PLGA-Based Microparticles for the Sustained Release of BMP-2

Poly(Lactic Acid)-Based Microparticles for Drug Delivery Applications: An Overview of Recent Advances

The sustained release of pharmaceutical substances remains the most convenient way of drug delivery. Hence, a great variety of reports can be traced in the open literature associated with drug delivery systems (DDS). Specifically, the use of microparticle systems has received special attention during the past two decades. Polymeric microparticles (MPs) are acknowledged as very prevalent carriers toward an enhanced bio-distribution and bioavailability of both hydrophilic and lipophilic drug substances. Poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), and their copolymers are among the most frequently used biodegradable polymers for encapsulated drugs. This review describes the current state-of-the-art research in the study of poly(lactic acid)/poly(lactic-co-glycolic acid) microparticles and PLA-copolymers with other aliphatic acids as drug delivery devices for increasing the efficiency of drug delivery, enhancing the release profile, and drug targeting of active pharmaceutical ingredients (API). Potential advances in generics and the constant discovery of therapeutic peptides will hopefully promote the success of microsphere technology.

Review of emerging nanotechnology in bone regeneration: progress, challenges, and perspectives

The application of nanotechnology to regenerative medicine has increased over recent decades. The development of materials that can influence biology at the nanoscale has gained interest as our understanding of the interactions between cells and biomaterials at the nanoscale has grown. Materials that are either nanostructured or influence the nanostructure of the cellular microenvironment have been developed and shown to have advantages over their microscale counterparts. There are several reviews which have been published that discuss how nanomaterials have been used in regenerative medicine, particularly in bone regeneration. Most of these studies have explored this concept in specific areas, such as the application of glass-based nanocomposites, nanotechnology for targeted drug delivery to stimulate bone repair, and the progress in nanotechnology for the treatment of osteoporosis. In this review paper, the impact of nanotechnology in biomaterials development for bone regeneration will be discussed highlighting specifically, nanostructured materials that influence mechanical properties, biocompatibility, and osteoinductivity.

Droplet Microfluidic Optimisation Using Micropipette Characterisation of Bio-Instructive Polymeric Surfactants

Droplet microfluidics can produce highly tailored microparticles whilst retaining monodispersity. However, these systems often require lengthy optimisation, commonly based on a trial-and-error approach, particularly when using bio-instructive, polymeric surfactants. Here, micropipette manipulation methods were used to optimise the concentration of bespoke polymeric surfactants to produce biodegradable (poly(d,l-lactic acid) (PDLLA)) microparticles with unique, bio-instructive surface chemistries. The effect of these three-dimensional surfactants on the interfacial tension of the system was analysed. It was determined that to provide adequate stabilisation, a low level (0.1% (w/v)) of poly(vinyl acetate-co-alcohol) (PVA) was required. Optimisation of the PVA concentration was informed by micropipette manipulation. As a result, successful, monodisperse particles were produced that maintained the desired bio-instructive surface chemistry.

Scaffold-based osteogenic dual delivery system with melatonin and BMP-2 releasing PLGA microparticles

The growing safety problems about the use of bone morphogenetic protein 2 (BMP-2) is one of the recent issues that was improved by using low doses of BMP-2 with the support of other osteoinductive agents and/or using appropriate carriers. The aim of the present study is to investigate the effect of scaffold-based dual release system including melatonin (MEL) and BMP-2 loaded polylactic-co-glycolic acid (PLGA) microparticles on the osteogenic activity of pre-osteoblastic MC3T3-E1 cells. MEL and BMP-2 loaded microparticles were prepared by double emulsion solvent evaporation method in the average diameters of ~2 µm and ~11 µm, respectively and loaded into chitosan/hydroxyapatite (HAp) scaffolds. In vitro MC3T3-E1 culture studies were carried out comparatively with blank scaffolds, single (BMP-2 or MEL) releasing groups and dual (BMP-2 and MEL) releasing group. Microscopic observations and hematoxylin/eosin staining showed enhanced number of cells and dense ECM in dual release group. The expressions of differentiation markers, Runt-related transcription factor 2 (RUNX2) and alkaline phosphatase (ALP) and also mineralization were higher in dual release group than that of the other groups. Our findings showed that BMP-2 at low doses (~20 ng per scaffold) was sufficient in terms of osteogenic activity with controlled release systems where it was used in combination with MEL (~10 µg per scaffold).

Experimental Study on the Effect of Allogeneic Endothelial Progenitor Cells on Wound Healing in Diabetic Mice

Endothelial progenitor cells (EPCs) are involved in the neovascularization in traumatic and ischemic sites, but EPCs are "detained" in bone marrow under diabetic conditions, which results in reduction of the number of EPCs and their biological activity in peripheral blood. Based on our previous study to mobilize autologous bone marrow EPCs by administering AMD3100+G-CSF to realize the optimal effect, our present study is aimed at exploring the effects of transplanting EPCs locally in a wound model of diabetic mice. First, we prepared and identified EPCs, and the biological functions and molecular characteristics were compared between EPCs from DB/+ and DB/DB mice. Then, we performed full-thickness skin resection in DB/DB mice and tested the effect of local transplantation of EPCs on skin wound healing. The wound healing process was recorded using digital photographs. The animals were sacrificed on postoperative days 7, 14, and 17 for histological and molecular analysis. Our results showed that DB/+ EPCs were biologically more active than those of DB/DB EPCs. When compared with the control group, local transplantation of EPCs accelerated wound healing in DB/DB mice by promoting wound granulation tissue formation, angiogenesis, and collagen fiber deposition, but there was no significant difference in wound healing between DB/+ EPCs and DB/DB EPCs transplanted into the wound. Furthermore, local transplantation of EPCs promoted the expression of SDF-1, CXCR4, and VEGF. We speculated that EPC transplantation may promote wound healing through the SDF-1/CXCR4 axis. This point is worth exploring further. Present data are of considerable significance because they raise the possibility of promoting wound healing by isolating autologous EPCs from the patient, which provides a new approach for the clinical treatment of diabetic wounds in the future.

