Tuning the biomimetic behavior of scaffolds for regenerative medicine through surface modifications

Tissue engineering and regenerative medicine rely extensively on biomaterial scaffolds to support cell adhesion, proliferation, and differentiation physically and chemically in vitro and in vivo. Changes to the surface characteristics of the scaffolds have the greatest impact on cell response. Here, we discuss five dominant surface modification approaches used to biomimetically improve the most common scaffolds for tissue engineering, those based on aliphatic polyesters. Scaffolds of aliphatic polyesters such as poly(l-lactic acid), poly(l-lactic-co-glycolic acid), and poly(ε-caprolactone) are often used in tissue engineering because they provide desirable, tunable properties such as ease of manufacturing, good mechanical properties, and nontoxic degradation products. However, cell-surface interactions necessary for tissue engineering are limited on these materials by their smooth postfabrication surfaces, hydrophobicity, and lack of recognizable biochemical binding sites. The surface modification techniques that have been developed for synthetic polymer scaffolds reduce initial barriers to cell adhesion, proliferation, and differentiation. Topographical modification, protein adsorption, mineral coating, functional group incorporation, and biomacromolecule immobilization each contribute through varying mechanisms to improving cell interactions with aliphatic polyester scaffolds. Furthermore, rational combination of methods from these categories can provide nuanced, specific environments for targeted tissue development.

Surface modification of poly(D,L-lactic acid) scaffolds for orthopedic applications: a biocompatible, nondestructive route via diazonium chemistry

Scaffolds made with synthetic polymers such as polyesters are commonly used in bone tissue engineering. However, their hydrophobicity and the lack of specific functionalities make their surface not ideal for cell adhesion and growth. Surface modification of these materials is thus crucial to enhance the scaffold's integration in the body. Different surface modification techniques have been developed to improve scaffold biocompatibility. Here we show that diazonium chemistry can be used to modify the outer and inner surfaces of three-dimensional poly(D,L-lactic acid) (PDLLA) scaffolds with phosphonate groups, using a simple two-step method. By changing reaction time and impregnation procedure, we were able to tune the concentration of phosphonate groups present on the scaffolds, without degrading the PDLLA matrix. To test the effectiveness of this modification, we immersed the scaffolds in simulated body fluid, and characterized them with scanning electron microscopy, X-ray photoelectron spectroscopy, Raman, and infrared spectroscopy. Our results showed that a layer of hydroxyapatite particles was formed on all scaffolds after 2 and 4 weeks of immersion; however, the precipitation was faster and in larger amounts on the phosphonate-modified than on the bare PDLLA scaffolds. Both osteogenic MC3T3-E1 and chondrogenic ATDC5 cell lines showed increased cell viability/metabolic activity when grown on a phosphonated PDLLA surface in comparison to a control PDLLA surface. Also, more calcium-containing minerals were deposited by cultures grown on phosphonated PDLLA, thus showing the pro-mineralization properties of the proposed modification. This work introduces diazonium chemistry as a simple and biocompatible technique to modify scaffold surfaces, allowing to covalently and homogeneously bind a number of functional groups without degrading the scaffold's polymeric matrix.

Combination of silica nanoparticles with hydroxyapatite reinforces poly (l-lactide acid) scaffolds without loss of bioactivity

Composite scaffolds of poly(l-lactide acid) and hydroxyapatite are of great interest in bone tissue engineering, but their mechanical properties are typically inferior to scaffolds of pure poly(l-lactide acid) due to agglomeration of the particles and weak interfacial component interaction. Fabrication strategies like double sonication of hydroxyapatite or increasing the amount of this inorganic filler do not effectively enhance the mechanical performance. In this study, poly(l-lactide acid) composites combining two types of fillers, mesoporous silica (SiO2) nanoparticles and hydroxyapatite, were developed to reinforce the poly(l-lactide acid) scaffold without any loss of bioactivity. A 5% addition of SiO2 nanoparticles to hydroxyapatite nanopowder and subjecting the scaffold formulation to double sonication increased the Young’s modulus from 5 MPa (pure poly(l-lactide acid) scaffold) to almost 7 MPa (poly(l-lactide acid)/hydroxyapatite/SiO2 scaffold). In addition, the composite was able to deposit a layer of biomimetic hydroxyapatite both on the surface and interior of the scaffold after 21 days of immersion in a simulated body fluid. The manufacturing method was straightforward and economically viable and does not require any chemical modification of the particles’ surfaces.

