Degradation of extracellular matrix regulates osteoblast migration: A microfluidic-based study

Bone regeneration is strongly dependent on the capacity of cells to move in a 3D microenvironment, where a large cascade of signals is activated. To improve the understanding of this complex process and to advance in the knowledge of the role of each specific signal, it is fundamental to analyze the impact of each factor independently. Microfluidic-based cell culture is an appropriate technology to achieve this objective, because it allows recreating realistic 3D local microenvironments by taking into account the extracellular matrix, cells and chemical gradients in an independent or combined scenario. The main aim of this work is to analyze the impact of extracellular matrix properties and growth factor gradients on 3D osteoblast movement, as well as the role of cell matrix degradation. For that, we used collagen-based hydrogels, with and without crosslinkers, under different chemical gradients, and eventually inhibiting metalloproteinases to tweak matrix degradation. We found that osteoblast's 3D migratory patterns were affected by the hydrogel properties and the PDGF-BB gradient, although the strongest regulatory factor was determined by the ability of cells to remodel the matrix.

Osteoblasts Adherence and Migration through Three-dimensional Porous Mineralized Collagen Based Composite: nHAC/PLA

Osteoblast cells were separated from the neonatal rat calvaria and co-cultured on a novel mineralized hydroxyapatite/collagen/poly(lactic acid) composite scaffold. By using this static cell culture, a three-dimensional osteoblasts/composite bone-like was constructed in vitro. The culture process was observed by scanning electron microscopy, fluorescence microscopy, confocal laser scanning microscopy, and histological analysis. Cells were observed to spread and proliferate throughout the inner-pores of the scaffold material. After a 12-day culture, the cells had grown into the interior scaffold about 200–400 μm depth of the composite by histological section observation. This mobile behavior of osteoblasts appeared to be similar to the composition and hierarchical structure of bone tissue. The adherence and migration of osteoblast cells in this three-dimensional composite is clinically important for large bone defect repair based on tissue engineering.

Biodegradable photo-crosslinked polymer substrates with concentric microgrooves for regulating MC3T3-E1 cell behavior

Both intrinsic material properties and topographical features are critical in influencing cell-biomaterial interactions. We present a systematic investigation of regulating mouse pre-osteoblastic MC3T3-E1 cell behavior on biodegradable polymer substrates with distinct mechanical properties and concentric microgrooves. The precursors for fabricating substrates used here were two poly(ϵ-caprolactone) triacrylates (PCLTAs) synthesized from poly(ϵ-caprolactone) triols with molecular weights of ∼7000 and ∼10000 g mol(-1) . These two PCLTAs were photo-crosslinked into PCL networks with distinct thermal, rheological, and mechanical properties at physiological temperature because of their different crystallinities and melting temperatures. Microgrooved substrates with four groove widths of 7.5, 16.1, 44.2, and 91.2 μm and three groove depths of 0.2, 1, and 10 μm were prepared through replica molding, i.e., photo-crosslinking PCLTA on micro-fabricated silicon wafers with pre-designed concentric groove patterns. MC3T3-E1 cell attachment and proliferation could be better supported by the stiffer substrates while not significantly influenced by the microgrooves. Microgroove dimensions could regulate MC3T3-E1 cell alignment, nuclear shape and distribution, mineralization, and gene expression. Among the microgrooves with a fixed depth of 10 μm, the smallest width of 7.5 μm could align and elongate the cytoskeleton and nuclei most efficiently. Strikingly, higher mineral deposition and upregulation of osteocalcin gene expression were found in the narrower microgrooves when the groove depth was 10 μm.

