In vitro evaluation of osteoblast responses to carbon nanotube-coated titanium surfaces

BACKGROUND

The effects of surface roughness and carboxyl functionalization of multi-walled carbon nanotubes (MWCNTs) mixed with collagen coated onto titanium (Ti) substrates on MC3T3-E1 osteoblasts were evaluated.

METHODS

The proliferation, differentiation, and matrix mineralization were investigated using (1) smooth-surfaced Ti discs, (2) Ti discs coated with collagen and MWCNT (Ti-MWCNT), and (3) Ti discs coated with collagen and MWCNT-COOH (Ti-MWCNT-COOH) for applications in orthodontic mini screw implants (MSIs). The coatings were uniform when analyzed using scanning electron microscopy (SEM), and surface roughness was evaluated by surface profilometry that demonstrated similar surface roughness (R a , mean ± SD) in the MWCNT (0.83 ± 0.02 μm) and MWCNT-COOH (0.84 ± 0.01 μm) groups. MTT (3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide) assay was performed after days 1, 3, and 7 to assess proliferation. Alkaline phosphatase (ALP)-specific activity was assessed after day 7 to quantify differentiation. Alizarin red staining was measured after day 28 to quantify matrix mineralization. All data were analyzed with JMP Pro11 software (SAS, USA) with a statistical significance of p  < 0.05.

RESULTS

Surface profilometry demonstrated similar surface roughness (R a , mean ± SD) in the MWCNT (0.83 ± 0.02 μm) and MWCNT-COOH (0.84 ± 0.01 μm) groups. On day 7, ALP assay showed that MWCNT-COOH (mean ± SD 0.98 ± 0.26 U/μg of protein) enhanced cell differentiation when compared to the uncoated group (p = 0.05). Alizarin red staining after 28 days of cell culture revealed that MWCNT-COOH (mean ± SD 1.5 ± 0.2 OD405) increased (p = 0.03) matrix mineralization when compared to the uncoated group (0.9 ± 0.09 OD405).

CONCLUSIONS

This study showed that coatings containing MWCNT-COOH (increased hydrophilic surface chemistry) influence osteoblast proliferation, differentiation, and matrix mineralization and should be further studied for applications in orthodontic MSIs.

Advances in Controlled Drug Delivery for Treatment of Osteoporosis

Osteoporosis, which is characterized by resorption of bone exceeding formation, remains a significant human health concern, and the impact of this condition will only increase with the "graying" of the worldwide population. This review focuses on current and emerging approaches for delivering therapeutic agents to restore bone remodeling homeostasis. Well-known antiresorptive and anabolic agents, such as estrogen, estrogen analogs, bisphosphonates, calcitonin, and parathyroid hormone, along with newer modulators and antibodies, are primarily administered orally, intravenously, or subcutaneously. Although these treatments can be effective, continuing problems include patient noncompliance and adverse systemic or remote-site effects. Controlled drug delivery via polymeric, targeted, and active release systems extends drug half-life by shielding against premature degradation and improves bioavailability while also providing prolonged, sustained, or intermittent release at therapeutic doses to more effectively treat osteoporosis and associated fracture risk.

Porous PLGA scaffolds for controlled release of naked and polyethyleneimine-complexed DNA

The ability to precisely control delivery of single or multiple bioactive molecules is critical in tissue engineering, and controlled release of plasmid coding for growth factors and their regulators can give cell-regulated, short-term expression of these therapeutic biomolecules. In this work, porous poly(lactic-co-glycolic acid) (PLGA) scaffolds comprising acid-terminated chains of either low (LMW; 10 kDa) or high molecular weight (HMW; 30 kDa) were developed for controlled release of naked or polyethyleneimine (PEI)-complexed DNA. The compressive strength of blank HMW and LMW scaffolds was 6 and 2 MPa, respectively, while the strength of PEI:DNA-containing HMW and LMW scaffolds was 7 and 1 MPa, respectively. LMW scaffolds degraded more quickly than HMW scaffolds, with 80-100% and 15-30% mass loss at 30 days, respectively. Encapsulation of plasmid, particularly PEI-complexed DNA, only modestly affected degradation. Release profiles showed bi- or triphasic patterns, with early burst release of surface-associated DNA, slower diffusion-mediated release, and degradation-related release at later time points. Complexation with PEI tended to a slow release of plasmids, likely because of interaction with the carboxyl groups of PLGA. Culturing rat bone marrow cells on blank PLGA scaffolds in the presence of IGF-I resulted in growth and chondrogenic differentiation of these cells. Porous scaffolds made of PLGA with the appropriate selection of hydrophobicity and molecular weight will allow controlled delivery of naked and condensed plasmid DNA for different tissue engineering applications.

Bioactivity of WLBU2 peptide antibiotic in combination with bioerodible polymer

WLBU2 is a peptide antibiotic designed for broad antimicrobial activity, including bacteria associated with periodontal disease. Although periodontitis is associated with various systemic conditions, ranging from cardiovascular disease to preterm birth, local therapy is needed to treat the source of infection. Biodegradable polymers are often used to control locally the amount and rate of delivery of drugs. In the present study, a bioerodible association polymer comprising cellulose acetate phthalate (CAP) and Pluronic F-127 (PF-127) was explored for its interaction with WLBU2. The intrinsic antimicrobial activity of CAP/PF-127 and the combined effects of the polymer and WLBU2 were examined using Streptococcus gordonii, a species involved in early colonisation of tooth surfaces. The polymer blend alone had dose-dependent bacteriostatic properties, resulting in a ≥ 2 log decrease in colonies at the highest concentrations tested, possibly due to the hydrophobicity of CAP disrupting the surface of bacteria. When WLBU2 was combined with CAP/PF-127, an apparent binding of peptide to polymer significantly decreased the activity compared with free WLBU2, which functions like other cationic peptides by destabilising the bacterial membrane. Formulation with sucrose as an excipient, which reduced the interaction between WLBU2 and polymer, restored the bactericidal activity of the peptide antibiotic as reflected by a > 3 log decrease in S. gordonii. WLBU2 can be locally delivered using CAP/PF-127 as a release vehicle, with the peptide's bactericidal activity dominating the polymer's bacteriostatic effect.

