A study on hydroxyapatite formation on/in the hydroxyl groups-bearing nonionic hydrogels

Use of tendon to produce decellularized sheets of mineralized collagen fibrils for bone tissue repair and regeneration

With demand for alternatives to autograft and allograft materials continuing to rise, development of new scaffolds for bone tissue repair and regeneration remains of significant interest. Engineered collagen‐calcium phosphate (CaP) constructs can offer desirable attributes, including absence of foreign body response and possession of inherent osteogenic potential. Despite their promise, current collagen‐CaP constructs are limited to nonload‐bearing applications. In this article, we describe a process for creating decellularized sheets of highly aligned, natively cross‐linked, and mineralized collagen fibrils, which may be useful for developing multilaminate collagen‐CaP constructs with improved mechanical properties. Decellularized bovine tendons were cryosectioned to produce thin sheets of aligned collagen fibrils. Mineralization of the sheets was then performed using an alternate soaking method incorporating a polymer‐induced liquid precursor (PILP) process to promote intrafibrillar mineralization, along with incorporation of physiologically relevant amounts of citrate, Mg, and carbonate. Characteristics of the produced scaffolds were assessed using energy‐dispersive X‐ray spectroscopy (EDX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Scaffolds were also compared with both native bovine cortical bone and pure hydroxyapatite using X‐ray powder diffraction (XRD), and Fourier transform infrared spectroscopy attenuated total reflection (FTIR‐ATR). Structural and chemical analyses show that the scaffold preparation process that we described is successful in creating mineralized collagen sheets, possessing a mineral phase similar to that found in bone as well as a close association between collagen fibrils and mineral plates.

Synthesis of silk fibroin-g-PAA composite using H2O2-HRP and characterization of the in situ biomimetic mineralization behavior

Silk fibroin (SF) as a bioactive protein can offer growth substrates for hydroxyapatite (HAp) deposition. In the current work, graft copolymerization of acrylic acid (AA) onto fibroin chains was carried out using hydrogen peroxide-horseradish peroxidase (H₂O₂-HRP) catalytic system, SF-g-polyacrylic acid (PAA) membranes was prepared subsequently, followed by in situ biomimetic mineralization in the Ca/P solutions, aiming at promoting the deposition of HAp and endowing the fibroin-based biocomposite with enhanced bioactivity. Meanwhile, p-hydroxyphenylacetamide (PHAD) and methyl acrylate (MA), as the model compounds of tyrosine residues in SF and vinyl monomer were used to disclose the mechanism of graft copolymerization. The data from FTIR and SEC chromatograms indicated that vinyl monomer was successfully graft copolymerized with SF during H₂O₂-HRP treatment. According to the results of XRD, SEM patterns and EDS-Mapping, mineral phases on the surfaces of SF-g-PAA membranes were detected after different cycles of biomimetic mineralization, and the mechanical property of SF-g-PAA/HAp membrane was noticeably improved. Cell viability and adhesion assays revealed that the composite of SF-g-PAA/HAp exhibited acceptable biocompatibility and outstanding adhesion property. The present work provides a novel method for preparation of the fibroin-based biomaterial for bone tissue engineering.

In Vitro Calcification Model (2): Apatite Formation on Segmented Polyurethane Thin Films by Using an Alternate Soaking Process: The Effect of Adsorbed Serum Proteins on Calcification

In the present study, the effect of serum proteins on the calcification (apatite deposition) of commercially available polyurethane (PU) films, Pellethane® 2363-80AE, Pellathane® 2363-55DE, and K-III, was evaluated by using an alternating soaking process. Bovine serum albumin, γ-globulin and fibrinogen were used as typical serum proteins. After the proteins were adhered to PU films, the films were used in the apatite formation process. The thin-film X-ray diffraction spectra of a fibrinogen adhered K-III sample, after 200 calcification sequences, showed HAp formation. The calcium content on the surface of all films to which protein adhered was greater than that of films incubated in protein free solutions. The films to which fibrinogen adhered showed remarkable apatite formation, though the amount of adhered fibrinogen was much smaller than the amounts of the other proteins. K-III films to which fibrinogen had adhered showed small globular apatite aggregations, whereas the Pellethane® 2363 series films showed confetti-shaped apatite. Different morphologies exhibited by the deposits depended on the types of serum protein and on the substrate chemistry. The reason for the different apatite morphology on different PU films is not clear, however, the kind of serum protein seems to play an important role in the calcification of a PU.

