Biomedical subjects

J R de Wijn

Publications and source records attributed to J R de Wijn.

At least 19 recordsLinked to original sources

PubMed2006

Dynamic mechanical properties of 3D fiber-deposited PEOT/PBT scaffolds: an experimental and numerical analysis.

Mechanical properties of three-dimensional (3D) scaffolds can be appropriately modulated through novel fabrication techniques like 3D fiber deposition (3DF), by varying scaffold's pore size and shape. Dynamic stiffness, in particular, can be considered as an important property to optimize the scaffold structure for its ultimate in vivo application to regenerate a natural tissue. Experimental data from dynamic mechanical analysis (DMA) reveal a dependence of the dynamic stiffness of the scaffold on the intrinsic mechanical and physicochemical properties of the material used, and on the overall porosity and architecture of the construct. The aim of this study was to assess the relationship between the aforementioned parameters, through a mathematical model, which was derived from the experimental mechanical data. As an example of how mechanical properties can be tailored to match the natural tissue to be replaced, articular bovine cartilage and porcine knee meniscus cartilage dynamic stiffness were measured and related to the modeled 3DF scaffolds dynamic stiffness. The dynamic stiffness of 3DF scaffolds from poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) copolymers was measured with DMA. With increasing porosity, the dynamic stiffness was found to decrease in an exponential manner. The influence of the scaffold architecture (or pore shape) and of the molecular network properties of the copolymers was expressed as a scaffold characteristic coefficient alpha, which modulates the porosity effect. This model was validated through an FEA numerical simulation performed on the structures that were experimentally tested. The relative deviation between the experimental and the finite element model was less than 15% for all of the constructs with a dynamic stiffness higher than 1 MPa. Therefore, we conclude that the mathematical model introduced can be used to predict the dynamic stiffness of a porous PEOT/PBT scaffold, and to choose the biomechanically optimal structure for tissue engineering applications.

Animals↗

PubMed2005

Three-dimensional fiber-deposited PEOT/PBT copolymer scaffolds for tissue engineering: influence of porosity, molecular network mesh size, and swelling in aqueous media on dynamic mechanical properties.

Among novel scaffold fabrication techniques, 3D fiber deposition (3DF) has recently emerged as a means to fabricate well-defined and custom-made scaffolds for tissue regeneration, with 100% interconnected pores. The mechanical behavior of these constructs is dependent not only on different three-dimensional architectural and geometric features, but also on the intrinsic chemical properties of the material used. These affect the mechanics of the solid material and eventually of 3D porous constructs derived from them. For instance, poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) block copolymers are known to have mechanical properties, depending on the PEOT/PBT weight ratio in block form and on the molecular weight of the initial poly(ethylene glycol) (PEG) blocks. These differences are enhanced even more by their different swelling properties in aqueous media. Therefore, this article examines the influence of copolymer compositions in terms of their swelling on dynamic mechanical properties of solid material and porous 3DF scaffolds. The molecular weight of the starting PEG blocks used in the copolymer synthesis varied from 300 to 1000 g/mol. The PEOT/PBT weight ratio in the blocks used varied from 55/45 to 80/20. This corresponded to an increase of the swelling ratio Q from 1.06 to 2.46, and of the mesh size xi from approximately 9 Angstrom to approximately 47 Angstrom. With increased swelling, dynamic mechanical analysis (DMA) revealed a decrease in elastic response and an increase of viscoelasticity. Thus, by coupling structural and chemical characteristics, the viscoelastic properties of PEOT/PBT 3DF scaffolds may be fine tuned to achieve mechanical requirements for a variety of engineered tissues. Ultimately, the combination of 3DF and DMA may be useful to validate the hypothesis that mimicking the biomechanical behavior of a specific tissue for its optimal replacement is an important issue for at least some tissue-engineering applications.

Animals↗

PubMed2005

3D fiber-deposited scaffolds for tissue engineering: influence of pores geometry and architecture on dynamic mechanical properties.