Infection, inflammation, and bone regeneration: a paradoxical relationship

Various strategies have been developed to promote bone regeneration in the craniofacial region. Most of these interventions utilize implantable materials or devices. Infections resulting from colonization of these implants may result in local tissue destruction in a manner analogous to periodontitis. This destruction is mediated via the expression of various inflammatory mediators and tissue-destructive enzymes. Given the well-documented association among microbial biofilms, inflammatory mediators, and tissue destruction, it seems reasonable to assume that inflammation may interfere with bone healing and regeneration. Paradoxically, recent evidence also suggests that the presence of certain pro-inflammatory mediators is actually required for bone healing. Bone injury (e.g., subsequent to a fracture or surgical intervention) is followed by a choreographed cascade of events, some of which are dependent upon the presence of pro-inflammatory mediators. If inflammation resolves promptly, then proper bone healing may occur. However, if inflammation persists (which might occur in the presence of an infected implant or graft material), then the continued inflammatory response may result in suboptimal bone formation. Thus, the effect of a given mediator is dependent upon the temporal context in which it is expressed. Better understanding of this temporal sequence may be used to optimize regenerative outcomes.

Formulations for intermittent release of parathyroid hormone (1-34) and local enhancement of osteoblast activities

The objective of these studies was to develop simple, implantable devices that intermittently release PTH(1-34) and thus could be used for locally stimulating bone formation. The formulations were based on the association polymer system of cellulose acetate phthalate and Pluronic F-127. Release profiles for intermittent devices showed five discrete peaks, whereas sustained devices exhibited zero-order kinetics. Osteoblastic activity was greater for cells intermittently treated with PTH(1-34) compared to sustained exposure. These controlled release devices delivering PTH(1-34) in an intermittent manner may be useful for affecting osteoblast activities in a localized area.

Immobilization of glycoproteins, such as VEGF, on biodegradable substrates

Attachment of growth factors to biodegradable polymers, such as poly(lactide-co-glycolide) (PLGA), may enhance and/or accelerate integration of tissue engineering scaffolds. Although proteins are commonly bound via abundant amino groups, a more selective approach may increase bioactivity of immobilized molecules. In this research, exposed carboxyl groups on acid-terminated PLGA were modified with dihydrazide spacer molecules. The number of hydrazide groups available for subsequent attachment of protein was dependent on dihydrazide length, with shorter molecules present at significantly greater surface densities. The potent angiogenic glycoprotein vascular endothelial growth factor (VEGF) was oxidized with periodate and the aldehyde moieties allowed to react with the hydrazide-derivatized PLGA. Derivatization initially affected the amount of protein bound to the surfaces, but differences were substantially reduced following overnight incubation in saline. More importantly, use of shorter dihydrazide spacers significantly enhanced accessibility of immobilized VEGF for binding neutralizing antibody and soluble VEGF receptor. Furthermore, immobilized growth factor enhanced endothelial cell proliferation, with surfaces having the shortest and longest spacers stimulating greater effects. The present work has not only demonstrated an alternative approach to immobilizing growth factors on biodegradable materials, but the scheme can be used to alter the amount of protein bound as well as its availability for subsequent biointeractions.

Protein Binding to Peptide-Imprinted Porous Silica Scaffolds

Of the many types of biomolecules used for molecular imprinting applications, proteins are some of the most useful, yet challenging, templates to work with. One method, termed the 'epitope approach', involves imprinting a short peptide fragment of the protein into the polymer to promote specific adsorption of the entire protein, similar to the way an antigen binds to an antibody via the epitope. Whole lysozyme or the 16 residue lysozyme C peptide was imprinted into porous silica scaffolds using sol-gel processing. After removing template, scaffolds were exposed to lysozyme and/or RNase A, which was used as a competitor molecule of comparable size. When comparing protein- to peptide-imprinted scaffolds, similar amounts of lysozyme and RNase were bound from single protein solutions. However, while whole lysozyme-imprinted scaffolds showed about 4:1 preferential binding of lysozyme to RNase, peptide-imprinted scaffolds failed to show statistical significance, even though a slight preferential binding trend was present. These initial studies suggest there is potential for using peptide-imprinting to create specific protein-binding sites on porous inorganic surfaces, although further development of the materials is needed.

Mechanical and degradation behavior of polymer-calcium sulfate composites

Calcium sulfate (CS) is one of the oldest bone graft materials still in use. Its main limitations are poor handling characteristics, poor mechanical properties, and a resorption rate that is too fast for some applications. The present study investigated the effect of viscous polymers, such as carboxymethylcellulose (CMC) and hyaluronan (HY), on the handling characteristics, mechanical properties, and degradation behavior of CS. CMC and HY were added to CS at concentrations from 1-10 wt%. Addition of CMC to CS at more than 4 wt% produced a putty-like material and decreased the density of the composite, while also increasing flexural and compressive strength at higher loadings. Incorporation of CMC produced a concentration-dependent increase in water absorption and degradation rate. At an equivalent loading, HY-containing CS composites showed better compressive strength than CS with CMC. Overall, addition of CMC or HY to CS resulted in composite materials with better handling characteristics and improved mechanical properties after set, however the degradation rate of the augmented materials was increased. These properties suggest that the enhanced CS materials may be useful in certain clinical situations, such as filling non-uniform bone defects and situations that require mechanical integrity of the bone graft substitute during implantation.