Hydroxyapatite nanorod-reinforced biodegradable poly(L-lactic acid) composites for bone plate applications

Novel PLLA composite fibers containing hydroxyapatite (HAp) nanorods with or without surface lactic acid grafting were produced by extrusion for use as reinforcements in PLLA-based bone plates. Fibers containing 0-50% (w/w) HAp nanorods, aligned parallel to fiber axis, were extruded. Lactic acid surface grafting of HAp nanorods (lacHAp) improved the tensile properties of composites fibers better than the non-grafted ones (nHAp). Best tensile modulus values of 2.59, 2.49, and 4.12 GPa were obtained for loadings (w/w) with 30% lacHAp, 10% nHAp, and 50% amorphous HAp nanoparticles, respectively. Bone plates reinforced with parallel rows of these composite fibers were molded by melt pressing. The best compressive properties for plates were obtained with nHAp reinforcement (1.31 GPa Young's Modulus, 110.3 MPa compressive strength). In vitro testing with osteoblasts showed good cellular attachment and spreading on composite fibers. In situ degradation tests revealed faster degradation rates with increasing HAp content. To our knowledge, this is the first study containing calcium phosphate-polymer nanocomposite fibers for reinforcement of a biodegradable bone plate or other such implants and this biomimetic design was concluded to have potential for production of polymer-based biodegradable bone plates even for load bearing applications.

Biocomposites and hybrid biomaterials based on calcium orthophosphates

The state-of-the-art of biocomposites and hybrid biomaterials based on calcium orthophosphates that are suitable for biomedical applications is presented in this review. Since these types of biomaterials offer many significant and exciting possibilities for hard tissue regeneration, this subject belongs to a rapidly expanding area of biomedical research. Through successful combinations of the desired properties of matrix materials with those of fillers (in such systems, calcium orthophosphates might play either role), innovative bone graft biomaterials can be designed. Various types of biocomposites and hybrid biomaterials based on calcium orthophosphates, either those already in use or being investigated for biomedical applications, are extensively discussed. Many different formulations, in terms of the material constituents, fabrication technologies, structural and bioactive properties as well as both in vitro and in vivo characteristics, have already been proposed. Among the others, the nanostructurally controlled biocomposites, those containing nanodimensional compounds, biomimetically fabricated formulations with collagen, chitin and/or gelatin as well as various functionally graded structures seem to be the most promising candidates for clinical applications. The specific advantages of using biocomposites and hybrid biomaterials based on calcium orthophosphates in the selected applications are highlighted. As the way from the laboratory to the hospital is a long one, and the prospective biomedical candidates have to meet many different necessities, this review also examines the critical issues and scientific challenges that require further research and development.

Biodegradable polymeric scaffolds. Improvements in bone tissue engineering through controlled drug delivery

Recent advances in biology, medicine, and engineering have led to the discovery of new therapeutic agents and novel materials for the repair of large bone defects caused by trauma, congenital defects, or bone tumors. These repair strategies often utilize degradable polymeric scaffolds for the controlled localized delivery of bioactive molecules to stimulate bone ingrowth as the scaffold degrades. Polymer composition, hydrophobicity, crystallinity, and degradability will affect the rate of drug release from these scaffolds, as well as the rate of tissue ingrowth. Accordingly, this chapter examines the wide range of synthetic degradable polymers utilized for osteogenic drug delivery. Additionally, the therapeutic proteins involved in bone formation and in the stimulation of osteoblasts, osteoclasts, and progenitor cells are reviewed to direct attention to the many critical issues influencing effective scaffold design for bone repair.

Manipulations in hydrogel degradation behavior enhance osteoblast function and mineralized tissue formation

Hydrogels were prepared by copolymerizing a degradable macromer, poly(lactic acid)-b-poly(ethylene glycol)-b-poly(lactic acid) endcapped with methacrylate groups (PEG-LA-DM), with a nondegradable macromer, poly(ethylene glycol) dimethacrylate (PEGDM). Copolymer networks consisted of 100:0, 83:17, 67:33, and 50:50 PEGDM:PEG-LA-DM mass%, essentially creating scaffolds that exhibit 0, 17, 33, and 50% degradation over the time course of the experiment. Osteoblasts were photoencapsulated in these copolymer hydrogels and cultured for 3 weeks in vitro. Metabolic activity, proliferation, and alkaline phosphatase production were enhanced by an increase PEG-LADM content and corresponding degradation. Gene expression of the cultured osteoblasts, normalized to beta-actin, was analyzed, and osteopontin and collagen type I gene expression increased with degradation. Finally, as a measure of mineralized tissue formation, calcium and phosphate deposition was analyzed biochemically and histologically. Mineralization increased with increasing concentration of PEG-LA-DM and biochemically resembled that of hydroxyapatite.

