Controlled release from coated polymer microparticles embedded in tissue-engineered scaffolds

Porous PLGA microparticles prepared with nanosized/micronized sugar particles as porogens

The objective of this study was to explore the use of nanosized/micronized sugar particles as porogens for preparing porous poly(lactide-co-glycolide) (PLGA) microparticles by a solid-in-oil-in-water (S/O/W) solvent evaporation method. Porous PLGA microparticles containing dexamethasone were prepared with different nanosized/micronized sugars (sucrose, trehalose and lactose), types of PLGA, and osmogens (NaCl or sucrose) in the external water phase. The microparticles were characterized for morphology, thermal properties, particle size, surface area, encapsulation efficiency and drug release/swelling during release. The addition of nanosized/micronized sugar particles resulted in porous PLGA microparticles with high encapsulation efficiencies. The porosity of the microparticles was caused both by the influx of water into the polymer droplets and the encapsulation and subsequent dissolution of sugar particles during the manufacturing process. The porosity (pore size) of the microparticles and, as a result, the drug release pattern could be well controlled by the particle size and weight fraction of the sugar particles. Because of a larger inner surface area, nanosized sugar particles were more efficient porogen than micronized sugar particles to obtain porous PLGA microparticles with flexible release patterns.

Engineering Bioactive Scaffolds for Skin Regeneration

Large skin wounds pose a major clinical challenge. Scarcity of donor site and postsurgical scarring contribute to the incomplete or partial loss of function and aesthetic concerns in skin wound patients. Currently, a wide variety of skin grafts are being applied in clinical settings. Scaffolds are used to overcome the issues related to the misaligned architecture of the repaired skin tissues. The current review summarizes the contribution of biomaterials to wound healing and skin regeneration and addresses the existing limitations in skin grafting. Then, the clinically approved biologic and synthetic skin substitutes are extensively reviewed. Next, the techniques for modification of skin grafts aiming for enhanced tissue regeneration are outlined, and a summary of different growth factor delivery systems using biomaterials is presented. Considering the significant progress in biomaterial science and manufacturing technologies, the idea of biomaterial-based skin grafts with the ability for scarless wound healing and reconstructing full skin organ is more achievable than ever.

Recent advances in biomaterials for 3D scaffolds: A review

Considering the advantages and disadvantages of biomaterials used for the production of 3D scaffolds for tissue engineering, new strategies for designing advanced functional biomimetic structures have been reviewed. We offer a comprehensive summary of recent trends in development of single- (metal, ceramics and polymers), composite-type and cell-laden scaffolds that in addition to mechanical support, promote simultaneous tissue growth, and deliver different molecules (growth factors, cytokines, bioactive ions, genes, drugs, antibiotics, etc.) or cells with therapeutic or facilitating regeneration effect. The paper briefly focuses on divers 3D bioprinting constructs and the challenges they face. Based on their application in hard and soft tissue engineering, in vitro and in vivo effects triggered by the structural and biological functionalized biomaterials are underlined. The authors discuss the future outlook for the development of bioactive scaffolds that could pave the way for their successful imposing in clinical therapy.

Synthetic polymer scaffolds for soft tissue engineering

Tissue engineering (TE) and regenerative medicine are progressively developed areas due to many novel tissue replacements and implementation strategies. Increasing knowledge involving the fabrication of biomaterials with advanced physicochemical and biological characteristics, successful isolation and preparation of stem cells, incorporation of growth and differentiation factors, and biomimetic environments gives us a unique opportunity to develop various types of scaffolds for TE. The current strategies for soft tissue reconstitution or regeneration highlight the importance of novel regenerative therapies in cases of significant soft tissue loss and in cases of congenital defects, disease, trauma and ageing. Various types of biomaterials and scaffolds have been tested for soft tissue regeneration. The synthetic types of materials have gained great attention due to high versatility, tunability and easy functionalization for better biocompatibility. This article reviews the current materials that are usually the most used for the fabrication of scaffolds for soft TE; in addition, the types of scaffolds together with examples of their applications for the regenerative purposes of soft tissue, as well as their major physicochemical characteristics regarding the increased applicability of these materials in medicine, are reviewed.