Non-invasive Production of Multi-Compartmental Biodegradable Polymer Microneedles for Controlled Intradermal Drug Release of Labile Molecules

Transdermal drug delivery represents an appealing alternative to conventional drug administration systems. In fact, due to their high patient compliance, the development of dissolvable and biodegradable polymer microneedles has recently attracted great attention. Although stamp-based procedures guarantee high tip resolution and reproducibility, they have long processing times, low levels of system engineering, are a source of possible contaminants, and thermo-sensitive drugs cannot be used in conjunction with them. In this work, a novel stamp-based microneedle fabrication method is proposed. It provides a rapid room-temperature production of multi-compartmental biodegradable polymeric microneedles for controlled intradermal drug release. Solvent casting was carried out for only a few minutes and produced a short dissolvable tip made of polyvinylpyrrolidone (PVP). The rest of the stamp was then filled with degradable poly(lactide-co-glycolide) (PLGA) microparticles (μPs) quickly compacted with a vapor-assisted plasticization. The outcome was an array of microneedles with tunable release. The ability of the resulting microneedles to indent was assessed using pig cadaver skin. Controlled intradermal delivery was demonstrated by loading both the tip and the body of the microneedles with model therapeutics; POXA1b laccase from Pleurotus ostreatus is a commercial enzyme used for the whitening of skin spots. The action and indentation of the enzyme-loaded microneedle action were assessed in an in vitro skin model and this highlighted their ability to control the kinetic release of the encapsulated compound.

Newly Designed Human-Like Collagen to Maximize Sensitive Release of BMP-2 for Remarkable Repairing of Bone Defects

Designing the “ideal” hydrogel/matrix which can load bone morphogenetic protein-2 (BMP-2) in a low dose and with a sustained release is the key for its successful therapeutic application to enhance osteogenesis. The current use of natural collagen sponges as hydrogel/matrix is limited due to the collagen matrix showing weak mechanical strength and unmanageable biodegradability. Furthermore, the efficiency and safe dose usage of the BMP-2 has never been seriously considered other than purely chasing the lowest dose usage and extended-release time. In this paper, we customized a novel enzymatically cross-linked recombinant human-like collagen (HLC) sponge with low immunogenicity, little risk from hidden viruses, and easy production. We obtained a unique vertical pore structure and the porosity of the HLC, which are beneficial for Mesenchymal stem cells (MSCs) migration into the HLC sponge and angiopoiesis. This HLC sponge loading with low dose BMP-2 (1 µg) possessed high mechanical strength along with a burst and a sustained release profile. These merits overcome previous limitations of HLC in bone repair and are safer and more sensitive than commercial collagens. For the first time, we identified that a 5 µg dose of BMP-2 can bring about the side effect of bone overgrowth through this sensitive delivery system. Osteoinduction of the HLC-BMP sponges was proved by an in vivo mouse ectopic bone model and a rat cranial defect repair model. The method and the HLC-BMP sponge have the potential to release other growth factors and aid other tissue regeneration. Additionally, the ability to mass-produce HLC in our study overcomes the current supply shortage, which limits bone repair in the clinic.

Formulation, Colloidal Characterization, and In Vitro Biological Effect of BMP-2 Loaded PLGA Nanoparticles for Bone Regeneration

Nanoparticles (NPs) based on the polymer poly (lactide-co-glycolide) acid (PLGA) have been widely studied in developing delivery systems for drugs and therapeutic biomolecules, due to the biocompatible and biodegradable properties of the PLGA. In this work, a synthesis method for bone morphogenetic protein (BMP-2)-loaded PLGA NPs was developed and optimized, in order to carry out and control the release of BMP-2, based on the double-emulsion (water/oil/water, W/O/W) solvent evaporation technique. The polymeric surfactant Pluronic F68 was used in the synthesis procedure, as it is known to have an effect on the reduction of the size of the NPs, the enhancement of their stability, and the protection of the encapsulated biomolecule. Spherical solid polymeric NPs were synthesized, showing a reproducible multimodal size distribution, with diameters between 100 and 500 nm. This size range appears to allow the protein to act on the cell surface and at the cytoplasm level. The effect of carrying BMP-2 co-adsorbed with bovine serum albumin on the NP surface was analyzed. The colloidal properties of these systems (morphology by SEM, hydrodynamic size, electrophoretic mobility, temporal stability, protein encapsulation, and short-term release profile) were studied. The effect of both BMP2-loaded NPs on the proliferation, migration, and osteogenic differentiation of mesenchymal stromal cells from human alveolar bone (ABSC) was also analyzed in vitro.

Porous PolyHIPE microspheres for protein delivery from an injectable bone graft

Delivery of osteoinductive factors such as bone morphogenetic protein 2 (BMP-2) has emerged as a prominent strategy to improve regeneration in bone grafting procedures. However, it remains challenging to identify a carrier that provides the requisite loading efficiency and release kinetics without compromising the mechanical properties of the bone graft. Previously, we reported on porous, polymerized high internal phase emulsion (polyHIPE) microspheres fabricated using controlled fluidics. Uniquely, this solvent-free method provides advantages over current microsphere fabrication strategies including in-line loading of growth factors to improve loading efficiency. In the current study, we utilized this platform to fabricate protein-loaded microspheres and investigated the effect of particle size (∼400 vs ∼800 μm) and pore size (∼15 vs 30 μm) on release profiles. Although there was no significant effect of these variables on the substantial burst release profile of the microspheres, the incorporation of the protein-loaded microspheres within the injectable polyHIPE resulted in a sustained release of protein from the bulk scaffold over a two-week period with minimal burst release. Bioactivity retention of encapsulated BMP-2 was confirmed first using a genetically-modified osteoblast reporter cell line. A functional assay with human mesenchymal stem cells established that the BMP-2 release from microspheres induced osteogenic differentiation. Finally, microsphere incorporation had minimal effect on the cure and compressive properties of an injectable polyHIPE bone graft. Overall, this work demonstrates that these microsphere-polyHIPE composites have strong potential to enhance bone regeneration through controlled release of BMP-2 and other growth factors. STATEMENT OF SIGNIFICANCE: This manuscript describes a method for solvent-free fabrication of porous microspheres from high internal phase emulsions using a controlled fluids setup. The principles of emulsion templating and fluid dynamics provide exceptional control of particle size and pore architecture. In addition to the advantage of solvent-free fabrication, this method provides in-line loading of protein directly into the pores of the microspheres with high loading efficiencies. The incorporation of the protein-loaded microspheres within an injectable polyHIPE scaffold resulted in a sustained release of protein over a two-week period with minimal burst release. Retention of BMP-2 bioactivity and incorporation of microspheres with minimal effect on scaffold compressive properties highlights the potential of these new bone grafts.