Effect of FGF and Polylactide Scaffolds on Calvarial Bone Healing With Growth Factor on Biodegradable Polymer Scaffolds

Repair of bone defects remains a major concern in reconstructive surgery. Synthetic biodegradable polymers have been used as scaffolds for guided bone regeneration. Fibroblast growth factors (FGFs) promote cell growth, differentiation, and tissue maintenance factors. They can stimulate the proliferation of osteogenic cells and chondrocytes, and also promote angiogenesis. Acidic and basic fibroblast growth factors (FGF-1 and FGF-2, respectively) are the best known members of this protein family. To evaluate the healing of experimental bone defects using poly-L/D-lactide (PLDLA) 96/4 scaffolds and FGF-1, 18 adult rats were operated on. A 6-mm diameter critical size defect (CSD) was made in the calvarial bone of each rat. The animals were divided into three treatment groups: 1) Neither scaffold nor FGF was used (control group); 2) scaffold only; and 3) scaffold with FGF-1. Follow-up time was eight weeks. Samples were embedded in methylmethacrylate and 5-microm thick sections from the middle of each specimen were stained with modified Masson-Goldner method. The shape and size of defects were evaluated radiologically. New bone formation was measured histologically and histomorphometrically. Radiologically, in the control group the shape of the defects changed from round to oval and edges were blunt. In the other groups the defects were round with sharp edges. Histomorphometrically, mean surface area of bone trabeculae was 1.05 mm (SD +/- 0.25) in group 1 (no implant), 1.35 mm (SD +/- 0.52) in group 2 (implant) and 0.79 mm (SD +/- 0.34) in group 3 (implant and FGF-1). Histological examinations revealed no or little osteoid in the groups 1 and 2, whereas in the group 3 samples had little or moderate new bone formation. Accordingly, no clear benefit of using knitted PLDLA scaffolds combined with FGF-1 on the healing of calvarial critical size defects in rats could be demonstrated.

Poly(lactic-co-glycolic acid) hollow fibre membranes for use as a tissue engineering scaffold

Mass transfer limitations of scaffolds are currently hindering the development of 3-dimensional, clinically viable, tissue engineered constructs. We have developed a poly(lactide-co-glycolide) (PLGA) hollow fibre membrane scaffold that will provide support for cell culture, allow psuedovascularisation in vitro and provide channels for angiogenesis in vivo. We produced P(DL)LGA flat sheet membranes using 1, 4-dioxane and 1-methyl-2-pyrrolidinone (NMP) as solvents and water as the nonsolvent, and hollow fibre membranes, using NMP and water, by dry/wet- and wet-spinning. The resulting fibres had an outer diameter of 700 micro m and an inner diameter of 250 micro m with 0.2-1.0 micro m pores on the culture surface. It was shown that varying the air gap and temperature when spinning changed the morphology of the fibres. The introduction of a 50 mm air gap caused a dense skin of 5 micro m thick to form, compared to a skin of 0.5 micro m thick without an air gap. Spinning at 40 degrees C produced fibres with a more open central section in the wall that contained more, larger macrovoids compared to fibres spun at 20 degrees C. Culture of the immortalised osteogenic cell line 560pZIPv.neo (pZIP) was carried out on the P(DL)LGA flat sheets in static culture and in a P(DL)LGA hollow fibre bioreactor under counter-current flow conditions. Attachment and proliferation was statistically similar to tissue culture polystyrene on the flat sheets and was also successful in the hollow fibre bioreactor. The P(DL)LGA hollow fibres are a promising scaffold to address the size limitations currently seen in tissue engineered constructs.

Cell distribution profiles in three-dimensional scaffolds with inverted-colloidal-crystal geometry: modeling and experimental investigations

Limited ingrowth of stromal cells is observed when a three-dimensionally ordered scaffold possessing inverted-colloidal-crystal geometry is used to culture adherent cells. In this work, a computational model explaining, as well as predicting, experimental cell distributions is developed. It incorporates a modified Contois cell-growth model that includes the effects of nutrient saturation, competitive product inhibition, and cell-contact inhibition to describe the scaffold-cell system. Our results agree with the hypothesis that the rapid growth of cells on the surface of the scaffold depletes the nutrient supply to the core, resulting in the preferential growth on the exterior of the scaffold. When the cells are cultured in a scaffold subjected to a uniform velocity field, they penetrate to a greater extent into the scaffold core. Alternative seeding and culture strategies are suggested and evaluated.