One of the main issues in tissue engineering is the fabrication of scaffolds that closely mimic the biomechanical properties of the tissues to be regenerated. Conventional fabrication techniques are not sufficiently suitable to control scaffold structure to modulate mechanical properties. Within novel scaffold fabrication processes 3D fiber deposition (3DF) showed great potential for tissue engineering applications because of the precision in making reproducible 3D scaffolds, characterized by 100% interconnected pores with different shapes and sizes. Evidently, these features also affect mechanical properties. Therefore, in this study we considered the influence of different structures on dynamic mechanical properties of 3DF scaffolds. Pores were varied in size and shape, by changing fibre diameter, spacing and orientation, and layer thickness. With increasing porosity, dynamic mechanical analysis (DMA) revealed a decrease in elastic properties such as dynamic stiffness and equilibrium modulus, and an increase of the viscous parameters like damping factor and creep unrecovered strain. Furthermore, the Poisson's ratio was measured, and the shear modulus computed from it. Scaffolds showed an adaptable degree of compressibility between sponges and incompressible materials. As comparison, bovine cartilage was tested and its properties fell in the fabricated scaffolds range. This investigation showed that viscoelastic properties of 3DF scaffolds could be modulated to accomplish mechanical requirements for tailored tissue engineered applications.

Biocompatible Materials↗

PubMed2005

Porous Ti6Al4V scaffolds directly fabricated by 3D fibre deposition technique: effect of nozzle diameter.

3D porous Ti6Al4V scaffolds were successfully directly fabricated by a rapid prototyping technology: 3D fibre deposition. In this study, the rheological properties of Ti6Al4V slurry was studied and the flow rate was analyzed at various pressures and nozzle diameters. Scaffolds with different fibre diameter and porosity were fabricated. ESEM observation and mechanical tests were performed on the obtained porous Ti6Al4V scaffolds with regard to the porous structure and mechanical properties. The results show that these scaffolds have 3D interconnected porous structure and a compressive strength which depends on porosity at constant fibre diameters and on the fibre diameter at constant porosity. These Ti6Al4V scaffolds are expected to be constructs for biomedical applications.

Alloys↗

PubMed2000

Use of bone-bonding hydrogel copolymers in bone: an in vitro and in vivo study of expanding PEO-PBT copolymers in goat femora.

Polyactive(R) [polyethylene oxide-polybuthylene terephtalate (PEO-PBT)] refers to a group of copolymers with bone-bonding properties. In reference to these properties, PEO-PBT copolymers are currently being investigated for their possible use in orthopedic surgery and dentistry. PEO-PBT copolymers exhibit hydrogel behavior. When swelling in fluid is prohibited by mechanical confinement, the copolymers exert a swelling pressure on surrounding structures. In the first part of this study, these swelling pressures were measured in vitro. Polymers with different ratios of PEO-PBT exerted a swelling pressure of more than 2 MPa when tested in fluid between the cross-heads of a Hounsfield test-bench. In the second part of the study, the biocompatibility of PEO-PBT 55-45 and the effect of continuous intramedullary pressure of these copolymers on bone was investigated. Large cylinders of dry PEO-PBT 55-45 were implanted with a tight fit in the distal part of goat femora. Preswollen cylinders of PEO-PBT implanted in the opposite femur served as a control. Although it was assumed that the pressure of dry PEO-PBT on the bone would reach more than 2 MPa with press-fit insertion, no immediate hazardous effects of the expanding polymer were noticed within the first days postoperatively. The goats were sacrificed after 3, 9, and 25 weeks. Histological examination showed good implant-bone contact at different follow-up times in the distal femora with the dry implanted implants. The femora in which the preswollen cylinders had been implanted showed a thin layer of soft tissue between the PEO-PBT implant and bone. The swelling pressure exerted by dry press-fit implanted PEO-PBT implants is an important factor in creating a strong interface bond between PEO-PBT and bone.

Animals↗

PubMed1999

Incorporation of bovine serum albumin in calcium phosphate coating on titanium.

Calcium phosphate (Ca-P) and bovine serum albumin (BSA) were coprecipitated as a coating on commercially pure titanium (cpTi) with a high protein loading (15 wt %) by employing a recently developed wet-chemistry technique. It was observed that the incorporation of BSA significantly modified the morphology, composition, and crystallinity of the Ca-P coating. The Ca-P coating without BSA is a mixture of hydroxyapatite (HA) and octacalcium phosphate (OCP) with sharp-edged thin OCP crystal plates on the top layer, whereas only an HA phase was detected in the Ca-P/BSA coating. The crystal plates in the latter had a more rounded appearance. The Ca-P/BSA coatings were immersed respectively in neutral (pH 7.4) and acidic (starting pH 4.0) phosphate-buffered saline (PBS) at 37 degrees C over a 14-day period. No protein release was detected in the neutral PBS during the immersion; however, a continuous release of BSA was measured in the acidic PBS, subsequently leading to the formation of a very dense and well-adherent composite coating of BSA and Ca-P on cpTi. The present study provides the possibility to achieve a long-term effective release of biologically active proteins from a Ca-P-coated metallic implant.