Bioactive polymeric scaffolds for tissue engineering

A variety of engineered scaffolds have been created for tissue engineering using polymers, ceramics and their composites. Biomimicry has been adopted for majority of the three-dimensional (3D) scaffold design both in terms of physicochemical properties, as well as bioactivity for superior tissue regeneration. Scaffolds fabricated via salt leaching, particle sintering, hydrogels and lithography have been successful in promoting cell growth in vitro and tissue regeneration in vivo. Scaffold systems derived from decellularization of whole organs or tissues has been popular due to their assured biocompatibility and bioactivity. Traditional scaffold fabrication techniques often failed to create intricate structures with greater resolution, not reproducible and involved multiple steps. The 3D printing technology overcome several limitations of the traditional techniques and made it easier to adopt several thermoplastics and hydrogels to create micro-nanostructured scaffolds and devices for tissue engineering and drug delivery. This review highlights scaffold fabrication methodologies with a focus on optimizing scaffold performance through the matrix pores, bioactivity and degradation rate to enable tissue regeneration. Review highlights few examples of bioactive scaffold mediated nerve, muscle, tendon/ligament and bone regeneration. Regardless of the efforts required for optimization, a shift in 3D scaffold uses from the laboratory into everyday life is expected in the near future as some of the methods discussed in this review become more streamlined.

Novel preparation of controlled porosity particle/fibre loaded scaffolds using a hybrid micro-fluidic and electrohydrodynamic technique

The purpose of this research was to produce multi-dimensional scaffolds containing biocompatible particles and fibres. To achieve this, two techniques were combined and used: T-Junction microfluidics and electrohydrodynamic (EHD) processing. The former was used to form layers of monodispersed bovine serum albumin (BSA) bubbles, which upon drying formed porous scaffolds. By altering the T-Junction processing parameters, bubbles with different diameters were produced and hence the scaffold porosity could be controlled. EHD processing was used to spray or spin poly(lactic-co-glycolic) (PLGA), polymethysilsesquioxane (PMSQ) and collagen particles/fibres onto the scaffolds during their production and after drying. As a result, multifunctional BSA scaffolds with controlled porosity containing PLGA, PMSQ and collagen particles/fibres were obtained. Product morphology was studied by optical and scanning electron microscopy. These products have potential applications in many advanced biomedical, pharmaceutical and cosmetic fields e.g. bone regeneration, drug delivery, cosmetic cream lathers, facial scrubbing creams etc.

Comparison of PLGA and lecithin/chitosan nanoparticles for dermal targeting of betamethasone valerate

Poly(lactide-co-glycolide) (PLGA) and lecithin/chitosan (LC) nanoparticles were prepared to evaluate the difference in the behavior upon administration on skin, for steroidal treatment. For this purpose, betamethasone-17-valerate (BMV)-loaded nanoparticles with a narrow size distribution and high entrapment efficiency were prepared. Permeation studies showed that both polymeric nanoparticles enhanced the amount of BMV in epidermis, which is the target site of topical steroidal treatment, when compared with commercial formulation. 1.58-Fold increase was determined in the epidermis concentration of BMV by LC nanoparticles with respect to PLGA nanoparticles. Nanoparticles were diluted in chitosan gel (10%, w/w) to prepare suitable formulation for topical application. Accumulation from both gel formulations were found significantly higher than commercial formulation in skin layers (p < 0.05). In addition, pharmacodynamic responses were also investigated as anti-inflammatory and skin-blanching parameters. Both formulations significantly improved these parameters although they contained 10 times less amount of BMV than commercial cream. Moreover, TEWL measurement exhibited no barrier function changes upon the application of nanoparticles on skin. Overall, both nanoparticles improved the localization of BMV within skin layers; but when compared with PLGA nanoparticles, the LC nanoparticles could be classified as a better candidate for topical delivery vehicle in the treatment of various dermatological inflammatory diseases.

