Cell-Laden Scaffolds towards Bioengineered Tissues and Organs

Engineering three-dimensional (3D) scaffolds for functional tissue and organ regeneration is a major challenge of the tissue engineering (TE) community. Great progress has been made in developing scaffolds to support cells in 3D, and to date, several implantable scaffolds are available for treating damaged and dysfunctional tissues, such as bone, osteochondral, cardiac and nerve. However, recapitulating the complex extracellular matrix (ECM) functions of native tissues is far from being achieved in synthetic scaffolds. Modular TE is an intriguing approach that aims to design and fabricate ECM-mimicking scaffolds by the bottom-up assembly of building blocks with specific composition, morphology and structural properties. This review provides an overview of the main strategies to build synthetic TE scaffolds through bioactive modules assembly and classifies them into two distinct schemes based on microparticles (µPs) or patterned layers. The µPs-based processes section starts describing novel techniques for creating polymeric µPs with desired composition, morphology, size and shape. Later, the discussion focuses on µPs-based scaffolds design principles and processes. In particular, starting from random µPs assembly, we will move to advanced µPs structuring processes, focusing our attention on technological and engineering aspects related to cell-free and cell-laden strategies. The second part of this review article illustrates layer-by-layer modular scaffolds fabrication based on discontinuous, where layers' fabrication and assembly are split, and continuous processes.

Engineering a multiphasic, integrated graft with a biologically developed cartilage-bone interface for osteochondral defect repair

Tissue engineering is a promising approach to repair osteochondral defects, yet successful reconstruction of different layers in an integrated graft, especially the interface remains challenging. The multiphasic, functionally integrated tissue engineering graft described herein mimics the entire osteochondral tissue in terms of structure and composition at the cartilage, bone and cartilage-bone interface layer to repair osteochondral defects. In this manuscript, we report the fabrication of a multiphasic graft via bonding of a cartilaginous hydrogel and a sintered poly(lactic-co-glycolic acid) microsphere scaffold by an endogenous fibrotic cartilaginous extracellular matrix. We demonstrated that culturing chondrocytes within the alginate hydrogel conjugated to the poly(lactic-co-glycolic acid) scaffold allows for (i) gradient transition and integration from the cartilage layer to the subchondral bone layer as assessed by scanning electron microscopy, histology and biochemistry, and (ii) superior tissue repair efficacy in a rabbit knee defect model. Industrialization of the graft remains an unsolved challenge as after decellularization the tissue repair efficacy of the graft decreased. Taken together, the multiphasic osteochondral graft repaired the osteochondral defects successfully and has the potential to be applied clinically as an implant in orthopaedic surgery.

Post-manufacture loading of filaments and 3D printed PLA scaffolds with prednisolone and dexamethasone for tissue regeneration applications

Strategies to load prednisolone or dexamethasone in preformed poly(L-lactic acid) (PLA) filaments and 3D printed scaffolds were explored as a way of personalizing the drug, the dose and the release profile for regenerative medicine purposes. Instead of starting from a PLA filament preloaded with a given content of drug, we explored two more versatile strategies. The first one involved the soaking of PLA filaments into a drug solution prepared in a solvent that reversibly swelled PLA; during 3D printing the melting of PLA contributed to the efficient integration (encapsulation) of the drug inside the printed strand. The second strategy consisted in first printing the 3D PLA scaffolds followed by soaking in a suitable drug solution in order to exploit the higher specific surface of the printed strands compared to the filament. Sustained release profiles were recorded when either prednisolone or dexamethasone were loaded in preformed PLA filaments, while rapid release was recorded for 3D PLA scaffolds loaded after printing. The combination of the two proposed methods reported here opened the possibility of creating concentration gradients of different drugs in the same scaffold exhibiting distinct release patterns. Namely, the strand core contained an active ingredient to be slowly released, while the surface was covered with other active ingredient that could be rapidly delivered. The feasibility of this approach was confirmed through dual loading of dexamethasone in the filament and of prednisolone on the preformed scaffold. Drug-loaded scaffolds were characterized in terms of printability, structural characteristics (DSC, XRD), mechanical properties, biodegradation, and ability to promote cell attachment and proliferation. Finally, anti-inflammatory response and osteoinductive properties were verified in cell cultures.