Modulated release of bioactive protein from multilayered blended PLGA coatings

The objective of this study was to develop a poly(D,L-lactic-co-glycolic acid) (PLGA)-based coating system for producing biologically-inspired delivery profiles. Protein-loaded microspheres were made from PLGA (50:50) terminated with carboxylic acid groups (PLGA-2A) blended either with more hydrophobic PLGA (50:50) having lauryl ester endcaps (PLGA-LE) or with the more hydrophilic Pluronic F-127 (PF-127). Dense coatings were formed by pressure-sintering the microspheres. Altering hydrophobicity changed the water concentration within coatings, and consequently the time to onset of polymer degradation and protein release was modulated. After blending up to 8% Pluronic, degradation by-products began accumulating immediately upon incubation in saline, whereas, degradation was delayed for up to 14 days with blending of up to 30% PLGA-LE. Primary protein release peaks from one-layer coatings could be created from 7 to 20 days using 8% PF-127 or 30% PLGA-LE blends, respectively. Multilayered coatings of different blends generated several release peaks, with their temporal occurrence remaining approximately the same when layers of other hydrophobicity were added above or below. To allow design of coatings for future use, results were used to construct a model based on Fourier analysis. This polymer blend system and model can be used to mimic temporally varying profiles of protein expression.

In vitro effects of combined and sequential delivery of two bone growth factors

Bone formation and repair occur by a complex cascade involving numerous growth factors and cytokines. In this study, two-layered heterogeneously loaded and crosslinked gelatin coatings were used to obtain combined and sequential delivery of two bone growth factors, BMP-2 and IGF-I, in cell cultures. Peak release from the top and bottom layers was localized around 1 and 6 days, respectively. For comparison, cells were also treated with soluble growth factors directly added to the culture medium. Pluripotent C3H10T1/2 (C3H) cells responded to soluble growth factor treatments with the greatest specific alkaline phosphatase (AP) activity resulting from addition of BMP-2 followed by IGF-I or by BMP-2+IGF-I. Altered loading and subsequent release of BMP-2 and IGF-I from gelatin coatings also affected AP activity in C3H cultures, and the coatings influenced AP activity and incorporation of calcium in the extracellular matrix of bone marrow stromal cell cultures. Early delivery of BMP-2 followed by increased release of BMP-2 and IGF-I after 5 days resulted in the largest, as well as earliest, elevation of AP activity and mineralized matrix formation compared to controls and other treatments. Simultaneous release of both growth factors from both layers did not significantly change AP activity or matrix calcium content compared to control coatings. These results demonstrate that temporally varying delivery of multiple growth factors can significantly affect cell behavior.

Cell responses to BMP-2 and IGF-I released with different time-dependent profiles

During wound healing, growth factors are expressed in time-dependent amounts. Constant delivery of biomolecules, however, is often used to influence cell and tissue behavior. In the present studies, a crosslinked gelatin-coating system was used to deliver bone morphogenetic protein 2 (BMP-2) or insulin-like growth factor (IGF-I) to three types of mesenchymal cells with three temporally varying release profiles. The "early" delivery profile released most of the growth factor within the first 2 days. The "pseudo-zero-order" profile approximated constant rate of delivery for about 5 days. The "late" delivery profile released most of the growth factor after about 5 days. Early delivery of IGF-I had the greatest effect on mitogenesis of SaOS-2 human osteosarcoma cells with a secondary effect noted nearly 5 days after delivery was completed. Late delivery of BMP-2 resulted in greatest alkaline phosphatase (AP) activity in mouse pluripotent C3H10T1/2 cells. Rat bone marrow stromal cells (BMCs) responded to all delivery profiles of BMP-2, with the duration of elevated AP activity increasing as the amount of BMP-2 delivered increased. In addition to an early increase in AP activity, late release also stimulated BMCs over a longer portion of the culture period. BMCs responded similarly to SaOS-2 cells when seeded on early IGF-I delivery coatings, increasing AP activity after delivery had ended. Overall, these studies further show the importance of delivery profile, specifically the characteristics of time and concentration, on cell and tissue responses.

Triphasic release model for multilayered gelatin coatings that can recreate growth factor profiles during wound healing

Multilayered gelatin coatings were created to mimic growth factor profiles that normally occur during fracture healing. A model was developed to relate crosslinking and loading of individual layers to protein release. Modeling was simplified by dividing release profiles into three phases. The diffusion-controlled phase was determined by calculating periods of constant diffusivity for each homogeneous layer within devices. Diffusivity was a power law function of crosslinking. Fick's second law of diffusion was then used to determine release during the diffusion-controlled phase. Secondary diffusivity was determined by summing resistances of each successive homogeneous layer. The initial burst phase was defined as events proceeding the diffusion-controlled phase. Percentage of drug burst was a linear function of crosslinking. Release during the degradation-controlled phase, events following diffusion-controlled phase, was estimated based on first order hydrolysis of crosslinks. The model predicted time-variant release of differently labeled protein measured experimentally, and it can be used to design coatings to recreate the cascade of biomolecules that determine natural bone repair.