Novel polymeric scaffolds using protein microbubbles as porogen and growth factor carriers

Polymeric tissue engineering scaffolds prepared by conventional techniques like salt leaching and phase separation are greatly limited by their poor biomolecule-delivery abilities. Conventional methods of incorporation of various growth factors, proteins, and/or peptides on or in scaffold materials via different crosslinking and conjugation techniques are often tedious and may affect scaffold's physical, chemical, and mechanical properties. To overcome such deficiencies, a novel two-step porous scaffold fabrication procedure has been created in which bovine serum albumin microbubbles (henceforth MB) were used as porogen and growth factor carriers. Polymer solution mixed with MB was phase separated and then lyophilized to create porous scaffold. MB scaffold triggered substantially lesser inflammatory responses than salt-leached and conventional phase-separated scaffolds in vivo. Most importantly, the same technique was used to produce insulin-like growth factor-1 (IGF-1)-eluting porous scaffolds, simply by incorporating IGF-1-loaded MB (MB-IGF-1) with polymer solution before phase separation. In vitro such MB-IGF-1 scaffolds were able to promote cell growth to a much greater extent than scaffold soaked in IGF-1, confirming the bioactivity of the released IGF-1. Further, such MB-IGF-1 scaffolds elicited IGF-1-specific collagen production in the surrounding tissue in vivo. This novel growth factor-eluting scaffold fabrication procedure can be used to deliver a range of single or combination of bioactive biomolecules to substantially promote cell growth and function in degradable scaffold.

Injectable, Biodegradable Hydrogels for Tissue Engineering Applications

Hydrogels have many different applications in the field of regenerative medicine. Biodegradable, injectable hydrogels could be utilized as delivery systems, cell carriers, and scaffolds for tissue engineering. Injectable hydrogels are an appealing scaffold because they are structurally similar to the extracellular matrix of many tissues, can often be processed under relatively mild conditions, and may be delivered in a minimally invasive manner. This review will discuss recent advances in the field of injectable hydrogels, including both synthetic and native polymeric materials, which can be potentially used in cartilage and soft tissue engineering applications.

In situ characterization of the degradation of PLGA microspheres in hyaluronic acid hydrogels by optical coherence tomography

The polymeric implant material poly(lactide-co-glycolide) (PLGA) degrades by a process of bulk degradation, which allows it to be used for the controlled release of therapeutic molecules from implants and microspheres. The temporal characterization of PLGA microsphere degradation has been limited by the need to destructively monitor the samples at each time point. In this study, a noninvasive imaging technology, optical coherence tomography (OCT), was utilized to characterize the in situ degradation of PLGA microspheres suspended within photo-crosslinked hyaluronic acid (HA) hydrogels. Microspheres with differing degradation rates were loaded with bovine serum albumin (BSA) as a marker protein, and temporal release of protein was correlated with morphological changes observed during 3-D OCT imaging. As proof-of-principle, a microsphere-loaded hydrogel scaffold was implanted in a modified rat calvarial critical size defect model and imaged using OCT. This animal model presents the opportunity to monitor microsphere degradation over time in living animals.

Preparation, characterization, and encapsulation/release studies of a composite nanofiber mat electrospun from an emulsion containing poly (lactic-co-glycolic acid)

The aim of this study was to investigate the preparation, characterization, and encapsulation/release performance of an electrospun composite nanofiber mat. The hypothesis was that the composite nanofiber mat with nano-scaled drug particles impregnated in biocompatible and biodegradable polymer nanofibers can serve as an innovative type of tissue engineering scaffold with desired and controllable drug encapsulation/release properties. To test the hypothesis, the composite nanofiber mat electrospun from an emulsion consisting of poly (lactic-co-glycolic acid) (PLGA) Rhodamine B (a model compound to simulate drugs), sorbitan monooleate (Span-80, a non-ionic emulsifier/surfactant that is presumably non-toxic/safe for cell-growth), chloroform, DMF, and distilled water was prepared and characterized; and the Rhodamine B encapsulation/release profile in phosphate buffered saline (pH = 7.4) was recorded and analyzed. For comparison purposes, two additional nanofiber mats electrospun from (1) a solution containing PLGA and Rhodamine B, and (2) a solution containing PLGA, Rhodamine B, and Span-80 were also prepared and assessed as the control samples. The results indicated that the composite nanofiber mat electrospun from the emulsion had the most desired and controllable Rhodamine B encapsulation/release profile and the excellent morphological sustainability; thus, it could be utilized as both a drug encapsulation/release vehicle and a tissue engineering scaffold.