Tethering of rhBMP-2 upon calcium phosphate cement via alendronate/heparin for localized, sustained and enhanced osteoactivity

rhBMP-2 was tethered on surface of calcium phosphate cement via alendronate–heparin. This novel delivery system can concurrently satisfy high bioactive immobilization and sustainable release of rhBMP-2, and consequently induce rapid bone regeneration.

Small molecules and their controlled release that induce the osteogenic/chondrogenic commitment of stem cells

Stem cell-based tissue engineering plays a significant role in skeletal system repair and regenerative therapies. However, stem cells must be differentiated into specific mature cells prior to implantation (direct implantation may lead to tumour formation). Natural or chemically synthesised small molecules provide an efficient, accurate, reversible, and cost-effective way to differentiate stem cells compared with bioactive growth factors and gene-related methods. Thus, investigating the influences of small molecules on the differentiation of stem cells is of great significance. Here, we review a series of small molecules that can induce or/and promote the osteogenic/chondrogenic commitment of stem cells. The controlled release of these small molecules from various vehicles for stem cell-based therapies and tissue engineering applications is also discussed. The extensive studies in this field represent significant contributions to stem cell-based tissue engineering research and regenerative medicine.

Cell-Mediated Dexamethasone Release from Semi-IPNs Stimulates Osteogenic Differentiation of Encapsulated Mesenchymal Stem Cells

Scaffold-based delivery of bioactive molecules capable of directing stem cell differentiation is critical to the development of point-of-care cell therapy for orthopedic repair. Dexamethasone-conjugated hyaluronic acid (HA-DXM) was synthesized and combined with hydrolytically degradable, photo-cross-linkable PEG-bis(2-acryloyloxy propanoate) (PEG-bis-AP) to form semi-IPNs. Dexamethasone (DX) release was limited in physiological buffer and substantially increased in the presence of encapsulated human mesenchymal stem cells (hMSCs) or exogenous hyaluronidase, confirming that release occurred primarily by a cell-mediated enzymatic mechanism. hMSCs encapsulated in PEG-bis-AP/HA-DXM semi-IPNs increased osteoblast-specific gene expression, alkaline phosphatase activity, and matrix mineralization, attaining levels that were not significantly different from positive controls consisting of hMSCs in PEG-bis-AP/native HA cultured with DX supplementation in the culture medium. These studies demonstrate that PEG-bis-AP/HA-DXM semi-IPNs can provide cell-mediated release of bioactive free DX that induces hMSC osteogenic differentiation. This approach offers an efficient system for local delivery of osteogenic molecules empowering point of care applications.

Engineering poly(lactic-co-glycolic acid)/hydroxyapatite microspheres with diverse macropores patterns and the cellular responses

Design macroporous topography on spherical substrates via a straightforward approach and investigate the corresponding cell responses.

Controllable promotion of chondrocyte adhesion and growth on PVA hydrogels by controlled release of TGF-β1 from porous PLGA microspheres