A technique to immobilize bioactive proteins, including bone morphogenetic protein-4 (BMP-4), on titanium alloy

Immobilization of biomolecules on surfaces enables both localization and retention of molecules at the cell-biomaterial interface. Since metallic biomaterials used for orthopedic and dental implants possess a paucity of reactive functional groups, biomolecular modification of these materials is challenging. In the present work, we investigated the use of a plasma surface modification strategy to enable immobilization of bioactive molecules on a "bioinert" metal. Conditions during plasma polymerization of allyl amine on Ti-6Al-4V were varied to yield 5 ("low")- and 12 ("high")-NH2/nm2. One- and two-step carbodiimide schemes were used to immobilize lysozyme, a model biomolecule, and bone morphogenetic protein-4 (BMP-4) on the aminated surfaces. Both schemes could be varied to control the amount of protein bound, but the one-step method destroyed the activity of immobilized lysozyme because of crosslinking. BMP-4 was then immobilized using the two-step scheme. Although BMP bound to both low- and high-NH2 surfaces was initially able to induce alkaline phosphatase activity in pluripotent C3H10T1/2 cells, only high amino group surfaces were effective following removal of weakly bound protein by incubation in cell culture medium.

Biomaterials and biomechanics of oral and maxillofacial implants: current status and future developments

Research in biomaterials and biomechanics has fueled a large part of the significant revolution associated with osseointegrated implants. Additional key areas that may become even more important--such as guided tissue regeneration, growth factors, and tissue engineering--could not be included in this review because of space limitations. All of this work will no doubt continue unabated; indeed, it is probably even accelerating as more clinical applications are found for implant technology and related therapies. An excellent overall summary of oral biology and dental implants recently appeared in a dedicated issue of Advances in Dental Research. Many advances have been made in the understanding of events at the interface between bone and implants and in developing methods for controlling these events. However, several important questions still remain. What is the relationship between tissue structure, matrix composition, and biomechanical properties of the interface? Do surface modifications alter the interfacial tissue structure and composition and the rate at which it forms? If surface modifications change the initial interface structure and composition, are these changes retained? Do surface modifications enhance biomechanical properties of the interface? As current understanding of the bone-implant interface progresses, so will development of proactive implants that can help promote desired outcomes. However, in the midst of the excitement born out of this activity, it is necessary to remember that the needs of the patient must remain paramount. It is also worth noting another as-yet unsatisfied need. With all of the new developments, continuing education of clinicians in the expert use of all of these research advances is needed. For example, in the area of biomechanical treatment planning, there are still no well-accepted biomaterials/biomechanics "building codes" that can be passed on to clinicians. Also, there are no readily available treatment-planning tools that clinicians can use to explore "what-if" scenarios and other design calculations of the sort done in modern engineering. No doubt such approaches could be developed based on materials already in the literature, but unfortunately much of what is done now by clinicians remains empirical. A worthwhile task for the future is to find ways to more effectively deliver products of research into the hands of clinicians.

Understanding and controlling the bone-implant interface

A goal of current implantology research is to design devices that induce controlled, guided, and rapid healing. In addition to acceleration of normal wound healing phenomena, endosseous implants should result in formation of a characteristic interfacial layer and bone matrix with adequate biomechanical properties. To achieve these goals, however, a better understanding of events at the interface and of the effects biomaterials have on bone and bone cells is needed. Such knowledge is essential for developing strategies to optimally control osseointegration. This paper reviews current knowledge of the bone-biomaterial interface and methods being investigated for controlling it. Morphological studies have revealed the heterogeneity of the bone-implant interface. One feature often reported, regardless of implant material, is an afibrillar interfacial zone, comparable to cement lines and laminae limitantes at natural bone interfaces. These electron-dense interfacial layers are rich in noncollagenous proteins, such as osteopontin and bone sialoprotein. Several approaches, involving alteration of surface physicochemical, morphological, and/or biochemical properties, are being investigated in an effort to obtain a desirable bone-implant interface. Of particular interest are biochemical methods of surface modification, which immobilize molecules on biomaterials for the purpose of inducing specific cell and tissue responses or, in other words, to control the tissue-implant interface with biomolecules delivered directly to the interface. Although still in its infancy, early studies indicate the value of this methodology for controlling cell and matrix events at the bone-implant interface.

Release and retention of biomolecules in collagen deposited on orthopedic biomaterials

Delivery of osteotropic biomolecules directly to the bone-implant interface can alter initial interactions between tissue and biomaterial. To this end, type I collagen coatings containing a model biomolecule, lysozyme, were deposited on Co-Cr-Mo and Ti-6Al-4V. Two deposition methods were examined. In the first, lysozyme was deposited concurrently with collagen, while in the second, protein was impregnated into previously deposited collagen coatings. The amount of collagen and the amount of lysozyme loaded into collagen were varied to provide different amounts of weakly and strongly bound protein. Release and retention of lysozyme were monitored over a 7 d period of incubation in physiological saline. For both methods, larger amounts of collagen in the coatings allowed incorporation of more lysozyme. Additionally, loading collagen coatings with greater amounts of lysozyme resulted in release of more protein. During the first 24-96 h of incubation, loosely bound protein was eluted, resulting in release of 2 micrograms to 55 mg (5-75% of the amount available) of enzymatically active lysozyme. This left 25-95% of the protein bound to the collagen-coated biomaterials and, thus, available for later release during degradation of the collagen.