Effect of layered double hydroxide intercalated with fluoride ions on the physical, biological and release properties of a dental composite resin

OBJECTIVES

The aim of this work was the preparation of a new fluoride-releasing dental material characterized by a release of fluoride relatively constant over time without any initial toxic burst effect. This type of delivery is obtained by a matrix controlled elution and elicits the beneficial effect of a low amount of fluoride on human dental pulp stem cells (hDPSCs) towards mature phenotype.

METHODS

The modified hydrotalcite intercalated with fluoride ions (LDH-F), used as filler, was prepared via ion exchange procedure and characterized by X-ray diffraction and FT-IR spectroscopy. The LDH-F inorganic particles (0.7, 5, 10, 20wt.%) were mixed with a photo-activated Bis-GMA/TEGDMA (45/55wt/wt) matrix and novel visible-light cured composites were prepared. The dynamic thermo-mechanical properties were determined by dynamic mechanical analyzer. The release of fluoride ions in physiological solution was determined using a ionometer. Total DNA content was measured by a PicoGreen dsDNA quantification kit to assess the proliferation rate of hDPSCs. Alkaline phosphatase activity (ALP) was measured in presence of fluoride resins.

RESULTS

Incorporation of even small mass fractions (e.g. 0.7 and 5wt.%) of the fluoride LDH in Bis-GMA/TEGDMA dental resin significantly improved the mechanical properties of the pristine resin, in particular at 37°C. The observed reinforcement increases on increasing the filler concentration. The release of fluoride ions resulted very slow, lasting months. ALP activity gradually increased for 28 days in hDPSCs cell grown, demonstrating that low concentrations of fluoride contributed to the cell differentiation.

CONCLUSIONS

The prepared composites containing different amount of hydrotalcite filler showed improved mechanical properties, slow fluoride release and promoted hDPSCs cell proliferation and cell differentiation.

Advances in biomimetic and nanostructured biohybrid materials

The rapid increase of interest in the field of biohybrid and biomimetic materials that exhibit improved structural and functional properties is attracting more and more researchers from life science, materials science, and nanoscience. Concomitant results offer valuable opportunities for applications that involve disciplines dealing with engineering, biotechnology, medicine and pharmacy, agriculture, nanotechnology, and others. In the current contribution we collect recent illustrative examples of assemblies between materials of biological origin and inorganic solids of different characteristics (texture, structure, and particle size). We introduce here a general overview on strategies for the preparation and conformation of biohybrids, the synergistic effects that determine the final properties of these materials, and their diverse applications, which cover areas as different as tissue engineering, drug delivery systems, biosensing devices, biocatalysis, green nanocomposites, etc.

An optimized β-tricalcium phosphate and agarose scaffold fabrication technique

Biodegradable scaffolds composed of beta-tricalcium phosphate, and a natural hydrogel, agarose, were prepared by a shaping method based on the thermal gelation of the polymeric component. This technique was modified to facilitate the inclusion, during the scaffold preparation stage, of therapeutic agents that could improve the graft performance. Vancomycin was included in materials containing different amounts of agarose and ceramic without affecting the scaffold consolidation process. These materials, easily injectable, behave like a reinforced hydrogel whose swelling behavior and drug release rate depend on their composition.

Mineralization of radiation-crosslinked polyvinyl alcohol/polyvinyl pyrrolidone hydrogels

A study of the calcification of the polyvinyl alcohol/polyvinyl pyrrolidone (PVA/PVP) hydrogels during their exposure to a calcium chloride solution or a simulated body fluid has been carried out. On the basis of the experiments, using a two-compartment permeation cell, the diffusion of calcium ions and their subsequent deposition in the hydrogels were elucidated. Steady-batch experiments were also performed to further elaborate the deposition pattern and the types of calcium deposits. It was demonstrated that Fick's second law of diffusion can describe the diffusion of calcium ions through PVA/PVP hydrogels at 310 K. The diffusion coefficient was determined to be (4.4+/-0.1)x10(-10) m2/s and the partition coefficient for the hydrogels was 0.06. Formation of calcium deposits was noticed taking place both on the surface and inside the hydrogels. The deposits formed on the surface have flake morphology, while the deposits inside the hydrogels are more like globular aggregates. Both types of deposits have been characterized as being comprised calcium and hydroxyl ion deficient apatites with chloride ions the most likely substituting species at the hydroxyl sites.