Poly(vinyl alcohol) (PVA) hydrogels have been candidate materials for cartilage tissue engineering. However, the cell non-adhesive nature of PVA hydrogels has been a limit. In this paper, the cell adhesion and growth on PVA hydrogels were promoted by compositing with transform growth factor-β1 (TGF-β1) loaded porous poly(D,L-lactide-co-glycolide) (PLGA) microspheres. The porous microspheres were fabricated by a modified double emulsion method with bovine serum albumin (BSA) as porogen. The average pore size of microspheres was manipulated by changing the BSA/PLGA ratio. Such controllable porous structures effectively influenced the encapsulation efficiency (Eencaps) and release profile of TGF-β1. By compositing PVA hydrogels with such TGF-β1-loaded PLGA microspheres, chondrocyte adhesion and proliferation were significantly promoted in a controllable manner, as confirmed by fluorescent imaging and quantitative CCK-8 assay. That is, the chondrocyte proliferation was favored by using PLGA microspheres with high Eencaps of TGF-β1 or by increasing the PLGA microsphere content in the hydrogels. These results demonstrated a facile method to improve the cell adhesion and growth on the intrinsically cell non-adhesive PVA hydrogels, which may find applications in cartilage substitution.

Bone tissue engineering and regenerative medicine: targeting pathological fractures

Patients with bone diseases have the highest risk of sustaining fractures and of suffering from nonunion bone healing due to tissue degeneration. Current fracture management strategies are limited in design and functionality and do not effectively promote bone healing within a diseased bone environment. Fracture management approaches include pharmaceutical therapy, surgical intervention, and tissue regeneration for fracture prevention, fracture stabilization, and fracture site regeneration, respectively. However, these strategies fail to accommodate the pathological nature of fragility fractures, leading to unwanted side effects, implant failures, and nonunions. To target fragility fractures, fracture management strategies should include bioactive bone substitutes designed for the pathological environment. However, the clinical outcome of these materials must be predictable within various disease environments. Initial development of a targeted treatment strategy should focus on simulating the physiological in vitro bone environment to predict clinical effectiveness of the engineered bone. An in vitro test system can facilitate reduction of implant failures and non-unions in fragility fractures.

Gold nanoparticle conjugated PLGA-PEG-SA-PEG-PLGA multiblock copolymer nanoparticles: synthesis, characterization, in vivo release of rifampicin

A series of succinate linearly linked PLGA-PEG-SA-PEG-PLGA multiblock copolymers were synthesized using direct melt polycondensation and characterized using inherent viscosity, gel permeation chromatography (GPC), FTIR and ¹H-NMR spectroscopy techniques. Gold nanoparticles (AuNPs) were synthesized using an as-synthesized citrate-PEG (CPEG) hybrid dendron, which acts as a reducing agent as well as a stabilizing agent. The CPEG capped AuNPs were characterized using UV-visible spectroscopy and TEM analysis. The Au-conjugated PLGA-PEG-SA-PEG-PLGA multiblock copolymer NPs were loaded with the tuberculosis drug rifampicin (RIF) using ultrasonication followed by solvent evaporation and were characterized by TEM, powder XRD and XPS analyses. The RIF loading efficiency and percentage drug content of RIF loaded Au-conjugated multiblock copolymer NPs were evaluated using UV-visible spectroscopy. The RIF loading efficiency and RIF content of the AuNP conjugated multiblock copolymer NPs were 41.8-75.7% and 11.5-17.7% respectively. The in vivo drug release studies in male Wistar rats show that AuNP conjugated multiblock copolymer NPs exhibit drug release up to 240 h. The nanoconjugates exhibit 18.13-29.41 μg mL^-1 of C_max with a delayed T_max of 72 h and the relative bioavailability is increased to 107-190.

One-step generation of engineered drug-laden poly(lactic-co-glycolic acid) micropatterned with Teflon chips for potential application in tendon restoration

Regulating cellular behaviors such as cellular spatial arrangement and cellular phenotype is critical for managing tissue microstructure and biological function for engineered tissue regeneration. We herein pattern drug-laden poly(lactic-co-glycolic acid) (PLGA) into grooves using novel Teflon stamps (that possess excellent properties of resistance to harsh organic solvents and molecular adsorption) for engineered tendon-repair therapeutics. The drug release and biological properties of melatonin-laden PLGA grooved micropatterns are investigated. The results reveal that fibroblasts cultured on the melatonin-laden PLGA groove micropatterns not only display significant cell alignment that mimics the cell behavior in native tendon, but also promote the secretion of a major extracellular matrix in tendon, type I collagen, indicating great potential for the engineering of functional tendon regeneration.