Construction of microenvironment onto titanium substrates to regulate the osteoblastic differentiation of bone marrow stromal cells in vitro and osteogenesis in vivo

To mimic the extracellular matrix of natural bone, apatite/gelatin composite was deposited onto nanostructured titanium substrates via a coprecipitation method, which was pretreated by potassium hydroxide and heat treatment to generate an anticorrosive nanostructured layer. The successful formation of the apatite/gelatin nanocomposite onto titanium surfaces was revealed by Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, atomic force microscopy (AFM), and thin film X-ray diffraction (TF-XRD) measurements, respectively. The immunofluorescence staining of vinculin revealed that the apatite/gelatin nanocomposite deposited titanium substrate was favorable for cell adhesion. More importantly, bone marrow stromal cells cultured onto the apatite/gelatin nanocomposite deposited titanium substrates displayed significantly higher (p < 0.05 or p < 0.01) proliferation and differentiation levels of alkaline phosphatase, mRNA expressions of osteocalcin (OC), osteopontin (OPN), and collagen type I (Col I), and OC content after culture for 7, 14, and 21 days, respectively, which was also revealed by the immunofluorescence analysis of OC and OPN expression. The deposition of apatite/gelatin nanocomposite improved bone density (p < 0.05) and bone-implant contact rate (p < 0.05), which was reflected by microcomputed tomography analysis and histological evaluation in vivo using a rabbit model. This work provides an approach to fabricate high-performance titanium-based implants with enhanced bone osseointegration.

Behavior of human cells in contact with a poly(d,l-lactic acid) porous matrix after calcification using phosphatidylserine

As part of a strategy aimed at improving bioresorbable scaffolds for the engineering of bony tissues, a route to deposit calcium phosphate onto surfaces of poly(dl-lactic acid)–based porous matrices was investigated. Porosity was generated using the NaCl-leaching technique. Calcification was achieved after deposition of phosphatidylserine, a nucleating agent of natural origin, onto pore surfaces, followed by incubation of the phospholipid-coated matrix in a pH 6.5 aqueous medium consisted of 3.5 mmol CaCl2 and 2.6 mmol KH2PO4 for 3 days. Calcified matrices were noncytotoxic according to the ISO10993-5 standard test and exhibited low inflammatory potential. To compare responses of human cells of different types, human osteogenic bone marrow cells from the femoral head, human chondrocytes from femoral cartilage collected after hip surgery, and human vascular endothelial cells isolated from an umbilical cord were allowed to grow in the presence of the calcified matrices in vitro. Articular chondrocytes adhered to and grew on the calcified matrices up to colony formation. In contrast, the other two types of cells attached and proliferated for approximately 3 days and then detached. These different cell behaviors are discussed with respect to the nature of the cells and to the release of calcium ions from the coating.

Biomimetic Apatite-coated PCL Scaffolds: Effect of Surface Nanotopography on Cellular Functions

In this study, polycaprolactone (PCL) scaffolds, consisting of agglomerated microspheres with nanotopographic surface structures, were fabricated by the freeze-drying method. These scaffolds were coated with bone-like apatite by using a calcium phosphate solution similar to saturated simulated body fluid (10× SBF-like) in two different immersion periods (6 and 24 h). Scanning electron microscopic views of the 6-h treatment in 10× SBF-like solution showed formation of calcium phosphate nucleation sites on the PCL scaffolds, while the apatite particles formed characteristic cauliflower-like morphology after 24 h. The X-ray diffraction (XRD) data showed that the mineral phase was made of hydroxyapatite (HA). The osteogenic activity of untreated and SBF-treated PCL scaffolds was examined by pre-osteoblastic MC3T3 cell culture studies. Cells had attached and spread on both the PCL scaffolds and the 6-h SBF immersion-treated scaffolds.

Injectable Poly(ethylene glycol) Dimethacrylate-based Hydrogels with Hydroxyapatite

Injectable hydrogels are attractive materials for tissue engineering as they provide fast reaction rates, low heat release, and biocompatibility for cell proliferation and permanent interface with surrounding tissue. A series of injectable poly(ethylene glycol) dimethacrylate (PEGDMA) hydrogels with four different weight fractions of hydroxyapatite (HA) particles were prepared and thermal and mechanical properties evaluated. The cytocompatibility was assessed by examining the viability and morphology of human mesenchymal stem cells (hMSCs) seeded on the hydrogels. The in situ crosslink process displayed a vast decrease in the maximal temperature and an increase in the maximal temperature time. Cytocompatibility evaluation by MTT assay, scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM) showed that the cells on the composite hydrogels possessed better viability and adherence than the hydrogels without HA. The results indicated that composite hydrogels have potential as injectable materials for tissue engineering application.

Fabrication and in Vitro Evaluation of Calcium Phosphate Combined with Chitosan Fibers for Scaffold Structures

A rapid prototyping and rapid tool technique-based method was developed to fabricate chitosan fiber calcium phosphate cement composites (CF/CPC) for bone tissue engineering scaffold applications. The products were characterized and the in vitro performance with canine bone marrow stem cells (BMCs) on CF/CPC scaffold with controlled fiber structures evaluated. The X-ray diffraction analysis showed that about 91% of the inorganic part of the CF/CPC scaffold was hydroxyapatite (HA) and the variation in CF had little effect on the percentage of HA content. The results from in vitro study demonstrated that the interconnected macropores rapidly formed inside the CF/CPC scaffolds and that the patterns were related to the fiber structures used. The differences in the fiber structures altered the morphology of the BMCs without affecting the proliferation of the BMCs.