Calcium Phosphates↗

PubMed1998

Collagen/apatite coating on 3-dimensional carbon/carbon composite.

A three-dimensional carbon/carbon composite (3D C/C) was studied as potential bone-repairing material; its major mechanical properties were found to be closer to those of human bone than other common bone-repairing materials available. In vitro calcification tests revealed that as-received 3D C/C is almost bioinert in simulated body fluid (SBF) over an immersion period of 4 weeks. To improve the bioactivity of 3D C/C, surface modification was accomplished through two practical routes: (1) grafting with polyethylene glycol (PEG) and (2) phosphorylation and precalcification. After grafting with alpha, omega di(aminopropyl) polyethylene glycol 800 (NH2-PEG-NH2), a continuous layer of calcium phosphate was formed on the surface of 3D C/C in SBF after 4 weeks. Phosphorylated 3D C/C samples have the ability to induce apatite precipitation after precalcification in a saturated Ca(OH)2 solution for 1 week. To speed up the coating process, a calcification solution with collagen was developed in which a collagen/apatite coating layer can be formed on 3D C/C in 9 h in ambient conditions.

Apatites↗

PubMed1998

A study on the grafting reaction of isocyanates with hydroxyapatite particles.

The surface grafting reactions of a series of isocyanates with hydroxyapatite particles at different temperatures were studied by Infrared spectrophotometry (IR) and thermal gravimetric analysis (TGA). The study results show that both hexamethylene diisocyanate (HMDI) and isocyanatoethyl methacrylate (ICEM) react readily with HA while ethyl isocyanate acetate (EIA) and butyl isocyanate (BIC) have lower reactivity towards HA particles. It also has been found that the reaction of ICEM with HA follows a second-order reaction mechanism, despite the heterogeneous nature of the reaction, while the reaction of HMDI with HA does not due to the complexity of the reaction. Based on this study, it is concluded that ICEM and HMDI are suitable agents for the coupling of polymers due to their reactivity towards HA.

Bone Substitutes↗

PubMed1998

Composite biomaterials with chemical bonding between hydroxyapatite filler particles and PEG/PBT copolymer matrix.

In an effort to make composites from hydroxyapatite and a PEG/PBT copolymer (Polyactive 70/30), chemical linkages were introduced between the filler particles and polymer matrix using hexamethylene diisocyanate as a coupling agent. Infrared spectra (IR) and thermal gravimetric analysis (TGA) confirmed the presence of Polyactive 70/30 on the surface of HA filler particles. The amount of chemically bound polymer was 4.7 wt.%, as determined by TGA. The mechanical properties of the composites, that is, tensile strength and Young's modulus, were improved significantly by the introduction of a chemical linkage between the filler particles and polymer matrix compared to control composites. This method provides an effective way to introduce chemical linkage between HA filler particles and a polymer matrix. By optimizing the grafting process, a further improvement of the mechanical properties in the composites can be expected.

Composite Resins↗

PubMed1998

Covalent bonding of PMMA, PBMA, and poly(HEMA)to hydroxyapatite particles.

In our earlier study, we showed that the surface hydroxyl groups of hydroxyapatite have the ability to react with organic isocyanate groups. In this study, the feasibility of grafting poly(methyl methacrylate) (PMMA), poly(n-butyl methacrylate) (PBMA), and Poly(hydroxyethyl methacrylate) [poly(HEMA)] by using the reaction of isocyanate groups with the hydroxyl groups on the surface of HA was investigated. Double bonds were introduced to the surface of HA via the coupling reaction of isocyanateoethyl methacrylate (ICEM) with HA, or through hexamethylene diisocyanate (HMDI) with hydroxyethyl methacrylate (HEMA) and HA, followed by radical polymerization in MMA, BMA, or HEMA. Infrared spectra indicated the existence of polymers on the surfaces of HA. Thermogravimetric analysis also confirmed the presence of grafted polymers on the surface of HA powder particles (20-26 wt%). The polymers gave typical PMMA, PBMA, or poly(HEMA) infrared spectra, with the exception of amide bands, a result of the coupling reaction of ICEM or HMDI with hydroxy groups of HA or HEMA. Therefore it is concluded that the polymers were chemically bonded to the surface of HA through the isocyanate groups of ICEM or HMDI.