Particle assemblies: toward new tools for regenerative medicine

Regenerative medicine is a demanding field in terms of design and elaboration of materials able to meet the specifications that this application imposes. The regeneration of tissue is a multiscale issue, from the signaling molecule through cell expansion and finally tissue growth requiring a large variety of cues that should be delivered in place and time. Hence, the materials should be able to accommodate cells with respect to their phenotypes, to allow cell division to the right tissue, to maintain the integrity of the surrounding sane tissue, and eventually use their signaling machinery to serve the development of the appropriate neo-tissue. They should also present the ability to deliver growth factors and regulate tissue development, to be degraded into safe products, in order not to impede tissue development, and finally be easily implanted/injected into the patients. In this context, colloid-based materials represent a very promising family of products because one can take advantage of their high specific area, their capability to carry/deliver bio-active molecules, and their capacity of assembling (eventually in vivo) into materials featuring other mechanical, rheological, physicochemical properties. Other benefits of great interest would be their ease of production even via high through-put processes and their potential manufacturing from safe, biodegradable and biocompatible parent raw material. This review describes the state-of-the-art of processes leading to complex materials from the assembly of colloids meeting, at least partially, the above-described specifications for tissue engineering and regenerative medicine.

Reciprocal induction of human dermal microvascular endothelial cells and human mesenchymal stem cells: time-dependent profile in a co-culture system

OBJECTIVES

Angiogenesis is closely associated with osteogenesis where reciprocal interactions between endothelial and osteoblast cells play an important role in bone regeneration. For these reasons, the aim of this work was to develop a co-culture system to study in detail any time-dependent interactions between human mesenchymal stem cells (HMSC) and human dermal microvascular endothelial cells (HDMEC), co-cultured in a 2D system, for 35 days.

MATERIALS AND METHODS

HMSC and HDMEC were co-cultured at a ratio of 1:4, respectively. Single-cell cultures were used as controls. Cell viability/proliferation was assessed using MTT, DNA quantification and calcein-AM assays. Cell morphology was monitored using confocal microscopy, and real time PCR was performed. Alkaline phosphatase activity and histochemical staining were evaluated. Matrix mineralization assays were also performed.

RESULTS

Cells were able to grow in characteristic patterns maintaining their viability and phenotype expression throughout culture time, compared to HMSC and HDMEC monocultures. HMSC differentiation seemed to be enhanced in the co-culture conditions, since it was observed an over expression of osteogenesis-related genes, and of ALP activity. Furthermore, presence of calcium phosphate deposits was also confirmed.

CONCLUSIONS

This work reports in detail the interactions between HMSC and HDMEC in a long-term co-culture 2D system. Endothelial and mesenchymal stem cells cultured in the present co-culture conditions ensured proliferation and phenotype differentiation of cell types, osteogenesis stimulation and over-expression of angiogenesis-related genes, in the same culture system. It is believed that the present work can lead to significant developments for bone tissue regeneration and cell biology studies.

Harnessing and modulating inflammation in strategies for bone regeneration

Inflammation is an immediate response that plays a critical role in healing after fracture or injury to bone. However, in certain clinical contexts, such as in inflammatory diseases or in response to the implantation of a biomedical device, the inflammatory response may become chronic and result in destructive catabolic effects on the bone tissue. Since our previous review 3 years ago, which identified inflammatory signals critical for bone regeneration and described the inhibitory effects of anti-inflammatory agents on bone healing, a multitude of studies have been published exploring various aspects of this emerging field. In this review, we distinguish between regenerative and damaging inflammatory processes in bone, update our discussion of the effects of anti-inflammatory agents on bone healing, summarize recent in vitro and in vivo studies demonstrating how inflammation can be modulated to stimulate bone regeneration, and identify key future directions in the field.