Microparticles for controlled growth differentiation factor 6 delivery to direct adipose stem cell-based nucleus pulposus regeneration

Currently, there is no effective long‐term treatment for intervertebral disc (IVD) degeneration, making it an attractive candidate for regenerative therapies. Hydrogel delivery of adipose stem cells (ASCs) in combination with controlled release of bioactive molecules is a promising approach to halt IVD degeneration and promote regeneration. Growth differentiation factor 6 (GDF6) can induce ASC differentiation into anabolic nucleus pulposus (NP) cells and hence holds promise for IVD regeneration. Here, we optimised design of novel poly(DL‐lactic acid‐co‐glycolic acid) (PLGA)–polyethylene glycol–PLGA microparticles to control GDF6 delivery and investigated effect of released GDF6 on human ASCs differentiation to NP cells. Recombinant human (rh)GDF6 was loaded into microparticles and total protein and rhGDF6 release assessed. The effect of microparticle loading density on distribution and gel formation was investigated through scanning electron microscopy. ASC differentiation to NP cells was examined after 14 days in hydrogel culture by quantitative polymerase chain reaction, histological, and immunohistochemical staining in normoxic and IVD‐like hypoxic conditions. RhGDF6 microparticles were distributed throughout gels without disrupting gelation and controlled rhGDF6 release over 14 days. Released GDF6 significantly induced NP differentiation of ASCs, with expression comparable with or exceeding media supplemented rhGDF6. Microparticle‐delivered rhGDF6 also up‐regulated sulphated glycosaminoglycan and aggrecan secretion in comparison with controls. In hypoxia, microparticle‐delivered rhGDF6 continued to effectively induce NP gene expression and aggrecan production. This study demonstrates the effective encapsulation and controlled delivery of rhGDF6, which maintained its activity and induced ASC differentiation to NP cells and synthesis of an NP‐like matrix suggesting suitability of microparticles for controlled growth factor release in regenerative strategies for treatment of IVD degeneration.

Scaffold-based lung tumor culture on porous PLGA microparticle substrates

Scaffold-based cancer cell culture techniques have been gaining prominence especially in the last two decades. These techniques can potentially overcome some of the limitations of current three-dimensional cell culture methods, such as uneven cell distribution, inadequate nutrient diffusion, and uncontrollable size of cell aggregates. Porous scaffolds can provide a convenient support for cell attachment, proliferation and migration, and also allows diffusion of oxygen, nutrients and waste. In this paper, a comparative study was done on porous poly (lactic-co-glycolic acid) (PLGA) microparticles prepared using three porogens—gelatin, sodium bicarbonate (SBC) or novel poly N-isopropylacrylamide [PNIPAAm] particles, as substrates for lung cancer cell culture. These fibronectin-coated, stable particles (19–42 μm) supported A549 cell attachment at an optimal cell seeding density of 250,000 cells/ mg of particles. PLGA-SBC porous particles had comparatively larger, more interconnected pores, and favored greater cell proliferation up to 9 days than their counterparts. This indicates that pore diameters and interconnectivity have direct implications on scaffold-based cell culture compared to substrates with minimally interconnected pores (PLGA-gelatin) or pores of uniform sizes (PLGA-PMPs). Therefore, PLGA-SBC-based tumor models were chosen for preliminary drug screening studies. The greater drug resistance observed in the lung cancer cells grown on porous particles compared to conventional cell monolayers agrees with previous literature, and indicates that the PLGA-SBC porous microparticle substrates are promising for in vitro tumor or tissue development.

Biodegradable polymer promotes osteogenic differentiation in immortalized and primary osteoblast-like cells

Biodegradable polymers have been broadly used as agents that can complex with and deliver osteoinductive agents, but osteoinductivity of the polymers themselves has been rarely studied. Here we report the osteoinductivity of poly(4-hydroxy-L-proline ester) (PHPE), a biodegradable cationic polymer with cell penetrating properties. Under physiological conditions, PHPE degrades into trans-4-hydroxy-L-proline (trans-Hyp), a non-coded amino acid with essential functions in collagen fibril formation and fibril stability. Treatment of SaOS-2 osteoblast-like cells and hFOB 1.19 primary osteoblast cells with PHPE promoted earlier collagen nodule formation and mineralization of the extracellular matrix compared to untreated cells, even when mineralization activators were absent in the growth medium. Our results indicate that PHPE is a potential osteoinductive agent in vitro that can favor bone regeneration. Moreover, this osteoinductive property could be partly attributed to the degradation product trans-Hyp, which could recapitulate some, but not all of the osteogenic activity. The primary findings of this study can be summarized as follows: treatment of cells with PHPE led to (1) the induction of COL1A1 expression, collagen synthesis and secretion in osteoblast-like cells, (2) mineralization of the ECM in both SaOS-2 and hFOB 1.19 primary osteoblasts, and (3) induction of BMP2 gene and protein expression in osteoblast-like cells, which can promote mineralization of the ECM and regeneration of the bone tissue. Our results suggest that PHPE is a non-cytotoxic polymer and can be potentially used to overcome collagenopathies such as osteogenesis imperfecta.