Quantitative analysis of three-dimensional fluid flow in rotating bioreactors for tissue engineering

Tissue engineering has emerged as a viable alternative to the problem of organ and tissue shortage. Our laboratory has developed matrices for bone tissue engineering based on sintered spherical particles and, using bioreactor technology, has demonstrated the ability to produce highly mineralized matrices in vitro. In this study, porous microcapsule scaffolds were developed for bone tissue engineering in the high aspect ratio vessel rotating bioreactor. The motion of individual microcapsules as well as scaffolds in the bioreactor were studied by numerical simulation and in situ imaging analysis. Results show that spherical microcapsules with density less than the surrounding fluid exhibited two motions: (1) a periodic circular orbit with tangential speed equal to the free fall speed of the particle, and (2) an inward radial migration of the circular orbit toward the center of the bioreactor vessel. Lighter-than-water scaffolds were fabricated by sintering poly(lactic-co-glycolic acid) hollow microcarriers with diameter from 500 to 860 microm into a fixed three-dimensional geometry with approximately 30% pore volume and 180 to 190 microm median pore size. Scaffolds were fabricated with aggregate densities ranging from 0.65 g/mL and 0.99 g/mL by appropriate combinations of hollow and solid microcarriers within the scaffold. Scaffold velocity in the bioreactor for the above range of densities was accurately predicted by numerical simulation and ranged from 100 mm/s to 3 mm/s. Maximum shear stress estimation due to media flow over the exterior of the scaffold ranged from 0.3 N/m(2) to 0.006 N/m(2). Internal perfusion velocity through scaffolds also was calculated and ranged from 13 mm/s to 0.2 mm/s. Estimates of maximum interior shear stress ranged from 0.03 to 0.0007 N/m(2). These analytical methods provide an excellent vehicle for the study of bone tissue synthesis in three-dimensional culture with fluid flow.

Attachment, proliferation, and migration of marrow stromal osteoblasts cultured on biomimetic hydrogels modified with an osteopontin-derived peptide

We prepared oligo(poly(ethylene glycol) fumarate) (OPF) hydrogels modified with a rat osteopontin-derived peptide (ODP), Asp-Val-Asp-Val-Pro-Asp-Gly-Arg-Gly-Asp-Ser-Leu-Ala-Try-Gly (DVDVPDGRGDSLAYG), as well as Gly-Arg-Gly-Asp-Ser (GRGDS) and investigated the modulation of marrow stromal osteoblast function on the peptide-modified hydrogels. Osteoblast attachment was competitively inhibited by a soluble peptide suggesting that the interaction of osteoblasts with the hydrogel was ligand specific. The proliferation index of osteoblasts relative to the initial seeding density was similar on the hydrogels modified with ODP (1.18+/-0.13) and GRGDS (1.27+/-0.12). However, fibroblasts proliferated faster on GRGDS-modified hydrogels than on ODP-modified hydrogels as evidenced by the proliferation indices of 4.89+/-0.03 and 2.42+/-0.16, respectively. A megacolony migration assay conducted for 3 days with a seeding density of 53,000 cells/cm(2) showed that osteoblasts migrated to a longer distance on ODP-modified hydrogels (0.23+/-0.06 mm/day) than on hydrogels modified with GRGDS (0.15+/-0.02 mm/day). In addition, osteoblasts migrated faster than fibroblasts seeded at the same density on ODP-modified hydrogels (0.15+/-0.11 mm/day). The migration of osteoblasts on the peptide-modified hydrogels was dependent on the peptide concentration of the hydrogels resulting in an increased migration distance with increasing the peptide concentration for the concentrations tested. These results show that OPF-based biomimetic hydrogels hold promise for modulating cell proliferation and migration for specific applications by altering the specific ligand and its concentration in the hydrogels.