Chondrogenesis, bone morphogenetic protein-4 and mesenchymal stem cells

OBJECTIVE

As adult cartilage has very limited potential to regenerate, cartilage repair is challenging. Available treatments have several disadvantages, including formation of fibrocartilage instead of hyaline-like cartilage, as well as eventual ossification of the newly formed tissue. The focus of this review is the application of bone morphogenetic protein-4 (BMP-4) and mesenchymal stem cells (MSCs) in cartilage repair, a combination that could potentially lead to the formation of permanent hyaline-like cartilage in the defect.

METHODS

This review is based on recent literature in the orthopaedic and tissue engineering fields, and is focused on MCSs and bone morphogenetic proteins (BMPs).

RESULTS

BMP-4, a stimulator of chondrogenesis, both in vitro and in vivo, is a potential therapeutic agent for cartilage regeneration. BMP-4 delivery can improve the healing process of an articular cartilage defect by stimulating the synthesis of the cartilage matrix constituents: type II collagen and aggrecan. BMP-4 has also been shown to suppress chondrogenic hypertrophy and maintain regenerated cartilage. Use of an appropriate carrier for BMP-4 is crucial for successful reconstruction of cartilage defects. Due to the relatively short half-life in vivo of BMP-4, there is a need to localize and maintain the delivery of BMP-4 to the injury site. Additionally, the delivery of MSCs to the wound site could improve cartilage regeneration; therefore, the carrier should function both as a cell and a protein delivery vehicle.

CONCLUSION

The role of BMP-4 in chondrogenesis is significant, and successful methods to deliver BMP-4, with or without MSCs, to the cartilage defect site are a promising therapy to treat cartilage defects.

Biomimetic materials for tissue engineering

Tissue engineering and regenerative medicine is an exciting research area that aims at regenerative alternatives to harvested tissues for transplantation. Biomaterials play a pivotal role as scaffolds to provide three-dimensional templates and synthetic extracellular matrix environments for tissue regeneration. It is often beneficial for the scaffolds to mimic certain advantageous characteristics of the natural extracellular matrix, or developmental or wound healing programs. This article reviews current biomimetic materials approaches in tissue engineering. These include synthesis to achieve certain compositions or properties similar to those of the extracellular matrix, novel processing technologies to achieve structural features mimicking the extracellular matrix on various levels, approaches to emulate cell-extracellular matrix interactions, and biologic delivery strategies to recapitulate a signaling cascade or developmental/wound healing program. The article also provides examples of enhanced cellular/tissue functions and regenerative outcomes, demonstrating the excitement and significance of the biomimetic materials for tissue engineering and regeneration.

In vitro characterization of a slow release system of polylactic acid and rhBMP2

The aim of the present report was to test a system for controlled release of recombinant human bone morphogenic protein (rhBMP2) incorporated into polylactic acid (PLA) implants. Incorporation of rhBMP2 into the polymer was accomplished by mixing rhBMP2 solution with granular powder of amorphous poly-DL-lactic acid, subsequent lyophilization, and high pressure CO(2) treatment at 100 bar. Porous cylindrical implants of 8 mm diameter and 3 mm thickness were fabricated with 100, 200, 400, and 800 microg BMP2/g polymer and submitted to in vitro testing. Polymer degradation was assessed during immersion of PLA implants into PBS for 176 days by measuring the inherent viscosity at days 0, 99, and 131. BMP2 release was evaluated by immersion of both the lyophilized powder and the implants into cell culture medium for up to 27 days. BMP2 release was assessed using a custom made ELISA. The biological activity of the released growth factor was determined by measuring the induction of alkaline phosphatase (AP) in C2C12 cells. There was a significant retardation in the release of BMP2 from the implants compared to the granular powder. Detectable amounts of BMP2 were found for all concentrations of BMP2 until the end of the observation period. Significant induction of AP was detected by BMP released from the implants after 3, 6, and 9 days. The present in-vitro study has shown that incorporation of rhBMP2 into PLA implants with subsequent slow release of biologically active growth factor is possible.