Room temperature synthesis of agarose/sol-gel glass pieces with tailored interconnected porosity

An original shaping technique has been applied to prepare porous bodies at room temperature. Agarose, a biodegradable polysaccharide, was added as binder of a sol-gel glass in powder form, yielding an easy to mold paste. Interconnected tailored porous bodies can be straightforwardly prepared by pouring the slurry into a polymeric scaffold, previously designed by stereolitography, which is subsequently eliminated by alkaline dissolution at room temperature. The so obtained pieces behave like a hydrogel with an enhanced consistency that makes them machinable and easy to manipulate. These materials generate an apatite-like layer when immersed in a simulated body fluid, indicating a potential in vivo bioactivity. The proposed method can be applied to different powdered materials to produce pieces, at room temperature, with various shapes and sizes and with tailored interconnected porosity.

Bone‐bonding ability of a hydroxyapatite coated zirconia–alumina nanocomposite with a microporous surface

Using a combination of hydroxyapatite (HA) coating and microporous surface treatment, bone-bonding ability was given to composites of ceria-stabilized tetragonal zirconia and alumina (CZA), which possesses excellent mechanical and wear properties and phase stability. Four types of CZA plates (2 x 10 x 15 mm3) were prepared for this study, which were CZA with a polished surface (group 1), a microporous surface prepared by hydrofluoric acid and heat treatment (group 2), a microporous surface with a submicron HA coating prepared by alternately soaking the plate from group 2 in aqueous CaCl2/HCl and Na2HPO4 solutions (group 3), and a microporous surface with a 4-microm HA coating prepared by the biomimetic method, where the plates from group 3 were soaked in simulated body fluid (group 4). Plates were implanted into rabbit tibia, and after 4, 8, and 16 weeks, tensile testing and histological examination of the bone-implant interface were conducted. At 4 weeks, group 4 had superior bone-bonding ability compared with other implants, which was maintained at the later postimplantation times. This HA-coated CZA with a microporous surface has the possibility of clinical use as a bearing material in cementless joint prostheses or as a load-bearing bone substitute.

Hydroxyapatite deposition by alternating soaking technique on poly(vinyl alcohol)-coated polyethylene films

A poly(vinyl alcohol) (PVA)-coating on polyethylene films, prepared by repetitive adsorption/drying in an aqueous PVA solution, accelerated hydroxyapatite (HAp) deposition by an altemate soaking in aqueous solutions containing Ca2+ and PO4(3-) ions. X-ray photoelectron spectra of the surface of the HAp-deposited film showed the presence of calcium and phosphorus of a suitable peak ratio for HAp formation. X-ray diffraction analyses also revealed peaks corresponding to HAp. Scanning electron microscopic observation showed the surface of the HAp layer to be smooth, with nano-ordered dotted threads in networks. A simple PVA coating on a surface will serve as a novel system for accelerated HAp formation via alternating soaking.

Effects of negatively charged groups (carboxymethyl) on the calcification of poly(2-hydroxyethyl methacrylate)

Poly(2-hydroxyethyl methacrylate) (pHEMA) has potentially wide biomedical applications: it is biocompatible, allows immobilization of cells or bioactive molecules and has a hardness comparable to bone. We previously reported that immobilization of alkaline phosphatase (AlkP) in pHEMA can initiate mineralization in a manner that mimics the calcification of cartilage and woven bone. Because numerous proteins known to initiate mineralization possess acidic species, we have modified the neutral electrical surface of pHEMA by carboxymethylation (CM). We have studied the effects of these negative groups on the calcification process in vitro. Calibrated pellets of pHEMA were prepared and carboxymethylated by soaking with 0.5 M bromoacetic acid in 2 M NaOH. Pellets of pHEMA, pHEMA-AlkP and pHEMA-CM were incubated during 5, 10 and 15 days in two types of body fluid: normal (1X) and 1.5X concentration of ions. Nodules of hydroxyapatite developed on pHEMA-AlkP and pHEMA-CM but not on pHEMA. Hydroxyapatite crystals were dissolved in HCl allowing calcium to be dosed. CM significantly increased the amount of deposited Ca by 1.8 folds in the 1X fluid and 15.8 folds in the 1.5X fluid. The presence of AlkP considerably increased the amount of deposited Ca: 25.9 folds in 1X and 23.3 in 1.5X. ROS 17/2.8 osteoblast-like cells were seeded on the materials and examined by confocal microscopy after phalloidin staining. Cells grown on pHEMA alone appeared round, while cells grown on the crystals deposited on the pHEMA-CM or pHEMA-AlkP were flattened. The presence of AlkP favours the mineralization process more than the existence of surface negative groups on the polymer. Cells preferentially adhere to the polymer when hydroxyapatite crystals were developed.