Soybean Lecithin-Mediated Nanoporous PLGA Microspheres with Highly Entrapped and Controlled Released BMP-2 as a Stem Cell Platform

Injectable polymer microsphere-based stem cell delivery systems have a severe problem that they do not offer a desirable environment for stem cell adhesion, proliferation, and differentiation because it is difficult to entrap a large number of hydrophilic functional protein molecules into the core of hydrophobic polymer microspheres. In this work, soybean lecithin (SL) is applied to entrap hydrophilic bone morphogenic protein-2 (BMP-2) into nanoporous poly(lactide-co-glycolide) (PLGA)-based microspheres by a two-step method: SL/BMP-2 complexes preparation and PLGA/SL/BMP-2 microsphere preparation. The measurements of their physicochemical properties show that PLGA/SL/BMP-2 microspheres had significantly higher BMP-2 entrapment efficiency and controlled triphasic BMP-2 release behavior compared with PLGA/BMP-2 microspheres. Furthermore, the in vitro and in vivo stem cell behaviors on PLGA/SL/BMP-2 microspheres are analyzed. Compared with PLGA/BMP-2 microspheres, PLGA/SL/BMP-2 microspheres have significantly higher in vitro and in vivo stem cell attachment, proliferation, differentiation, and matrix mineralization abilities. Therefore, injectable nanoporous PLGA/SL/BMP-2 microspheres can be potentially used as a stem cell platform for bone tissue regeneration. In addition, SL can be potentially used to prepare hydrophilic protein-loaded hydrophobic polymer microspheres with highly entrapped and controlled release of proteins.

A dual-application poly (dl-lactic-co-glycolic) acid (PLGA)-chitosan composite scaffold for potential use in bone tissue engineering

The development of patient-friendly alternatives to bone-graft procedures is the driving force for new frontiers in bone tissue engineering. Poly (dl-lactic-co-glycolic acid) (PLGA) and chitosan are well-studied and easy-to-process polymers from which scaffolds can be fabricated. In this study, a novel dual-application scaffold system was formulated from porous PLGA and protein-loaded PLGA/chitosan microspheres. Physicochemical and in vitro protein release attributes were established. The therapeutic relevance, cytocompatibility with primary human mesenchymal stem cells (hMSCs) and osteogenic properties were tested. There was a significant reduction in burst release from the composite PLGA/chitosan microspheres compared with PLGA alone. Scaffolds sintered from porous microspheres at 37 °C were significantly stronger than the PLGA control, with compressive strengths of 0.846 ± 0.272 MPa and 0.406 ± 0.265 MPa, respectively (p < 0.05). The formulation also sintered at 37 °C following injection through a needle, demonstrating its injectable potential. The scaffolds demonstrated cytocompatibility, with increased cell numbers observed over an 8-day study period. Von Kossa and immunostaining of the hMSC-scaffolds confirmed their osteogenic potential with the ability to sinter at 37 °C in situ.

Controlled release of GAG-binding enhanced transduction (GET) peptides for sustained and highly efficient intracellular delivery

Controlled release systems for therapeutic molecules are vital to allow the sustained local delivery of their activities which direct cell behaviour and enable novel regenerative strategies. Direct programming of cells using exogenously delivered transcription factors can by-pass growth factor signalling but there is still a requirement to deliver such activity spatio-temporally. We previously developed a technology termed GAG-binding enhanced transduction (GET) to efficiently deliver a variety of cargoes intracellularly, using GAG-binding domains which promote cell targeting, and cell penetrating peptides (CPPs) which allow cell entry. Herein we demonstrate that GET system can be used in controlled release systems to mediate sustained intracellular transduction over one week. We assessed the stability and activity of GET peptides in poly(dl-lactic acid-co-glycolic acid) (PLGA) microparticles (MPs) prepared using a S/O/W double emulsion method. Efficient encapsulation (∼65%) and tailored protein release profiles could be achieved, however intracellular transduction was significantly inhibited post-release. To retain GET peptide activity we optimized a strategy of co-encapsulation of l-Histidine, which may form a complex with the PLGA degradation products under acidic conditions. Simulations of the polymer microclimate showed that hydrolytic acidic PLGA degradation products directly inhibited GET peptide transduction activity, and use of l-Histidine significantly enhanced released protein delivery. The ability to control the intracellular transduction of functional proteins into cells will facilitate new localized delivery methods and allow approaches to direct cellular behaviour for many regenerative medicine applications.

STATEMENT OF SIGNIFICANCE

The goal for regenerative medicine is to restore functional biological tissue by controlling and augmenting cellular behaviour. Either Transcription (TFs) or growth factors (GFs) can be presented to cells in spatio-temporal gradients for programming cell fate and gene expression. Here, we have created a sustained and controlled release system for GET (Glycosaminoglycan-enhanced transducing)-tagged proteins using S/O/W PLGA microparticle fabrication. We demonstrated that PLGA and its acidic degradants inhibit GET-mediated transduction, which can be overcome by using pH-activated l-Histidine. l-Histidine inhibits the electrostatic interaction of GET/PLGA and allows enhanced intracellular transduction. GET could provide a powerful tool to program cell behaviour either in gradients or with sustained delivery. We believe that our controlled release systems will allow application of GET for tissue regeneration directly by TF cellular programming.