Biochemical surface modification of Ti-6Al-4V for the delivery of protein to the cell-biomaterial interface

Biochemical surface modification involves delivery of biomolecules to the tissue-implant interface to induce desired cell and tissue responses. We have previously had success in immobilizing and retaining bioactive molecules on Co-Cr-Mo but not on Ti-6Al-4V. The purpose of this study was to modify the gamma-aminopropyltriethoxysilane (APS) scheme to enable successful attachment of protein to the surface of Ti-6Al-4V. Ti-6Al-4V samples were silanized with organic (acetone) solutions of APS and dried at increasing temperatures. Concentrations resulting in 2-4 NH2 per nm2 of nominal surface area were incubated in physiological saline for up to 96 hr to assess retention of amino groups. A model protein, trypsin, was coupled to silanized Ti-6Al-4V via glutaraldehyde. The samples were then incubated in saline, and the activity of residual immobilized enzyme was quantified. After drying at 45, 80, or 115 degrees C, the NH2 groups were lost from the surface by 24 hr of incubation in saline. On samples dried at 150 degrees C, with 4% APS, the number of NH2 groups increased after 8 hr and remained relatively constant through 96 hr. Likewise, at 150 degrees C with 2% APS the surface density of NH2 groups reached a maximum at 24 hr and remained relatively constant up to 96 hr. When incubated for 96 hr, Ti-6Al-4V with 4% APS and dried at 150 degrees C retained approximately 31% of the activity initially immobilized, whereas protein on 45 degrees C or adsorbed samples was lost by 24-48 hr.

In vitro cellular responses to bioerodible particles loaded with recombinant human bone morphogenetic protein-2

Porous 50:50 poly(d,l lactide-co-glycolide) microspheres containing varying amounts of "free" recombinant human bone morphogenetic protein-2 (rhBMP-2) were evaluated for their ability to induce/enhance expression of osteoblastic characteristics by pluripotent mesenchymal cells in vitro. "Free" protein (Fp) is defined as protein present on the surface and within the porous matrix of the microspheres. Four preparations of bioerodible particles (BEP) were used: blank--without rhBMP-2; low Fp--24 microg of free rhBMP-2 per g of particles; medium Fp--403 microg/g; and high Fp--884 microg/g. C3H10T1/2 cells (C3H) and bone marrow stromal cells (BMC) were cultured with 1 mg of BEP for up to 4 weeks, and cell growth and expression of osteogenic responses were determined weekly. For both cell types, control cultures (neither BEP nor rhBMP-2) and cultures with blank BEP exhibited no or minimal osteoblastic characteristics. Compared to control and blank BEP cultures, C3H cells responded to particles having medium and high amounts of free rhBMP-2 with increased cell growth and alkaline phosphatase activity, but osteocalcin secretion and mineralization were not markedly influenced. Low Fp BEP enhanced only the alkaline phosphatase activity of C3H cells. In contrast, although growth was not affected, rhBMP-2-loaded BEP increased alkaline phosphatase activity, osteocalcin secretion, and mineralization in BMC cultures in a dose-dependent manner (i.e., blank < low < medium < high Fp).

Retention of enzymatic activity immobilized on silanized Co-Cr-Mo and Ti-6Al-4V

Biochemical surface modification of biomaterials utilizes immobilized biomolecules to induce preferred tissue responses. Operational stability, or retention of biological activity, of biochemically modified biomaterials is a fundamental determinant of their usefulness. The present study investigated retention of enzymatic activity immobilized on silanized Co-Cr-Mo and Ti-6Al-4V. A model enzyme, trypsin, was immobilized on monolayers and multilayers of silane deposited from aqueous or organic solutions of gamma-aminopropyltriethoxysilane (APS). Trypsin-conjugated biomaterials were incubated in cell culture medium at 37 degrees C for up to 96 h, and the residual immobilized activity was measured. Retention of bioactivity in physiological saline was dependent on the type of material and on the method of silanization. Activity of enzyme adsorbed on the metals was lost within 24-48 h. Both mono- and multilayers of APS deposited on Co-Cr-Mo by aqueous silanization effectively retained enzymatic activity throughout the 96 h incubation period. The monolayer retained approximately 23% of the activity initially present, and the multilayers retained approximately 50% of the initial activity. Organic silanization of Co-Cr-Mo was marginally effective as it initially slowed the loss of activity. However, all activity was lost by 48-72 h of incubation. Neither organic nor aqueous silanization enhanced retention of enzymatic activity on Ti-6Al-4V.

Dependence of mesenchymal cell responses on duration of exposure to bone morphogenetic protein-2 in vitro

Bone morphogenetic proteins (BMPs) induce osteoblastic responses in cultures of pluripotent mesenchymal cells. The effects of chronic treatment of these cells with BMPs and of withdrawal following exposure, however, have not been fully elucidated. Thus, the aim of this study was to obtain information about the duration of exposure to recombinant human BMP-2 (rhBMP-2) required for expression and retention of osteoblastic characteristics with subsequent formation of a mineralized extracellular matrix in mesenchymal cell cultures. C3H1OT1/2 cells and bone marrow stromal cells were cultured with 1 mug/ml rhBMP-2 for either 0, 7, 14, 21, or 28 days, with the remainder of the 4 week total culture period in the absence of rhBMP-2. Growth and expression of osteoblastic characteristics were examined at the end of each week. C3H1OT1/2 cells responded to increasing duration of exposure to rhBMP-2 with increased cell growth. Additionally, the longer the cells were exposed to rhBMP-2, the more fully they expressed and sustained osteoblastic traits, i.e., they exhibited duration of exposure-dependent higher levels of alkaline phosphatase and osteocalcin and larger total amounts of mineral in the matrix. In comparison, exposure of bone marrow stromal cells to rhBMP-2 for at least 14 days restrained cell growth and prevented detachment. With respect to osteoblastic traits, stromal cells exposed to rhBMP-2 also exhibited a dependence on the duration of exposure, however, cultures treated for 14, 21, or 28 days exhibited similar levels of alkaline phosphatase activity and comparable amounts of calcium in the mineralizing matrix.