Controlled release of bioactive doxorubicin from microspheres embedded within gelatin scaffolds

We have encapsulated the chemotherapeutic agent doxorubicin into biodegradable polymer microspheres, and incorporated these microspheres into gelatin scaffolds, resulting in a controlled delivery system. Doxorubicin was encapsulated in poly(D,L-lactide-co-glycolide) (PLGA) using a double emulsion/solvent extraction method. Characterization of the microspheres including diameter, surface morphology, and in vitro drug release was determined. The release of doxorubicin up to 30 days in phosphate buffered solution was assessed by measuring the absorbance of the releasate solution. Gelatin scaffolds were crosslinked using glutaraldehyde and microspheres were added to gelatin during gelation. The murine mammary mouse tumor cell line, 4T1, was treated with various doses of doxorubicin. A propidium iodide assay was utilized to visualize dead cells. Using a Transwell basket assay, PLGA microspheres and gelatin constructs were suspended above 4T1 cells for 48 h. Viable cells were determined using the CyQUANT cell proliferation assay. Results indicate that the release was controlled by the incorporation of PLGA microspheres into gelatin constructs. A significant difference was seen in the cumulative release over days 5-16 (p < 0.05). The bioactivity of doxorubicin released from the microspheres and scaffolds was maintained as proven by significant reduction in viable cells after treatment with PLGA microspheres as well as with the gelatin constructs (p < 0.001). The drug-polymer conjugate can be used as a controlled drug delivery system in a biocompatible scaffold that could potentially promote preservation of soft tissue contour.

Inductive tissue engineering with protein and DNA-releasing scaffolds

Cellular differentiation, organization, proliferation and apoptosis are determined by a combination of an intrinsic genetic program, matrix/substrate interactions, and extracellular cues received from the local microenvironment. These molecular cues come in the form of soluble (e.g. cytokines) and insoluble (e.g. ECM proteins) factors, as well as signals from surrounding cells that can promote specific cellular processes leading to tissue formation or regeneration. Recent developments in the field of tissue engineering have employed biomaterials to present these cues, providing powerful tools to investigate the cellular processes involved in tissue development, or to devise therapeutic strategies based on cell replacement or tissue regeneration. These inductive scaffolds utilize natural and/or synthetic biomaterials fabricated into three-dimensional structures. This review summarizes the use of scaffolds in the dual role of structural support for cell growth and vehicle for controlled release of tissue inductive factors, or DNA encoding for these factors. The confluence of molecular and cell biology, materials science and engineering provides the tools to create controllable microenvironments that mimic natural developmental processes and direct tissue formation for experimental and therapeutic applications.

Poly(lactide-co-glycolide) microspheres as a moldable scaffold for cartilage tissue engineering

This study demonstrates the use of biodegradable poly(lactide-co-glycolide) (PLG) microspheres as a moldable scaffold for cartilage tissue engineering. Chondrocytes were delivered to a cylindrical mold with or without PLG microspheres and cultured in vitro for up to 8 weeks. Cartilagenous tissue formed using chondrocytes and microspheres maintained thickness, shape, and chondrocyte collagen type II phenotype, as indicated by type II collagen staining. The presence of microspheres further enhanced total tissue mass and the amount of glycosaminoglycan that accumulated. Evaluation of microsphere composition demonstrated effects of polymer molecular weight, end group chemistry, and buffer inclusion on tissue-engineered cartilage growth. Higher molecular weight PLG resulted in a larger mass of cartilage-like tissue formed and a higher content of proteoglycans. Cartilage-like tissue formed using microspheres made from low molecular weight and free carboxylic acid end groups did not display increases in tissue mass, yet a modest increased proteoglycan accumulation was detected. Microspheres comprised of PLG with methyl ester end groups yielded a steady increase in tissue mass, with no real increase in matrix accumulation. The microencapsulation of Mg(OH)(2) had negative effects on tissue mass and matrix accumulation. The data herein reflect the potential utility of a moldable PLG-chondrocyte system for tissue-engineering applications.