Bacteriostatic properties of biomatrices against common orthopaedic pathogens

Tissue-engineered grafts for tissue regeneration include either mature or progenitor cells seeded onto biomatrices that provide shape and support for developing tissue. Popular biomaterials used in orthopaedic surgery include collagen type I, hyaluronic acid, hydroxyapatite, and polylactic polyglycolic acid (PLGA). Biomatrices with bacteriostatic properties may be beneficial in promoting tissue-engineered graft survival in patients susceptible to infection. We evaluated the bacteriostatic effects of these biomaterials on the growth of the four most common orthopaedic bacterial pathogens: Staphylococcus aureus, Staphylococcus epidermidis, beta-hemolytic Streptococcus, and Pseudomonas aeruginosa. Hyaluronic acid demonstrated the largest bacteriostatic effect on these pathogens by inhibiting bacterial growth by an average of 76.8% (p = 0.0005). Hydroxyapatite and collagen inhibited growth on average by 49.7% (p = 0.011) and 37.5% (p = 0.102), respectively. PLGA exhibited the least bacteriostasis with an average inhibition of 9.8% (NS) and actually accelerated the growth of beta-hemolytic Streptococcus and P. aeruginosa.

Protein adsorption, attachment, growth and activity of primary rat osteoblasts on polylactide membranes with defined surface characteristics

The adsorption of proteins and growth and activity of primary rat osteoblasts cultured for 1, 2 and 3 weeks on nonporous and porous resorbable poly(L/DL-lactide) 80/20% membranes with defined surface characteristics were investigated. The growth and activity of cells were estimated from the measurements of DNA, alkaline phosphatase activity and the total amount of protein in the cell lysate. The cell morphology was assessed from scanning electron microscopy and rhodamine staining. The protein adsorption to the membrane surface was assessed from the amide I peak at 1640-1660 cm(-1) and the amide II peak at 1540-1560 cm(-1) in the attenuated total reflection infrared spectra. The relative amount of proteins adsorbed on the nonporous and porous membranes was comparable. The cells growing on the nonporous and porous membranes maintained the phenotype and revealed morphology typical for osteoblasts. The mineralized noduli were larger in size on the porous membranes. The number of cells, the amount of DNA, the alkaline phosphatase activity, and the total amount of protein increased with time of the experiment and were higher for the porous membranes than for the nonporous ones.

Recent Developments in Ring Opening Polymerization of Lactones for Biomedical Applications

Aliphatic polyesters prepared by ring-opening polymerization of lactones are now used worldwide as bioresorbable devices in surgery (orthopaedic devices, sutures, stents, tissue engineering, and adhesion barriers) and in pharmacology (control drug delivery). This review presents the various methods of the synthesis of polyesters and tailoring the properties by proper control of molecular weight, composition, and architecture so as to meet the stringent requirements of devices in the medical field. The effect of structure on properties and degradation has been discussed. The applications of these polymers in the biomedical field are described in detail.

Tissue engineered bone: Measurement of nutrient transport in three-dimensional matrices

The classic paradigm for in vitro tissue engineering of bone involves the isolation and culture of donor osteoblasts or osteoprogenitor cells within three-dimensional (3D) scaffold biomaterials under conditions that support tissue growth and mineralized osteoid formation. Our studies focus on the development and utilization of new dynamic culture technologies to provide adequate nutrient flux within 3D scaffolds to support ongoing tissue formation. In this study, we have developed a basic one-dimensional (1D) model to characterize the efficiency of passive nutrient diffusion and transport flux to bone cells within 3D scaffolds under static and dynamic culture conditions. Internal fluid perfusion within modeled scaffolds increased rapidly with increasing pore volume and pore diameter to a maximum of approximately 1% of external fluid flow. In contrast, internal perfusion decreased significantly with increasing pore channel tortuosity. Calculations of associated nutrient flux indicate that static 3D culture and some inappropriately designed dynamic culture environments lead to regions of insufficient nutrient concentration to maintain cell viability, and can result in steep nutrient concentration gradients within the modeled constructs. These quantitative studies provide a basis for development of new dynamic culture methodologies to overcome the limitations of passive nutrient diffusion in 3D cell-scaffold composite systems proposed for in vitro tissue engineering of bone.