Effect of metal ions on the formation and function of osteoclastic cells in vitro

To determine if metal ions play a contributing role in loosening of orthopedic implants, the present work investigated whether sublethal concentrations of ions affect the formation and function of osteoclasts in vitro. Rat bone marrow cells were cultured on slices of devitalized bone and in the presence of ions associated with Co-Cr-Mo and Ti-6A1-4V alloys for up to four weeks. Cultures were assayed for total intracellular protein, used as measure of cell growth, and resorption activity of osteoclastic cells derived from hematopoietic stem cells was quantified using image analysis. Although Co2+ caused delayed toxicity not previously observed during short-term experiments, none of the other ions affected cell proliferation, indicating that the chosen concentrations were sublethal. In general, exposure of bone marrow cultures to ions caused either a decrease or no change in the total area of bone resorption. A decrease in the number of resorption pits formed by osteoclastic cells was primarily responsible for the decrease in total amount of resorption. Therefore, even though cells continued to grow over the entire culture period, less osteoclastic activity was observed. Findings indicate that if metal ions play a role in periprosthetic pathology, they may contribute to implant failure by impairing bone repair while allowing fibrous tissue formation following debris-induced osteolysis.

Stability of trypsin immobilized on inorganic orthopedic biomaterials

Biochemical surface modification of biomaterials utilizes immobilized biomolecules to induce preferred tissue responses. Although several techniques are available for immobilizing biomolecules on organic substrates, comparatively few are available for use with inorganic materials, such as those used in many orthopedic applications. The present study investigated the stability/elutability of a model enzyme immobilized on Co-Cr-Mo and Ti-6Al-4V alloys using p-nitrophenyl chloroformate (p-NPC). Trypsin-conjugated biomaterials were incubated in cell culture medium at 37 degrees C for up to 96 hr, and the residual immobilized activity was measured. Although all samples initially bound enzymatically active trypsin, significant decreases were observed within the first 2 hr of incubation. Immobilization of trypsin on Co-Cr-Mo treated with 0.65 mg p-NPC/cm2 of nominal surface area gave significantly higher residual activity than on untreated samples at 24-96 hr of incubation and prevented the nearly complete loss of enzymatic activity that was observed with free (not immobilized) enzyme. Derivatization of Ti-6Al-4V with p-NPC was not beneficial to the level of immobilized enzymatic activity after incubation in medium for longer than 6 hr.

Ti-6Al-4V ion solution inhibition of osteogenic cell phenotype as a function of differentiation timecourse in vitro

Metal ions released from the implant surface are suspected of playing some contributing role in loosening of hip and knee prostheses. previous work in this laboratory demonstrated that sublethal doses of the ionic constituents of Ti-6Al-4V alloy suppressed expression of the osteoblastic phenotype and deposition of a mineralized matrix. The purpose of this work was to further explore this suppression as a function of the normal time-course of phenotype expression. Bone marrow stromal cells were harvested from juvenile rats and exposed to time-staggered doses of a solution of ions representing Ti-6Al-4V alloy. Cells were cultured for four weeks and assayed for total protein, alkaline phosphatase, intra-and extracellular osteocalcin, and calcium. Ti-6Al-4V solutions were found to produce little difference from control solutions for total protein or alkaline phosphatase levels, but strongly inhibited osteocalcin synthesis. Calcium levels were reduced when ions were added before a critical point of osteoblastic differentiation (between 2 and 3 weeks after seeding). These results indicate that ions associated with Ti-6Al-4V alloy inhibited the normal differentiation of bone marrow stromal cells to mature osteoblasts in vitro, suggesting that ions released from implants in vivo may contribute to implant failure by impairing normal bone deposition.

Use of p-nitrophenyl chloroformate chemistry to immobilize protein on orthopedic biomaterials

Biochemical surface modification involves covalently immobilizing biomolecules onto biomaterial surfaces to induce specific biological responses. This approach may be useful for enhancing the fixation of orthopedic implants. p-Nitrophenyl chloroformate (p-NPC) was used to immobilize protein on bulk samples of Co-Cr-Mo and Ti-6Al-4V. Activation of both materials was dependent on the concentration of p-NPC, with a maximum of approximately 1.5 active groups/nm2 of nominal surface area. Trypsin was used as a model protein because much is known about its structure and mode of action. Derivatization with 0.65 mg p-NPC/cm2 resulted in significantly greater enzymatic activity (7.4 BAEE [N-(alpha)-benzoyl-L-arginine ethyl ester hydrochloride] units) on the Co-Cr-Mo samples compared with higher concentrations of p-NPC (5 BAEE units) and with simple adsorption of trypsin (1.5 BAEE units). An activity of 10.5 BAEE units was measured on both adsorbed and p-NPC-activated Ti-6Al-4V, with the exception of samples derivatized with 1.95 mg p-NPC/cm2, on which activity was significantly lower (4 BAEE units). In probing the linkages between trypsin and biomaterial by treatment with chaotropic agents, guanidine hydrochloride (GuHCl) was observed to eliminate more enzymatic activity than was urea. On Co-Cr-Mo samples, GuHCl removed nearly all the trypsin activity, while urea significantly decreased the activity only at a concentration of 0.65 mg p-NPC/cm2. Treatment of Ti-6Al-4V samples with GuHCl caused a trend of decreasing activity with increasing concentration of p-NPC, whereas urea had no effect on immobilized trypsin activity.