A trial to prepare biodegradable collagen-hydroxyapatite composites for bone repair

This paper is a trial to prepare collagen-hydroxyapatite composites in vitro by an alternate immersion method. Collagen sponges of different biodegradabilities were prepared through chemical cross-linking of Type I collagen with glutaraldehyde (GA) at concentrations of 0.2, 1.0, and 2.0 wt%. The sponges were immersed at 37 degrees C in Tris-HCl-buffered solution containing 200 mM CaCl2 (pH 7.4) for 2 h and then in an aqueous solution of 120 mM Na2HPO4 (pH 9.3) for a 2 h further (one immersion cycle). The alternate immersion cycle was repeated for different times to obtain collagen-hydroxyapatite composites. The characterization of the resulting composites was performed by Fourier transform infrared spectroscopy (FT-IR). X-ray diffraction (XRD), and scanning electron microscopy (SEM). The weight of composites increased with an increase in immersion cycles and the rate of increase became greater with higher GA cross-linking levels for collagen sponge preparation. The pH of the phosphate solution decreased with the immersion cycle, which suggests H+ generation accompanied hydroxyapatite formation. Irrespective of the GA concentration and immersion cycle, every composite showed IR absorption bands attributable to phosphate and hydroxyl groups at 950-1100 or 550-650 and 3000-3500 cm(-1) and broad peaks specific to hydroxyapatite on the XRD charts. SEM study revealed small white clusters of hydroxyapatite interspersed uniformly on/in the collagen framework without any preferential orientation. The composite prepared from 0.2 wt% GA cross-linked collagen sponge which showed favourable characteristics was applied to a rat skull defect to evaluate its osteoconductivity as well as biodegradability. The formation of new bone tissue was histologically observed at the defect 12 weeks after application in marked contrast to the collagen sponge alone. The composite degraded without any inflammation reaction. It is concluded that the collagen-hydroxyapatite composite prepared by the present method is a biodegradable biomaterial of osteoconductivity applicable to bone repair.

Improvement of swelling properties of poly(2-hydroxyethyl methacrylate) hydrogel by means of biomimetic method

Poly(2-hydroxyethyl methacrylates) (PHEMAs) structurally modified by means of polymer blends and random copolymers are intensively studied in order to improve mechanical properties. It was recently shown that a hydroxyapatite coating, which should improve the bonding of this biomaterial to the bone, can be obtained by means of the biomimetic method. When PHEMA is submitted to the biomimetic method, its swelling ratio is improved. This can be ascribed to the deposition of a silicatic layer, which improves the hydrophilicity, on the surface of the internal pores during the first stage of the method. This appears to be a valuable result for producing modified PHEMAs with improved mechanical properties and good swelling. The experimental results indicate the following: stronger interactions with the water molecules are set up, an induction period is observed that is linked to the rate of the reactions occurring at the surface of the glass and the establishment of a convenient concentration of the silicate ions at the external surface of the polymer, and the diffusion of simulated body fluid into the pores is the limiting stage of the process.