In situ forming lactic acid based orthopaedic biomaterials: influence of oligomer chemistry on osteoblast attachment and function

The ability of osteoblasts to attach and function normally on scaffolds fabricated from synthetic materials is essential for musculoskeletal tissue engineering applications. In this study, the osteoconductivity of polymer networks formed from multifunctional lactic acid oligomers was assessed. These oligomers form highly crosslinked networks via a photoinitiated polymerization, which provides potential advantages for many orthopaedic applications. Depending on the initial oligomer chemistry and the resultant polymer hydrophobicity, protein adsorption and osteoblast function varied significantly between the various lactic acid based polymer chemistries. Results were compared to control polymers of tissue culture polystyrene (TCPS) and 50:50 poly(lactic-co-glycolic acid) (PLGA). The viability of osteoblasts attached to poly(2EG10LA) and poly(2EG6LA) was close to the TCPS and PLGA after 7 and 14 days of culture, whereas cell viability was approximately 50% lower on poly(8EG6LA). Additionally, the alkaline phosphatase activity and mineralization of attached osteoblasts were similar on poly(2EG10LA) and PLGA, whereas these markers of bone formation were significantly lower for poly(2EG6LA) and poly(8EG6LA). For example, the alkaline phosphatase activity of rat calvarial osteoblasts attached to poly(2EG10LA) was 0.048 +/- 0.006 micromol mg(-1) protein-min, but only 0.030 +/- 0.003 micromol mg(-1) protein-min for osteoblasts attached to poly(8EG6LA) after 14 days of culture. Finally, osteoblasts were seeded onto three-dimensional scaffolds to demonstrate the applicability of the scaffolds for bone tissue engineering.

Biologically and chemically optimized composites of carbonated apatite and polyglycolide as bone substitution materials

We report on the development and characterization of a new composite material consisting of amorphous carbonated apatite, Ca(5)(PO(4), CO(3))(3)(OH), and microstructured poly(hydroxyacetic acid), polyglycolide (PGA). This material is able to keep the pH of a surrounding solution within the physiological range (7.2-7.6). This was achieved by chemical fine-tuning of the counterplay between the acidic degradation of the polyester and the basic dissolution of calcium phosphate. Microporous samples with pore sizes of <1 microm and compact samples were prepared. The biological behavior was assayed in vitro by long-term osteoblast culture. Morphological and biochemical analyses of cell differentiation revealed excellent biocompatibility, leading to cell attachment, collagen and osteocalcin expression, and mineral deposition. This material could be of use as a biodegradable bone substitution material and as a scaffold for tissue engineering.

Polymer concepts in tissue engineering

Traumatic injuries, cancer treatment, and congenital abnormalities are often associated with abnormal bone shape or segmental bone loss. Restoration of normal structure and function in these cases requires replacement of the missing bone that may be accomplished by surgical transfer of natural tissue from an uninjured location elsewhere in the body. However, this procedure is limited by availability, adequate blood supply, and secondary deformities at the donor site. One strategy to overcome these problems is to develop living tissue substitutes based on synthetic biodegradable polymers. Three methods of bone regeneration using biodegradable polymers are being studied in our laboratory: tissue induction, cell transplantation, and fabrication of vascularized bone flaps. Injectable polymers are used for filling skeletal defects and guiding bone tissue growth. Their main advantage is minimizing the surgical intervention or the severity of the surgery. Polymer-cell constructs also hold great promise in the field of tissue engineering. They provide a scaffold on which cells grow and organize themselves. As the cells begin to secrete their own extracellular matrix, the polymer degrades and is eventually eliminated from the body, resulting in completely natural tissue replacement. Bone flaps can be fabricated ectopically into precise shapes and sizes. With an attached vascular supply, these flaps can be transferred into areas deficient in vascularity. This article discusses polymer concepts regarding bone tissue engineering and reviews recent advances of our laboratory on guided bone regeneration using biodegradable polymer scaffolds.