Biochemical surface modification of Co-Cr-Mo

Because of the limited mechanical properties of tissue substitutes formed by culturing cells on polymeric scaffolds, other approaches to tissue engineering must be explored for applications that require complete and immediate ability to bear weight, e.g. total joint replacements. Biochemical surface modification offers a way to partially regulate events at the bone-implant interface to obtain preferred tissue responses. Tresyl chloride, gamma-aminopropyltriethoxysilane (APS) and p-nitrophenyl chloroformate (p-NPC) immobilization schemes were used to couple a model enzyme, trypsin, on bulk samples of Co-Cr-Mo. For comparison, samples were simply adsorbed with protein. The three derivatization schemes resulted in different patterns and levels of activity. Tresyl chloride was not effective in immobilizing active enzyme on Co-Cr-Mo. Aqueous silanization with 12.5% APS resulted in optimal immobilized activity. Activity on samples derivatized with 0.65 mg p-NPC cm-2 was four to five times greater than that on samples simple adsorbed with enzyme or optimally derivatized with APS and was about eight times that on tresylated samples. This work demonstrates that, although different methods have different effectiveness, chemical derivatization can be used to alter the amount and/or stability of biomolecules immobilized on the surface of Co-Cr-Mo.

Activity of enzyme immobilized on silanized Co-Cr-Mo

The surface of an orthopedic biomaterial was modified by the covalent immobilization of biomolecules. Derivatization of Co-Cr-Mo samples with organic and aqueous solutions of gamma-aminopropyltriethoxysilane (APS) resulted in a concentration-dependent number of reactive NH2 groups on the surface available for coupling to protein. The enzyme trypsin was used as a model biomolecule to investigate the effect of immobilization on proteolytic activity. Trypsin was coupled to the silanized samples by formation of Schiff's base linkages via glutaraldehyde. The nature of the interaction between trypsin and biomaterial was then probed by treatment with concentrated guanidine hydrochloride (GuHCl) and urea. Residual activity (following treatment with chaotropic agents) of trypsin immobilized on silanized Co-Cr-Mo was dependent both on the nature of the silane solution and on the type of chaotropic agent. Organic silanization with APS required a minimum density of approximately 49 NH2 per nm2 of nominal surface area (> 0.021 M APS) for residual activity of immobilized trypsin. For aqueous silanization, approximately 5.4 NH2/nm2 (0.51 M APS) resulted in maximal residual trypsin activity. Treatment with GuHCl removed more trypsin activity from Co-Cr-Mo samples silanized with organic solutions of APS than did treatment with urea. On the contrary, with aqueous silanization the samples possessed greater residual activity following treatment with GuHCl than following urea. Compared to simple adsorption with protein onto Co-Cr-Mo, both methods of silanization with APS resulted in superior residual immobilized enzyme activity.

Effects of sublethal metal ion concentrations on osteogenic cells derived from bone marrow stromal cells

Ions released from implant surfaces are suspected of playing some role in osteolysis surrounding metal prostheses. To understand how ions may affect osteogenesis, previous work exposed osteogenic cells to metal ions to study acute cytotoxic responses. The purpose of this study was to assess the long-term effects of sublethal ion concentrations on osteogenic cell proliferation and function. Bone marrow stromal cells were harvested from juvenile rats and exposed to solutions of ions associated with Co-Cr-Mo and Ti-6Al-4V implants. Cells were cultured for up to 4 weeks and assayed for total protein, alkaline phosphatase, osteocalcin, and calcium. Other than V+5, none of the ions affected cell proliferation, indicating that the chosen concentrations were sublethal as desired. V+5 elicited delayed gross toxicity not previously observed during acute experiments. At the chosen concentrations, Co+2, Cr+6, Mo+6, and Co-Cr-Mo alloy elicited little effect on cell proliferation and moderate effects on matrix mineralization. Cultures exposed to Ti+4, Al+3, and Ti-6Al-4V alloy also showed no decrease in cell number, but did show near total suppression of osteocalcin secretion and matrix mineralization. These results suggest that ions released from Ti alloy implants may interfere with osteoblastic cell differentiation, contributing to periprosthetic osteolysis by impairing normal osteogenesis.

Acute toxicity of metal ions in cultures of osteogenic cells derived from bone marrow stromal cells

The effects of metal ions released from orthopedic implants on nearby bone cells remain largely unknown. The purpose of this study was to examine the acute toxicity of metal ions on osteogenic cells derived from bone marrow. Bone marrow stromal cells were cultured with metal ions found in commonly used orthopedic implants, that is, Ti-6Al-4V, Co-Cr-Mo, and 316L stainless steel. Solutions of individual ions and combinations representing the alloy composition were prepared from atomic absorption standards and added to the cultures to give concentrations ranging from 50 ppb to 50 ppm. After a 48-h period of exposure to ions, the bone marrow cultures were examined for effects of cytotoxicity by measuring total cell number, total cell protein, and mitochondrial activity. Cr6+ was grossly cytotoxic; Co2+, Mo6+, Fe3+, and Ni2+ were moderately cytotoxic; and Ti4+, Al3+, V5+, and Mn2+ were minimally toxic, as determined by the assays used. Ion solutions representing Co-Cr-Mo and 316L stainless steel were moderately toxic; solutions representing Ti-6Al-4V were toxic at only the highest concentrations used. The observed cytotoxicity was time-dependent, with irreversible toxic effects being initiated following as short as a 3- to 6-hour exposure. These results show that metal ions associated with Co-Cr-Mo and 316L stainless steel are toxic to osteogenic cells at concentrations approximating those measured in the fibrous membrane encapsulating orthopedic implants.

Osteoblast attachment monitored with a quartz crystal microbalance

A quartz crystal microbalance is used in aqueous solutions to monitor the rate of attachment of osteoblasts, bone-forming cells, to the surface of the crystal. Changes in resonant frequency of the crystal are measured for various surface coverages by osteoblasts. Crystal surface coverages are determined by digital image processing of scanning electron micrographs. A linear relationship is established between the surface coverages and the changes in resonant frequency of the crystal. The osteoblasts are observed to behave viscoelastically. Hence, the Sauerbrey equation can not be used to describe the relationship between the change in mass of osteoblasts on the surface and the change in resonant frequency of the crystal. Apparent viscosities at 5.0 MHz are also determined for osteoblasts.