Poly(2-hydroxy ethyl methacrylate)-alkaline phosphatase: a composite biomaterial allowing in vitro studies of bisphosphonates on the mineralization process

We have immobilized the mineralizing agent alkaline phosphatase (AlkP) in a hydrophilic polymer: poly(2-hydroxy ethyl methacrylate) - (pHEMA) - in a copolymerization technique. Histochemical study on polymer sections revealed that AlkP has retained its enzymic activity. The image analysis of sections using a tessellation method showed a lognormal distribution of the area of the tiles surrounding AlkP particles, thus confirming a homogeneous distribution of the enzyme in the polymer. Pellets of pHEMA-AlkP were incubated with a synthetic body fluid containing organic phosphates (beta-glycerophosphate). Mineral deposits with a rounded shape (calcospherites) were obtained in about 17 days. We have investigated the effects of three bisphosphonic pharmacological compounds (etidronate, alendronate and tiludronate) on this system which mimics the mineralization process of cartilage and woven bone. Bisphosphonates at a concentration of 10(-2) M totally inhibited AlkP in solution at a concentration of 10(-4) mg/ml. Inhibition has been reported being due to the chelation of a metal cofactor (Zn2+). Etidronate and alendronate appeared to similarly inhibit the calcospherite deposition onto the pHEMA-AlkP material. Both bisphosphonates possess three sites for the mineral complexion by Ca chemisorbtion. On the other hand, tiludronate having only two sites, was associated with a reduced inhibitory effect on mineralization but larger crystals were obtained. The pHEMA-AlkP material contains an immobilized enzyme in a hydrogel and mimics the physiological conditions of matrix vesicles entrapped within the cartilage (or bone) matrix. It provides an interesting method to study the effects of pharmacological compounds on the mineralization process in bone and cartilage in a non cellular and protein-free model.

Preparation and characterization of apatite deposited on silk fabric using an alternate soaking process

Apatite-deposited silk fabric composite materials were developed using a new alternate soaking process. The characteristics of deposited apatite were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrophotometry (FTIR), and X-ray photoelectron spectroscopy (XPS). Apatite weight increased with alternating soaking in a calcium solution [200 mM aqueous calcium chloride solution buffered with tris(hydroxymethyl) aminomethane and HCl (pH 7.4)] and a phosphate solution (120 mM aqueous disodium hydrogenphosphate) changed every hour. SEM showed that apatite deposited after 21 or more repeated soakings was over 20 microm thick. XRD showed that with alternate soakings, the apatite crystals deposited on silk fabric elongated along the c axis. FTIR and XPS indicated the existence of carbonate, HPO(4)(2-), and Na(+) ions in addition to constituent ions of hydroxyapatite. A loss of HPO(4)(2-) and Na(+) ions in the deposit upon further soaking might be associated with an increasing apatite crystallinity. Apatite deposited on silk by the alternate soaking process was a deficient apatite containing carbonate, HPO(4)(2-), and Na(+) ions as in a natural bone tissue. Thus, this apatite-silk composite material might be potentially bioactive.

Growth of continuous bonelike mineral within porous poly(lactide-co-glycolide) scaffolds in vitro

Strategies to engineer bone have focused on the use of natural or synthetic degradable materials as scaffolds for cell transplantation or as substrates to guide bone regeneration. The basic requirements of the scaffold material are biocompatibility, degradability, mechanical integrity, and osteoconductivity. A major design problem is satisfying each of these requirements with a single scaffold material. This study addresses this problem by describing an approach to combine the biocompatibility and degradability of a polymer scaffold with the osteoconductivity and mechanical reinforcement of a bonelike mineral film. We report the nucleation and growth of a continuous carbonated apatite mineral on the interior pore surfaces of a porous, degradable polymer scaffold via a one step, room temperature incubation process. A 3-dimensional, porous scaffold of the copolymer 85:15 poly(lactide-co-glycolide) was fabricated by a solvent casting, particulate leaching process. Fourier transform IR spectroscopy and scanning electron microscopy (SEM) analysis after different incubation times in a simulated body fluid (SBF) demonstrate the growth of a continuous bonelike apatite layer within the pores of the polymer scaffold. Quantification of phosphate on the scaffold displays the growth and development of the mineral film over time with an incorporation of 0.43 mg of phosphate (equivalent to 0.76 mg of hydroxyapatite) per scaffold after 14 days in SBF. The compressive moduli of polymer scaffolds increased fivefold with formation of a mineral film after a 16-day incubation time as compared to control scaffolds. In summary, this biomimetic treatment provides a simple, one step, room temperature method for surface functionalization and subsequent mineral nucleation and growth on biodegradable polymer scaffolds for tissue engineering.