Colloid, adhesive and release properties of nanoparticular ternary complexes between cationic and anionic polysaccharides and basic proteins like bone morphogenetic protein BMP-2

Herein we describe an interfacial local drug delivery system for bone morphogenetic protein 2 (BMP-2) based on coatings of polyelectrolyte complex (PEC) nanoparticles (NP). The application horizon is the functionalization of bone substituting materials (BSM) used for the therapy of systemic bone diseases. Nanoparticular ternary complexes of cationic and anionic polysaccharides and BMP-2 or two further model proteins, respectively, were prepared in dependence of the molar mixing ratio, pH value and of the cationic polysaccharide. As further proteins chymotrypsin (CHY) and papain (PAP) were selected, which served as model proteins for BMP-2 due to similar isoelectric points and molecular weights. As charged polysaccharides ethylenediamine modified cellulose (EDAC) and trimethylammonium modified cellulose (PQ10) were combined with cellulose sulphatesulfate (CS). Mixing diluted cationic and anionic polysaccharide and protein solutions according to a slight either anionic or cationic excess charge colloidal ternary dispersions formed, which were cast onto germanium model substrates by water evaporation. Dynamic light scattering (DLS) demonstrated, that these dispersions were colloidally stable for at least one week. Fourier Transform Infrared (FTIR) showed, that the cast protein loaded PEC NP coatings were irreversibly adhesive at the model substrate in contact to HEPES buffer and solely CHY, PAP and BMP-2 were released within long-term time scale. Advantageously, out of the three proteins BMP-2 showed the smallest initial burst and the slowest release kinetics and around 25% of the initial BMP-2 content were released within 14days. Released BMP-2 showed significant activity in the myoblast cells indicating the ability to regulate the formation of new bone. Therefore, BMP-2 loaded PEC NP are suggested as novel promising tool for the functionalization of BSM used for the therapy of systemic bone diseases.

Microparticles for Sustained Growth Factor Delivery in the Regeneration of Critically-Sized Segmental Tibial Bone Defects

This study trialled the controlled delivery of growth factors within a biodegradable scaffold in a large segmental bone defect model. We hypothesised that co-delivery of vascular endothelial growth factor (VEGF) and platelet derived growth factor (PDGF) followed by bone morphogenetic protein-2 (BMP-2) could be more effective in stimulating bone repair than the delivery of BMP-2 alone. Poly(lactic-co-glycolic acid) (PLGA ) based microparticles were used as a delivery system to achieve a controlled release of growth factors within a medical-grade Polycaprolactone (PCL) scaffold. The scaffolds were assessed in a well-established preclinical ovine tibial segmental defect measuring 3 cm. After six months, mechanical properties and bone tissue regeneration were assessed. Mineralised bone bridging of the defect was enhanced in growth factor treated groups. The inclusion of VEGF and PDGF (with BMP-2) had no significant effect on the amount of bone regeneration at the six-month time point in comparison to BMP-2 alone. However, regions treated with VEGF and PDGF showed increased vascularity. This study demonstrates an effective method for the controlled delivery of therapeutic growth factors in vivo, using microparticles.

Monitoring model drug microencapsulation in PLGA scaffolds using X-ray powder diffraction

The microencapsulation of three model drugs; metronidazole, paracetamol and sulphapyridine into Poly (dl-Lactide-Co-Glycolide) (PLGA) scaffolds were probed using X-ray Powder Diffraction (XRPD). Changes in the diffraction patterns of the PLGA scaffolds after encapsulation was suggestive of a chemical interaction between the pure drugs and the scaffolds and not a physical intermixture.

In Vivo Assessment of Bone Regeneration in Alginate/Bone ECM Hydrogels with Incorporated Skeletal Stem Cells and Single Growth Factors

The current study has investigated the use of decellularised, demineralised bone extracellular matrix (ECM) hydrogel constructs for in vivo tissue mineralisation and bone formation. Stro-1-enriched human bone marrow stromal cells were incorporated together with select growth factors including VEGF, TGF-β3, BMP-2, PTHrP and VitD3, to augment bone formation, and mixed with alginate for structural support. Growth factors were delivered through fast (non-osteogenic factors) and slow (osteogenic factors) release PLGA microparticles. Constructs of 5 mm length were implanted in vivo for 28 days within mice. Dense tissue assessed by micro-CT correlated with histologically assessed mineralised bone formation in all constructs. Exogenous growth factor addition did not enhance bone formation further compared to alginate/bone ECM (ALG/ECM) hydrogels alone. UV irradiation reduced bone formation through degradation of intrinsic growth factors within the bone ECM component and possibly also ECM cross-linking. BMP-2 and VitD3 rescued osteogenic induction. ALG/ECM hydrogels appeared highly osteoinductive and delivery of angiogenic or chondrogenic growth factors led to altered bone formation. All constructs demonstrated extensive host tissue invasion and vascularisation aiding integration and implant longevity. The proposed hydrogel system functioned without the need for growth factor incorporation or an exogenous inducible cell source. Optimal growth factor concentrations and spatiotemporal release profiles require further assessment, as the bone ECM component may suffer batch variability between donor materials. In summary, ALG/ECM hydrogels provide a versatile biomaterial scaffold for utilisation within regenerative medicine which may be tailored, ultimately, to form the tissue of choice through incorporation of select growth factors.

Tailoring sub-micron PLGA particle release profiles via centrifugal fractioning

Poly(D,L-lactic-co -glycolic) acid (PLGA)-based sub-micron particles are uniquely posed to overcome limitations of conventional drug delivery systems. However, tailoring cargo/payload release profiles from PLGA micro/nanoparticles typically requires optimization of the multi-parameter formulation, where small changes may cause drastic shifts in the resulting release profiles. In this study, we aimed to establish whether refining the average diameter of sub-micron particle populations after formulation alters protein release profiles. PLGA particles were first produced via double emulsion-solvent evaporation method to encapsulate bovine serum albumin. Particles were then subjected to centrifugal fractioning protocols varying in both spin time and force to determine encapsulation efficiency and release profile of differently sized populations that originated from a single batch. We found the average particle diameter was related to marked alterations in encapsulation efficiencies (range: 36.4-49.4%), burst release (range: 15.8-49.1%), and time for total cargo release (range: 38-78 days). Our data corroborate previous reports relating PLGA particle size with such release characteristics, however, this is the first study, to our knowledge, to directly compare particle population size while holding all formulation parameters constant. In summary, centrifugal fractioning to selectively control the population distribution of sub-micron PLGA particles represents a feasible tool to tailor release characteristics. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part A 104A: 688-696, 2016.