Examination of osteoblast-orthopaedic biomaterial interactions using molecular techniques

Molecular techniques can be used to elucidate the effects of extended periods of cell-biomaterial interactions on the time-course and level of expression of particular genes which determine cellular phenotype. We used the polymerase chain reaction to demonstrate the expression of genes for the bone-related proteins osteocalcin, osteonectin and osteopontin by neonatal rat calvarial osteoblasts. In addition, Northern blotting was subsequently used to show that messenger RNAs encoding osteonectin and osteopontin were consistently expressed during a 5 wk period of interaction of osteoblasts with Ti-6Al-4V, a commercial brand of hydroxyapatite, and tissue culture polystyrene.

Mechanisms of fibronectin-mediated attachment of osteoblasts to substrates in vitro

Adhesive proteins of plasma and the extracellular matrix, such as fibronectin, adsorbed onto surfaces mediate cell/substrate adhesion. In a series of experiments, the roles of the type III connecting segment (IIICS) adhesion sites (specifically, CS1 and CS5 peptides) of fibronectin, heparan sulfate proteoglycan, endogenous proteins, and passive attachment in fibronectin-mediated osteoblast attachment were examined in vitro. The CS1 and CS5 peptides of the IIICS of fibronectin had no effect on osteoblast attachment. Blocking the heparin-binding domains of fibronectin inhibited osteoblast attachment by 40-45%, which is complementary to inhibition results previously obtained with the RGDS tetrapeptide. Endogenously synthesized and secreted proteins played a role in maintaining and repairing the osteoblast surface. Osteoblast attachment to fibronectin, but not to the nonadhesive protein albumin, occurred via active mechanisms in that the process was dependent on free sulfhydryl groups, divalent cations and temperature.

Formation of focal contacts by osteoblasts cultured on orthopedic biomaterials

The nature of the contact sites formed during the adhesion of osteoblasts to orthopedic implant materials was investigated by fluorescence microscopy. More specifically, the cytoskeletal organization of and the focal contact formation by neonatal rat calvarial osteoblasts attaching to and spreading on 316L stainless steel, Ti-6Al-4V, Co-Cr-Mo, Synamel (hydroxyapatite), alumina, and borosilicate glass were examined. Focal contacts are regions where the plasma membrane approaches the substrate to within 10-15 nm and where bundles of cytoskeletal microfilaments terminate. Fluorescent-labeling of F-actin-containing microfilaments demonstrated a typical sequence of events as rounded, suspended osteoblasts spread onto the substrates. Immunofluorescent-labeling of the protein vinculin, which is found at the cytoplasmic face of focal contacts, initially showed the formation of streak-like focal patches. On the biomaterials, the vinculin staining subsequently extended up and along, but ventral to, the microfilament bundles. The fibrillar patterns observed at later times may evidence the formation of extracellular matrix contacts.

Osteoblasts on hydroxyapatite, alumina and bone surfaces in vitro: morphology during the first 2 h of attachment

The morphological responses of individual osteoblasts as they attached and spread on hydroxyapatite, bovine bone, alumina with rough and polished surfaces, and tissue culture polystyrene in vitro were examined with scanning electron microscopy. Depending on the surface tested two different morphological sequences were observed during 2 h of adhesion. On alumina, both rough and smooth, bone, and tissue culture polystyrene the cells were round after 0.5 h, and spread radially during the next 1.5 h until they were almost flat, with a nuclear bulge. On hydroxyapatite, however, the cells were flat and circular at 0.5 h, and the edge of the cytoplasm was hardly discernable. This morphology did not change much during the subsequent 1.5 h. The observed cellular morphological response may be related to the bioreactivity of hydroxyapatite.

Osteoblast responses to orthopedic implant materials in vitro

Responses of neonatal rat calvarial osteoblasts to a variety of orthopedic implant materials were examined in vitro. Attachment, proliferation, and collagen synthesis of a well-characterized line of osteoblasts with 316L stainless steel, Ti-6Al-4V, Co-Cr-Mo, PMMA, hydroxyapatite, borosilicate glass, and tissue culture polystyrene were studied. Cell adhesion and growth were similar on nonapatitic materials. In contrast, attachment and growth of osteoblasts were significantly lower and slower, respectively, on hydroxyapatite. Collagen synthesis per cell and relative collagen synthesis, however, were comparable on all the materials tested.

RGDS tetrapeptide binds to osteoblasts and inhibits fibronectin-mediated adhesion

The mechanisms of osteoblast attachment to surfaces were probed using the adhesive tetrapeptide RGDS (Arg-Gly-Asp-Ser) and the related but non-adhesive RGES (Arg-Gly-Glu-Ser). Specifically, RGDS and RGES were investigated for their ability both to bind to a suspension of well-characterized neonatal rat calvarial osteoblasts and to inhibit cell attachment to fibronectin-coated microtiter plates. RGDS bound to the cells with an average Kd approximately 9.4 x 10(-4) M, and RGES bound with an average Kd approximately 3.0 x 10(-4) M; at saturation, the osteoblasts bound almost twice as much RGDS as RGES. RGDS partially inhibited cell adhesion (55% to 60%) in a competitive, dose-dependent manner. In contrast, RGES had minimal effect on cell attachment. Since complete inhibition of attachment was not observed, it is likely that a synergistic adhesion site in the fibronectin molecule and/or cell surface molecules such as proteoglycans are active in mediating osteoblast/substrate adhesion.