Growth of continuous bonelike mineral within porous poly(lactide-co-glycolide) scaffolds in vitro

Strategies to engineer bone have focused on the use of natural or synthetic degradable materials as scaffolds for cell transplantation or as substrates to guide bone regeneration. The basic requirements of the scaffold material are biocompatibility, degradability, mechanical integrity, and osteoconductivity. A major design problem is satisfying each of these requirements with a single scaffold material. This study addresses this problem by describing an approach to combine the biocompatibility and degradability of a polymer scaffold with the osteoconductivity and mechanical reinforcement of a bonelike mineral film. We report the nucleation and growth of a continuous carbonated apatite mineral on the interior pore surfaces of a porous, degradable polymer scaffold via a one step, room temperature incubation process. A 3-dimensional, porous scaffold of the copolymer 85:15 poly(lactide-co-glycolide) was fabricated by a solvent casting, particulate leaching process. Fourier transform IR spectroscopy and scanning electron microscopy (SEM) analysis after different incubation times in a simulated body fluid (SBF) demonstrate the growth of a continuous bonelike apatite layer within the pores of the polymer scaffold. Quantification of phosphate on the scaffold displays the growth and development of the mineral film over time with an incorporation of 0.43 mg of phosphate (equivalent to 0.76 mg of hydroxyapatite) per scaffold after 14 days in SBF. The compressive moduli of polymer scaffolds increased fivefold with formation of a mineral film after a 16-day incubation time as compared to control scaffolds. In summary, this biomimetic treatment provides a simple, one step, room temperature method for surface functionalization and subsequent mineral nucleation and growth on biodegradable polymer scaffolds for tissue engineering.

In Vitro Calcification Model—Part 1: Apatite Formation on Segmented Polyurethane Containing Silicone Using an Alternate Soaking Process

The effect of silicone on calcification on polyetherpolyurethane (PU) surfaces was studied using an alternate soaking process in vitro. The process is based on a wet process of hydroxyapatite formation that involves alternately soaking in CaCl2/Tris-HCl (pH 7.4) and Na2HPO4 solutions (pH 7.4) at 37°C. We used K-III and Pellethane® 2363 as samples. K-III is a complex of PU containing dimethyldiacetoxysilane and methyltriacetoxysilane. Pellethane® 2363 series contain less Si. Si content was assessed by X-ray photoelectron spectroscopy (XPS). The gravimetric measurements and scanning electron microscopic observations with energy dispersed X-ray analyzer (EDX) were performed after specific reaction cycles. Calcified deposits formed on the surface of K-III were 20 μg/cm2 after 10 reaction cycles. EDX results showed remarkable Ca (Kα, Kb), P (Kα), and Si (Kα) peaks by the deposits formed on the surface of K-III. On the other hand, no peaks for Ca of P were observed on the surface of Pellethane® after ten reaction cycles. The reason for calcified deposit formation on PU is not clear. However, it was suggested that Si on the surface of PU is one of the key factors for calcified deposit formation. These results suggest that an alternate soaking process could be a useful evaluation method for an accelerated evaluation of fatigue induced by calcification on polymer substrates.

Apatite formation on/in hydrogel matrices using an alternate soaking process (III): effect of physico-chemical factors on apatite formation on/in poly(vinyl alcohol) hydrogel matrices

The aim of this study is to clarify the physico-chemical factors which influence apatite formation on/in a hydrogel during a novel alternate soaking process. A poly(vinyl alcohol) (PVA) gel was used as a model matrix. The amount of apatite formed on/in PVA gels decreased with an increase in the reaction temperature during the same reaction cycles. This suggested that the equilibrium swelling ratios decreased with increasing reaction temperatures; that is, the diffusion of calcium and phosphate ions reduced at high reaction temperature. However, the crystallinity of apatite formed on/in PVA gels was greater at higher reaction temperatures. The amount of apatite formed on/in PVA gels increased with an increase in the calcium and phosphate solution concentrations, and increased by shaking at the first three reaction cycles. A few influences could be observed when the solution volume was changed, however, the soaking order was not effective in this study. These results indicate that the amount of apatite formation on/in PVA gels can be controlled by changing the reaction temperature and the Ca- and P-solution concentrations, and that the crystallinity of apatite can be also changed by controlling the reaction temperatures.