Overcoming translational challenges - The delivery of mechanical stimuli in vivo

Despite major medical advances, non-union bone fractures and skeletal defects continue to place significant burden on the patient, the clinicians and the healthcare system as a whole. Current bone substitute approaches are still limited in effectiveness and to date no adequate bone substitute material has been developed for routine clinical application. Tissue engineering presents a novel approach to tackling this clinical burden and developing an acceptable solution for the treatment of skeletal defects. Over the past three decades the field has evolved to appreciate the key biological, material and physical parameters influencing the development of a cell-based tissue engineered therapy and to create associated technologies to exploit such parameters. In recent years a number of therapies have started progressing along the pre-clinical pipeline to build a case for regulatory approval and ultimately clinical adoption. However, little emphasis has been given to the translational challenges faced when moving from "bench-to-bedside". One particular challenge lies in the delivery of functional mechanical stimuli to implanted cell populations to activate and promote osteogenic activities. This review introduces novel bio-magnetic approaches to overcoming this challenge.

Research Advances in Tissue Engineering Materials for Sustained Release of Growth Factors

Growth factors are a class of cytokines that stimulate cell growth and are widely used in clinical practice, such as wound healing, revascularization, bone repair, and nervous system disease. However, free growth factors have a short half-life and are instable in vivo. Therefore, the search of excellent carriers to enhance sustained release of growth factors in vivo has become an area of intense research interest. The development of controlled-release systems that protect the recombinant growth factors from enzymatic degradation and provide sustained delivery at the injury site during healing should enhance the growth factor's application in tissue regeneration. Thus, this study reviews current research on commonly used carriers for sustained release of growth factors and their sustained release effects for preservation of their bioactivity and their accomplishment in tissue engineering approaches.

The scope and sequence of growth factor delivery for vascularized bone tissue regeneration

Bone regeneration is a complex process, that in vivo, requires the highly coordinated presentation of biochemical cues to promote the various stages of angiogenesis and osteogenesis. Taking inspiration from the natural healing process, a wide variety of growth factors are currently being released within next generation tissue engineered scaffolds (in a variety of ways) in order to heal non-union fractures and bone defects. This review will focus on the delivery of multiple growth factors to the bone regeneration niche, specifically 1) dual growth factor delivery signaling and crosstalk, 2) the importance of growth factor timing and temporal separation, and 3) the engineering of delivery systems that allow for temporal control over presentation of soluble growth factors. Alternative methods for growth factor presentation, including the use of gene therapy and platelet-rich plasma scaffolds, are also discussed.

Cell and protein compatible 3D bioprinting of mechanically strong constructs for bone repair

Rapid prototyping of bone tissue engineering constructs often utilizes elevated temperatures, organic solvents and/or UV light for materials processing. These harsh conditions may prevent the incorporation of cells and therapeutic proteins in the fabrication processes. Here we developed a method for using bioprinting to produce constructs from a thermoresponsive microparticulate material based on poly(lactic-co-glycolic acid) at ambient conditions. These constructs could be engineered with yield stresses of up to 1.22 MPa and Young's moduli of up to 57.3 MPa which are within the range of properties of human cancellous bone. Further study showed that protein-releasing microspheres could be incorporated into the bioprinted constructs. The release of the model protein lysozyme from bioprinted constructs was sustainted for a period of 15 days and a high degree of protein activity could be measured up to day 9. This work suggests that bioprinting is a viable route to the production of mechanically strong constructs for bone repair under mild conditions which allow the inclusion of viable cells and active proteins.

Stem Cells for Cutaneous Wound Healing

Optimum healing of a cutaneous wound involves a well-orchestrated cascade of biological and molecular processes involving cell migration, proliferation, extracellular matrix deposition, and remodelling. When the normal biological process fails for any reason, this healing process can stall resulting in chronic wounds. Wounds are a growing clinical burden on healthcare systems and with an aging population as well as increasing incidences of obesity and diabetes, this problem is set to increase. Cell therapies may be the solution. A range of cell based approaches have begun to cross the rift from bench to bedside and the supporting data suggests that the appropriate administration of stem cells can accelerate wound healing. This review examines the main cell types explored for cutaneous wound healing with a focus on clinical use. The literature overwhelmingly suggests that cell therapies can help to heal cutaneous wounds when used appropriately but we are at risk of clinical use outpacing the evidence. There is a need, now more than ever, for standardised methods of cell characterisation and delivery, as well as randomised clinical trials.

Bone Morphogenetic Protein-2 Adsorption onto Poly-ɛ-caprolactone Better Preserves Bioactivity In Vitro and Produces More Bone In Vivo than Conjugation Under Clinically Relevant Loading Scenarios

BACKGROUND

One strategy to reconstruct large bone defects is to prefabricate a vascularized flap by implanting a biomaterial scaffold with associated biologics into the latissimus dorsi and then transplanting this construct to the defect site after a maturation period. This strategy, similar to all clinically and regulatory feasible biologic approaches to surgical reconstruction, requires the ability to quickly (<1 h within an operating room) and efficiently bind biologics to scaffolds. It also requires the ability to localize biologic delivery. In this study, we investigated the efficacy of binding bone morphogenetic protein-2 (BMP2) to poly-ɛ-caprolactone (PCL) using adsorption and conjugation as a function of time.