Marrow stromal osteoblast function on a poly(propylene fumarate)/beta-tricalcium phosphate biodegradable orthopaedic composite

The objective of this study was to assess the osteoconductivity of a poly(propylene fumarate)/beta-tricalcium phosphate (PPF/beta-TCP) composite in vitro. We examined whether primary rat marrow stromal cells would attach, proliferate, and express differentiated osteoblastic function when seeded on PPF/beta-TCP substrates. Attachment studies showed that a confluent monolayer of cells had adhered to the substrates within an 8 h time frame for marrow stromal cells seeded at confluent numbers. Proliferation and differentiated function of the cells were then investigated for a period of 4 weeks for an initial seeding density of 42,000 cells/cm2. Rapid proliferation during the first 24 h as determined by 3H-thymidine incorporation was mirrored by an initial rapid increase in total cell number by DNA assay. A lower proliferation rate and a gradual increase in cell number persisted for the remainder of the study, resulting in a final cell number of 128,000 cells/cm2. Differentiated cell function was assessed by measuring alkaline phosphatase (ALP) activity and osteocalcin (OC) production throughout the time course. Both markers of osteoblastic differentiation increased significantly over a 4-week period. By day 28, cells grown on PPF/beta-TCP reached a maximal ALP activity of 11 (+/- 1) x 10(-7) micromol/min/cell, while the OC production reached 40 (+/- 1) x 10(-6) ng/cell. These data show that a PPF/beta-TCP composite exhibits in vitro osteoconductivity similar to or better than that of control tissue culture polystyrene.

Bioresorbable bone graft substitutes of different osteoconductivities: a histologic evaluation of osteointegration of poly(propylene glycol-co-fumaric acid)-based cement implants in rats

Bioresorbable bone graft substitutes may significantly reduce the disadvantages associated with autografts, allografts and other synthetic materials currently used in bone graft procedures. We investigated the biocompatibility and osteointegration of a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene-glycol-co-fumaric acid), or simply poly(propylene fumarate), PPF, which is crosslinked in the presence of soluble and insoluble calcium filler salts. Four sets of animals each having three groups of 8 were evaluated by grouting bone graft substitutes of varying compositions into 3-mm holes that were made into the anteromedial tibial metaphysis of rats. Four different formulations varying as to the type of soluble salt filler employed were used: set 1--calcium acetate, set 2--calcium gluconate, set 3--calcium propionate, and set 4--control with hydroxapatite, HA, only. Animals of each of the three sets were sacrificed in groups of 8 at postoperative week 1, 3, and 7. Histologic analysis revealed that in vivo biocompatibility and osteointegration of bone graft substitutes was optimal when calcium acetate was employed as a soluble salt filler. Other formulations demonstrated implant surface erosion and disintegration which was ultimately accompanied by an inflammatory response. This study suggested that PPF-based bone graft substitutes can be designed to provide an osteoconductive pathway by which bone will grow in faster because of its capacity to develop controlled porosities in vivo. Immediate applicability of this bone graft substitute, the porosity of which can be tailored for the reconstruction of defects of varying size and quality of the recipient bed, is to defects caused by surgical debridement of infections, previous surgery, tumor removal, trauma, implant revisions and joint fusion. Clinical implications of the relation between developing porosity, resulting osteoconduction, and bone repair in vivo are discussed.

Synthetic biodegradable polymers for orthopaedic applications

Synthetic biodegradable polymers offer an alternative to the use of autografts, allografts, and nondegradable materials for bone replacement. They can be synthesized with tailored mechanical and degradative properties. They also can be processed to porous scaffolds with desired pore morphologic features conducive to tissue ingrowth. Moreover, functionalized polymers can modulate cellular function and induce tissue ingrowth. This review focuses on four classes of polymers that hold promise for orthopaedic applications: poly alpha-hydroxy esters, polyphosphazenes, polyanhydrides, and polypropylene fumarate crosslinked networks.