A Novel Osteochondral Scaffold Fabricated via Multi-nozzle Low-temperature Deposition Manufacturing

A functional-region/separate-interface/single-cell-type of tissue engineering pathway was evaluated to regenerate osteochondral defects that are deep in the marrow cavity. A gradient osteochondral scaffold fabricated via a rapid prototyping technology, called multi-nozzle low-temperature deposition manufacturing, was composed of three parts with different materials and pore-structures, respectively, for bone, cartilage and a separate interface between them. The separate interface was composed of micro-pores, that were less than 5 urn and with low porosity, to reduce or to avoid the destruction of the micro-environment in vivo by preventing blood and cells and reducing the amount of oxygen and nutrients moving from the marrow cavity to the articulate marrow. The preliminary results after 6 weeks of implantation into 4 mm diameter osteochondral defects in the knee joints of rabbits showed that the defects with the scaffold/cells composition had bone-like or cartilage-like tissue filling the defects with smooth surface, while the defects with nothing (blank) showed only fibrous tissue.

Hybrid Nanoapatite by Polysaccharide Nanogel-templated Mineralization

Self-assembled nanogels of cholesterol-bearing mannan (CHM) containing phosphodiester bonds in its structure were used as templates for calcium phosphate mineralization to prepare novel organic—inorganic hybrid nanocarriers for drug-delivery systems. These hybrid materials were synthesized from a solution of calcium phosphate in the presence of CHM nanogels. The size and crystallinity of the nanomaterials were controlled by changing the calcium ion concentration. Spherical amorphous calcium phosphate (ACP) nanoparticles of ~20 nm in size and needle-shaped hydroxyapatite (HAp) crystals ~80 nm in length were formed from solutions of [Ca2+] = 0.8 and 2.0 mM, respectively. These hybrid nanomaterials showed excellent colloidal stability in solution, which was supported by the polymer chains of CHM on the outer surface of the hybrid nanomaterials. It was also found that an amorphous-to-crystalline transformation occurred during the formation of CHM nanogel-HAp hybrid nanomaterials.

Rat Cartilage Repair Using Nanophase PLGA/HA Composite and Mesenchymal Stem Cells

Biodegradable polymer/bioceramic composite poly(lactide- coglycolide)/hydroxyapatite(PLGA/HA) was studied for bone tissue engineering. The PLGA/HA composite was evaluated as a scaffold with the ability for mesenchymal stem cells (MSC) to participate in cartilage repair. The PLGA/HA composite and the PLGA/HA composite cultured with MSC were transplanted into cartilage defects created in rats. The PLGA/HA-MSC and PLGA/HA had better tissue morphology, structure integrity, matrix staining, and much thicker newly formed cartilage than the control group. The histological score for PLGA/ HA-MSC better than that for PLGA/HA; it appears that the MSC plays an important role in tissue repair. Based on these results, using PLGA/HA as the tissue scaffold and the addition of MSC significantly enhances cartilage repair.

In Vitro Osteogenic Differentiation of Rat Mesenchymal Stem Cells in a Microgravity Bioreactor

Mesenchymal stem cells (MSCs) are multipotent progenitor cells with the ability to differentiate into osteoblasts, chondroblasts, myocytes, and adipocytes. They have potential for bone tissue engineering by the utilization of in vitro expanded cells with osteogenic capacity and their delivery to the appropriate sites via biomaterial scaffolds. The objective was to evaluate the potential of rat bone marrow MSCs to form 3D bone-like tissue by the use of mineralized poly(DL-lactic-co-glycolic acid) (PLGA) foam and osteoinductive medium under rotating culture conditions. PLGA foams were prepared by solvent casting and particulate leaching, then mineralized by incubating in simulated body fluid. MSCs isolated from the bone marrow of young Wistar rats were expanded and seeded on the mineralized scaffolds. The cell-polymer constructs were then cultured in a slow turning lateral vessel-type rotating bioreactor for 4 weeks under the effect of osteogenic inducers, β-glycerophosphate, ascorbic acid and dexamethasone. Mineralization was evaluated using FT-IR and increases in dry mass; morphology changes of the mineralized foams and cell adhesion was characterized by SEM; cell viability was monitored by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide). Osteogenic differentiation was determined by using immunohistochemistry (anti-osteopontin). Results indicate the feasibility of bone tissue engineering with MSCs and mineralized PLGA scaffolds supporting cell adhesion, viability and osteogenic differentiation properties of cells in hybrid structures under appropriate bioreactor conditions.