Cytarabine release from comatrices of albumin microspheres in a poly(lactide–co-glycolide) film: in vitro and in vivo studies

Cytarabine (ara-C) was included in albumin microspheres and these microspheres were immersed in a poly(lactide-co-glycolide) (PLGA) film to constitute a comatrix system to develop a prolonged form of release. Cytarabine-loaded albumin microspheres were synthesized by emulsion, and 25 or 50 mg of drug were included in the disperse phase. Thus, microspheres with 46+/-4 microg drug/mg microspheres and 50+/-5 microg drug/mg microspheres were obtained, which means a percentage of incorporation efficiency of 42+/-4% and 25+/-2%, respectively. These cytarabine-loaded microspheres were used to prepare PLGA-comatrices. Kinetic release studies indicated that total cytarabine release only takes place in the presence of protease, probably due to the fact that glutaraldehyde establishes covalent links with the amine side group of the drug and cross-links it with the protein matrix. A slower kinetic release of the drug was obtained from PLGA-comatrices, although only 80% of the included cytarabine was released on day 7. The comatrices were subcutaneously implanted in the back of rats and in both cases the ara-C administered dose was 36 mg of ara-C per kg of body weight. The drug was detected in plasma 10 days. The mean residence time (MRT) of the drug administered by these comatrices was 87-91 times larger when compared to the value obtained when the drug was administered in solution by intraperitoneal injection. The histological studies show that a degradative process of the comatrices takes place. The comatrices do not damage surrounding tissue; a normal regeneration of the implanted zone was observed.

Sustained release of ascorbate-2-phosphate and dexamethasone from porous PLGA scaffolds for bone tissue engineering using mesenchymal stem cells

The purpose of this research was to develop porous poly(D,L-lactide-co-glycolide) (PLGA) scaffolds from which ascorbate-2-phosphate (AsAP) and dexamethasone (Dex) are continuously released for a month for osteogenesis of mesenchymal stem cells for bone tissue engineering. Porous PLGA matrices containing AsAP and Dex were prepared by solvent casting/particulate leaching method. In vitro release and water uptake studies were performed in Dulbecco's phosphate buffered saline at 37 degrees C and 15 rpm. Drug loading and release rates were determined by high performance liquid chromatography. Release studies of Dex and AsAP showed that, after an initial burst release lasting 4 and 9 days, respectively, release rates followed zero order kinetics with high correlation coefficients at least until 35 days. Incorporation of AsAP into the scaffolds increased the release rates of Dex and AsAP, and the scaffold water uptake. When mesenchymal stem cells (MSCs) were cultured in the AsAP and Dex containing scaffolds in vitro, the amount of mineralization was significantly higher than in control scaffolds. In conclusion, AsAP and Dex were incorporated into porous PLGA scaffolds and continuously released over a month and osteogenesis of MSCs was increased by culture in these scaffolds.

Surface studies of coated polymer microspheres and protein release from tissue-engineered scaffolds

The controlled release of growth factors from porous, polymer scaffolds is being studied for potential use as tissue-engineered scaffolds. Biodegradable polymer microspheres were coated with a biocompatible polymer membrane to permit the incorporation of the microspheres into tissue-engineered scaffolds. Surface studies with poly(D,L-lactic-co-glycolic acid) [PLGA], and poly(vinyl alcohol) [PVA] were conducted. Polymer films were dip-coated onto glass slides and water contact angles were measured. The contact angles revealed an initially hydrophobic PLGA film, which became hydrophilic after PVA coating. After immersion in water, the PVA coating was removed and a hydrophobic PLGA film remained. Following optimization using these 2D contact angle studies, biodegradable PLGA microspheres were prepared, characterized, and coated with PVA. X-ray photoelectron spectroscopy was used to further characterize coated slides and microspheres. The release of the model protein bovine serum albumin from PVA-coated PLGA microspheres was studied over 8 days. The release of BSA from PVA-coated PLGA microspheres embedded in porous PLGA scaffolds over 24 days was also examined. Coating of the PLGA microspheres with PVA permitted their incorporation into tissue-engineered scaffolds and resulted in a controlled release of BSA.

Composition options for tissue-engineered bone

The logical assembly of tissue-engineered bone is ultimately directed by the clinical status of the patient. The basic elements for tissue-engineered bone should include signaling molecules, cells, and extracellular matrix. The assembly of these basic elements may need to be modified by tissue engineers to account for patient variables of age, gender, health, systemic conditions, habits, and anatomical implant. Moreover, different regions of the body will have different functional loads and vascularity. This review discusses several basic options that may be necessary to engineer bone, including spatial and temporal assembly of signaling factors, cells, and biomimetic extracellular matrices. Moreover, the importance of the health care status of the patient who may be receiving the tissue-engineered composition is emphasized.