METHODS

BMP2 was adsorbed (Ads) or conjugated (Conj) to PCL scaffolds with the same three-dimensional printed architecture while altering exposure time (0.5, 1, 5, and 16 h), temperature (4°C, 23°C), and BMP2 concentration (1.4, 5, 20, and 65 μg/mL). The in vitro release was quantified, and C2C12 cell alkaline phosphatase (ALP) expression was used to confirm bioactivity. Scaffolds with either 65 or 20 μg/mL Ads or Conj BMP2 for 1 h at 23°C were implanted subcutaneously in mice to evaluate in vivo bone regeneration. Micro-computed tomography, compression testing, and histology were performed to characterize bone regeneration.

RESULTS

After 1 h exposure to 65 μg/mL BMP2 at 23°C, Conj and Ads resulted in 12.83 ± 1.78 and 10.78 ± 1.49 μg BMP2 attached, respectively. Adsorption resulted in a positive ALP response and had a small burst release; whereas conjugation provided a sustained release with negligible ALP production, indicating that the conjugated BMP2 may not be bioavailable. Adsorbed 65 μg/mL BMP2 solution resulted in the greatest regenerated bone volume (15.0 ± 3.0 mm³), elastic modulus (20.1 ± 3.0 MPa), and %bone ingrowth in the scaffold interior (17.2% ± 5.4%) when compared with conjugation.

CONCLUSION

Adsorption may be optimal for the clinical application of prefabricating bone flaps due to BMP2 binding in a short exposure time, retained BMP2 bioactivity, and bone growth adhering to scaffold geometry and into pores with healthy marrow development.

Injectable and porous PLGA microspheres that form highly porous scaffolds at body temperature

Injectable scaffolds are of interest in the field of regenerative medicine because of their minimally invasive mode of delivery. For tissue repair applications, it is essential that such scaffolds have the mechanical properties, porosity and pore diameter to support the formation of new tissue. In the current study, porous poly(dl-lactic acid-co-glycolic acid) (PLGA) microspheres were fabricated with an average size of 84±24μm for use as injectable cell carriers. Treatment with ethanolic sodium hydroxide for 2min was observed to increase surface porosity without causing the microsphere structure to disintegrate. This surface treatment also enabled the microspheres to fuse together at 37°C to form scaffold structures. The average compressive strength of the scaffolds after 24h at 37°C was 0.9±0.1MPa, and the average Young's modulus was 9.4±1.2MPa. Scaffold porosity levels were 81.6% on average, with a mean pore diameter of 54±38μm. This study demonstrates a method for fabricating porous PLGA microspheres that form solid porous scaffolds at body temperature, creating an injectable system capable of supporting NIH-3T3 cell attachment and proliferation in vitro.

Evaluation of skeletal tissue repair, part 1: assessment of novel growth-factor-releasing hydrogels in an ex vivo chick femur defect model

Current clinical treatments for skeletal conditions resulting in large-scale bone loss include autograft or allograft, both of which have limited effectiveness. In seeking to address bone regeneration, several tissue engineering strategies have come to the fore, including the development of growth factor releasing technologies and appropriate animal models to evaluate repair. Ex vivo models represent a promising alternative to simple in vitro systems or complex, ethically challenging in vivo models. We have developed an ex vivo culture system of whole embryonic chick femora, adapted in this study as a critical size defect model to investigate the effects of novel bone extracellular matrix (bECM) hydrogel scaffolds containing spatio-temporal growth factor-releasing microparticles and skeletal stem cells on bone regeneration, to develop a viable alternative treatment for skeletal degeneration. Alginate/bECM hydrogels combined with poly (d,l-lactic-co-glycolic acid) (PDLLGA)/triblock copolymer (10-30% PDLLGA-PEG-PDLLGA) microparticles releasing VEGF, TGF-β3 or BMP-2 were placed, with human adult Stro-1+ bone marrow stromal cells, into 2mm central segmental defects in embryonic chick femurs. Alginate/bECM hydrogels loaded with HSA/VEGF or HSA/TGF-β3 demonstrated a cartilage-like phenotype, with minimal collagen I deposition, comparable to HSA-only control hydrogels. The addition of BMP-2 releasing microparticles resulted in enhanced structured bone matrix formation, evidenced by increased Sirius red-stained matrix and collagen expression within hydrogels. This study demonstrates delivery of bioactive growth factors from a novel alginate/bECM hydrogel to augment skeletal tissue formation and the use of an organotypic chick femur defect culture system as a high-throughput test model for scaffold/cell/growth factor therapies for regenerative medicine.

Evaluation of skeletal tissue repair, part 2: enhancement of skeletal tissue repair through dual-growth-factor-releasing hydrogels within an ex vivo chick femur defect model

There is an unmet need for improved, effective tissue engineering strategies to replace or repair bone damaged through disease or injury. Recent research has focused on developing biomaterial scaffolds capable of spatially and temporally releasing combinations of bioactive growth factors, rather than individual molecules, to recapitulate repair pathways present in vivo. We have developed an ex vivo embryonic chick femur critical size defect model and applied the model in the study of novel extracellular matrix (ECM) hydrogel scaffolds containing spatio-temporal combinatorial growth factor-releasing microparticles and skeletal stem cells for bone regeneration. Alginate/bovine bone ECM (bECM) hydrogels combined with poly(d,l-lactic-co-glycolic acid) (PDLLGA)/triblock copolymer (10-30% PDLLGA-PEG-PLDLGA) microparticles releasing dual combinations of vascular endothelial growth factor (VEGF), chondrogenic transforming growth factor beta 3 (TGF-β3) and the bone morphogenetic protein BMP2, with human adult Stro-1+bone marrow stromal cells (HBMSCs), were placed into 2mm central segmental defects in embryonic day 11 chick femurs and organotypically cultured. Hydrogels loaded with VEGF combinations induced host cell migration and type I collagen deposition. Combinations of TGF-β3/BMP2, particularly with Stro-1+HBMSCs, induced significant formation of structured bone matrix, evidenced by increased Sirius red-stained matrix together with collagen expression demonstrating birefringent alignment within hydrogels. This study demonstrates the successful use of the chick femur organotypic culture system as a high-throughput test model for scaffold/cell/growth factor therapies in regenerative medicine. Temporal release of dual growth factors, combined with enriched Stro-1+HBMSCs, improved the formation of a highly structured bone matrix compared to single release modalities. These studies highlight the potential of a unique alginate/bECM hydrogel dual growth factor release platform for bone repair.