Bone formation by three-dimensional stromal osteoblast culture in biodegradable polymer scaffolds

Bone formation was investigated in vitro by culturing stromal osteoblasts in three-dimensional (3-D), biodegradable poly(DL-lactic-co-glycolic acid) foams. Three polymer foam pore sizes, ranging from 150-300, 300-500, and 500-710 microns, and two different cell seeding densities, 6.83 x 10(5) cells/cm2 and 22.1 x 10(5) cells/cm2, were examined over a 56-day culture period. The polymer foams supported the proliferation of seeded osteoblasts as well as their differentiated function, as demonstrated by high alkaline phosphatase activity and deposition of a mineralized matrix by the cells. Cell number, alkaline phosphatase activity, and mineral deposition increased significantly over time for all the polymer foams. Osteoblast foam constructs created by seeding 6.83 x 10(5) cells/cm2 on foams with 300-500 microns pores resulted in a cell density of 4.63 x 10(5) cells/cm2 after 1 day in culture; they had alkaline phosphatase activities of 4.28 x 10(-7) and 2.91 x 10(-6) mumol/cell/min on Days 7 and 28, respectively; and they had a cell density that increased to 18.7 x 10(5) cells/cm2 by Day 56. For the same constructs, the mineralized matrix reached a maximum penetration depth of 240 microns from the top surface of the foam and a value of 0.083 mm for mineralized tissue volume per unit of cross sectional area. Seeding density was an important parameter for the constructs, but pore size over the range tested did not affect cell proliferation or function. This study suggests the feasibility of using poly(alpha-hydroxy ester) foams as scaffolding materials for the transplantation of autogenous osteoblasts to regenerate bone tissue.

Ectopic bone formation by marrow stromal osteoblast transplantation using poly(DL-lactic-co-glycolic acid) foams implanted into the rat mesentery

Porous biodegradable poly(DL-lactic-co-glycolic acid) foams were seeded with rat marrow stromal cells and implanted into the rat mesentery to investigate in vivo bone formation at an ectopic site. Cells were seeded at a density of 6.83 x 10(5) cells/cm2 onto polymer foams having pore sizes ranging from either 150 to 300 to 710 microns and cultured for 7 days in vitro prior to implantation. The polymer/cell constructs were harvested after 1, 7, 28, or 49 days in vivo and processed for histology and gel permeation chromatography. Visual observation of hematoxylin and eosin-stained sections and von Kossa-stained sections revealed the formation of mineralized bonelike tissue in the constructs within 7 days postimplantation. Ingrowth of vascular tissue was also found adjacent to the islands of bone, supplying the necessary metabolic requirements to the newly formed tissue. Mineralization and bone tissue formation were investigated by histomorphometry. The average penetration depth of mineralized tissue in the construct ranged from 190 +/- 50 microns for foams with 500-710-microns pores to 370 +/- 160 microns for foams with 150-300-microns pores after 49 days in vivo. The mineralized bone volume per surface area and total bone volume per surface area had maximal values of 0.28 +/- 0.21 mm (500-710-microns pore size, day 28) and 0.038 +/- 0.024 mm (150-300-microns, day 28), respectively. As much as 11% of the foam volume penetrated by bone tissue was filled with mineralized tissue. No significant trends over time were observed for any of the measured values (penetration depth, bone volume/surface area, or percent mineralized bone volume). These results suggest the feasibility of bone formation by osteoblast transplantation in an orthotopic site where not only bone formation from transplanted cells but also ingrowth from adjacent bone may occur.

Inhibition of smooth muscle cell growth in vitro by an antisense oligodeoxynucleotide released from poly(DL-lactic-co-glycolic acid) microparticles

We fabricated poly(DL-lactic-co-glycolic acid) (PLGA) 50:50 microparticles loaded with an antisense (AS) oligodeoxy-nucleotide (ODN) against the rat tenascin mRNA and determined the effect in vitro of the AS-ODN released on smooth muscle cell (SMC) proliferation and migration. AS-ODN was entrapped using a double-emulsion-solvent-extraction technique with high efficiency. Release of AS-ODN was characterized by a small initial-burst effect followed by a period of controlled AS-ODN release for up to 20 days. SMC proliferation studies exhibited dose-dependent growth inhibition with AS-ODN-loaded microparticles. Microparticles loaded with scrambled (SC) ODN showed less growth inhibition than AS-ODN. Moreover, only the AS-ODN-loaded microparticles inhibited migration. These results demonstrate the feasibility of entrapping an AS-ODN to rat tenascin in PLGA microparticles for controlled delivery to inhibit SMC proliferation and migration.