Biocompatible Materials↗

PubMed1998

Preparation of calcium phosphate coatings on titanium implant materials by simple chemistry.

A two-step chemical treatment has been developed in our group to prepare commercially pure titanium (cpTi) surfaces that will allow calcium phosphate (Ca-P) precipitation during immersion in a supersaturated calcification solution (SCS) with ion concentrations of [Ca2+] = 3.10 mM and [HPO4(2-)] = 1.86 mM. It was observed that a precalcification (Pre-Ca) procedure prior to immersion could significantly accelerate the Ca-P deposition process. In this work, the bioactivity of chemically treated cpTi and Ti6Al4V was further verified by applying commercially available Hanks' balanced salt solution (HBSS), an SCS with very low ion concentrations of [Ca2+] = 1.26 mM and [HPO4(2-)] = 0.779 mM, as the immersion solution. It was found that a uniform and very dense apatite coating magnesium impurities was formed if the Pre-Ca procedure was performed before immersion, as compared with the loose Ca-P layer obtained from the abovementioned high concentration of SCS. The formation of a microporous titanium dioxide thin surface layer on cpTi or Ti6Al4V by the two-step chemical treatment could be the main reason for the induction of apatite nucleation and growth from HBSS. Variations of pH values, Ca and P concentrations, and immersion time in HBSS were investigated to reveal the detailed process of Ca-P deposition. The described treatments provide a simple chemical method to prepare Ca-P coatings on both cpTi and Ti6Al4V.

Alloys↗

PubMed1998

Surface modification of nano-apatite by grafting organic polymer.

Since surface properties of hydroxyapatite (HA) play an important role in its performance, surface modification of HA has gained much attention from researchers. Silane coupling agents have been the focus of the research. In this study, an effective surface modification method was developed using hexamethylene diisocyanate as a coupling agent. Polyethylene glycol (Mw = 1500) was successfully coupled to the surface of nano-size apatite particles (nano-apatite). Various methods were used to characterize the surface-modified nano-apatite. Infra-red spectra confirmed the existence of a layer of polymer with both urethane and ether linkage on the surface of nano-apatite. The amount of grafted polymer as determined by total organic carbon analysis (TOC) and thermal gravimetric analysis (TGA) was about 20% in weight. Solid 1H MAS NMR spectra indicated that the amount of hydroxyl groups of nano-apatite was decreased by 7.7% after surface grafting reaction. It is concluded that the surface hydroxyl groups of nano-apatite have the reactivity towards isocyanate groups.

Carbon↗

PubMed1998

Polyacids as bonding agents in hydroxyapatite polyester-ether (Polyactive 30/70) composites.

A previously developed method to improve the interface between hydroxyapatite (HA) and a polyester-ether (Polyactive 70/30) by using polyacrylic acid or poly(ethylene-co-maleic acid) has been applied to HA/Polyactive 30/70 composites noting that polyactive 30/70 contains less polyethylene glycol (PEG) segments and a higher concentration of rigid poly(butylene terephthalate) (PBT) segments. The mobility of the PEG segments is significantly affected by the existence of a high concentration of rigid PBT segments. Our experimental results show that this method is indeed suitable for making HA/Polyactive 30/70 composites. The hydrogen bond/dipole interaction forming ability of the PEG segment is not affected by the existence of relatively large amounts of PBT segments. By using these coupling agents, the mechanical properties of the composite can be significantly improved both in dry and wet states. A fractographical study of the fracture surfaces revealed that the surface modified HA particles maintain better contact at fracture. It also showed that larger HA particles may initiate cracks and that such particles may be responsible for a decrease in the tensile strength of the composites.

Journal Article↗

PubMed1998

Evaluation of copolymers of polyethylene oxide and polybutylene terephthalate (polyactive): mechanical behaviour.