Remotely Activated Mechanotransduction via Magnetic Nanoparticles Promotes Mineralization Synergistically With Bone Morphogenetic Protein 2: Applications for Injectable Cell Therapy

Bone requires dynamic mechanical stimulation to form and maintain functional tissue, yet mechanical stimuli are often lacking in many therapeutic approaches for bone regeneration. Magnetic nanoparticles provide a method for delivering these stimuli by directly targeting cell-surface mechanosensors and transducing forces from an external magnetic field, resulting in remotely controllable mechanotransduction. In this investigation, functionalized magnetic nanoparticles were attached to either the mechanically gated TREK1 K+ channel or the (integrin) RGD-binding domains of human mesenchymal stem cells. These cells were microinjected into an ex vivo chick fetal femur (embryonic day 11) that was cultured organotypically in vitro as a model for endochondral bone formation. An oscillating 25-mT magnetic field delivering a force of 4 pN per nanoparticle directly against the mechanoreceptor induced mechanotransduction in the injected mesenchymal stem cells. It was found that cells that received mechanical stimuli via the nanoparticles mineralized the epiphyseal injection site more extensively than unlabeled control cells. The nanoparticle-tagged cells were also seeded into collagen hydrogels to evaluate osteogenesis in tissue-engineered constructs: in this case, inducing mechanotransduction by targeting TREK1 resulted in a 2.4-fold increase in mineralization and significant increases in matrix density. In both models, the combination of mechanical stimulation and sustained release of bone morphogenetic protein 2 (BMP2) from polymer microspheres showed a significant additive effect on mineralization, increasing the effectiveness of BMP2 delivery and demonstrating that nanoparticle-mediated mechanotransduction can be used synergistically with pharmacological approaches for orthopedic tissue engineering to maximize bone formation.

Accelerating protein release from microparticles for regenerative medicine applications

There is a need to control the spatio-temporal release kinetics of growth factors in order to mitigate current usage of high doses. A novel delivery system, capable of providing both structural support and controlled release kinetics, has been developed from PLGA microparticles. The inclusion of a hydrophilic PLGA–PEG–PLGA triblock copolymer altered release kinetics such that they were decoupled from polymer degradation. A quasi zero order release profile over four weeks was produced using 10% w/w PLGA–PEG–PLGA with 50:50 PLGA whereas complete and sustained release was achieved over ten days using 30% w/w PLGA–PEG–PLGA with 85:15 PLGA and over four days using 30% w/w PLGA–PEG–PLGA with 50:50 PLGA. These three formulations are promising candidates for delivery of growth factors such as BMP-2, PDGF and VEGF. Release profiles were also modified by mixing microparticles of two different formulations providing another route, not previously reported, for controlling release kinetics. This system provides customisable, localised and controlled delivery with adjustable release profiles, which will improve the efficacy and safety of recombinant growth factor delivery.

Preparation and optimization of matrix metalloproteinase-1-loaded poly(lactide-co-glycolide-co-caprolactone) nanoparticles with rotatable central composite design and response surface methodology

Matrix metalloproteases are key regulatory molecules in the breakdown of extracellular matrix and in inflammatory processes. Matrix metalloproteinase-1 (MMP-1) can significantly enhance muscle regeneration by promoting the formation of myofibers and degenerating the fibrous tissue. Herein, we prepared novel MMP-1-loaded poly(lactide-co-glycolide-co-caprolactone) (PLGA-PCL) nanoparticles (NPs) capable of sustained release of MMP-1. We established quadratic equations as mathematical models and employed rotatable central composite design and response surface methodology to optimize the preparation procedure of the NPs. Then, characterization of the optimized NPs with respect to particle size distribution, particle morphology, drug encapsulation efficiency, MMP-1 activity assay and in vitro release of MMP-1 from NPs was carried out. The results of mathematical modeling show that the optimal conditions for the preparation of MMP-1-loaded NPs were as follows: 7 min for the duration time of homogenization, 4.5 krpm for the agitation speed of homogenization and 0.4 for the volume ratio of organic solvent phase to external aqueous phase. The entrapment efficiency and the average particle size of the NPs were 38.75 ± 4.74% and 322.7 ± 18.1 nm, respectively. Further scanning electron microscopy image shows that the NPs have a smooth and spherical surface, with mean particle size around 300 nm. The MMP-1 activity assay and in vitro drug release profile of NPs indicated that the bioactivity of the enzyme can be reserved where the encapsulation allows prolonged release of MMP-1 over 60 days. Taken together, we reported here novel PLGA-PCL NPs for sustained release of MMP-1, which may provide an ideal MMP-1 delivery approach for tissue reconstruction therapy.

Scaffolds for Growth Factor Delivery as Applied to Bone Tissue Engineering

There remains a substantial shortfall in the treatment of severe skeletal injuries. The current gold standard of autologous bone grafting from the same patient has many undesirable side effects associated such as donor site morbidity. Tissue engineering seeks to offer a solution to this problem. The primary requirements for tissue-engineered scaffolds have already been well established, and many materials, such as polyesters, present themselves as potential candidates for bone defects; they have comparable structural features, but they often lack the required osteoconductivity to promote adequate bone regeneration. By combining these materials with biological growth factors, which promote the infiltration of cells into the scaffold as well as the differentiation into the specific cell and tissue type, it is possible to increase the formation of new bone. However due to the cost and potential complications associated with growth factors, controlling the rate of release is an important design consideration when developing new bone tissue engineering strategies. This paper will cover recent research in the area of encapsulation and release of growth factors within a variety of different polymeric scaffolds.