Polyether-polyester segmented block copolymers (Polyactive) on the basis of polybutylene terephthalate (PBT) and polyethylene oxide (PEO) were mechanically tested. Tensile strength and modulus of elasticity in compressive and tensile deformation were recorded according to ASTM standards. These tests were done in vitro under dry and wet conditions, and after 3, 9 and 25 wk subcutaneous implantation of these materials in goats. Strength and modulus of elasticity were higher with increased contents of PBT in the copolymers. After water uptake, the polymer displayed a lower strength and stiffness. Disintegration of the materials with 70% PEO content and dumb-bell shape was noted at 3 wk. Disintegration of the cylinders of the same material was seen after 25 wk implantation. Of the materials with 60% PEO content, only four of the five dumb-bells had disintegrated after 25 wk implantation. The in vivo test results of all other implants did not show a clinically relevant decrease of strength and stiffness with time after implantation of the copolymers in the goats. Mechanical behavior of the various copolymers seemed mainly determined by the amount and integrity of the PBT phase.

Journal Article↗

PubMed1997

In vitro calcium phosphate formation on a natural composite material, bamboo.

A natural self-reinforced composite material, bamboo, is studied for the first time as a biomedical material. Its anatomical structure was investigated and its mechanical properties were measured and compared with those of some common bone-bonding or bone-repairing biomaterials. It is found that, among all kinds of biomaterials, bamboo has the closest modulus of elasticity to human long bone. The cytotoxicity of bamboo was tested using the agar overlay method before and after heat or chemical treatments. The results reveal that ethanol, methanol and toluene can remove toxic leachable components from bamboo to some extent through extraction. After grafting a polymer whose molecule includes poly(ethylene glycol), alpha,omega-di(aminopropyl)poly(ethylene glycol) 800 on bamboo, bamboo has the ability to form a calcium phosphate coating after being immersed in calcification solution (simulated body fluid and accelerated calcification solution). The characteristics and the morphology of the mineral formed on bamboo were studied by infrared spectroscopy and scanning electron microscopy.

Apatites↗

PubMed1997

Nano-apatite/polymer composites: mechanical and physicochemical characteristics.

Hydrothermally synthesized acicular nano-apatite (Nap) was used as filler to make composites with a polyethylene glycol/poly(butylene terephthalate) (PEG/PBT) block copolymer (Polyactive 70:30). The Nap had a particle diameter of 9-25 nm and a length of 80-200 nm. The mechanical properties and the physiochemical characteristics of the composites, such as Young's modulus, swelling degree in water and the calcification behaviour, have been determined. It was found that Nap had a strong ability to promote the calcification of composites when incorporated into Polyactive 70:30, while poly(acrylic acid) (PAA) coating of Nap had an adverse effect on the calcification of composites, presumably due to the formation of complexes between PAA and PEG segments. Nap had a prominent stiffening effect for Polyactive 70:30 in the dry state, but had a poor stiffening effect for composites in an aqueous environment due to the hygroscopic nature and/or the formation of aggregates. PAA coating on Nap had almost no additional effect on the mechanical properties of composites either in the dry state or in an aqueous environment. To reinforce the polymer by Nap, achieving a more homogeneous dispersion of Nap in the polymer matrix and surface modifications to render the powders less hygroscopic appear to be necessary.

Acrylic Resins↗

PubMed1997

Fast precipitation of calcium phosphate layers on titanium induced by simple chemical treatments.

A simple two-step chemical treatment, i.e. etching with HCl and H2SO4 followed by immersion in boiling dilute NaOH solution, has been developed by our group to prepare bioactive microporous titanium surfaces allowing fast deposition of a calcium phosphate layer (CPL) from an in vitro supersaturated calcification solution (SCS). In this work, a precalcification (Pre-Ca) procedure was applied by soaking the two-step treated titanium in Na2HPO4 and then saturated Ca(OH)2 solution before immersion in SCS to accelerate further the CPL precipitation. The treated titanium surfaces with Pre-Ca were characterized after 1, 2, 4, 8 and 16 h of immersion in SCS by means of scanning electron microscopy together with energy dispersive X-ray analysis, X-ray diffraction and infrared absorption analysis. It was observed that the CPL precipitation rate with Pre-Ca averaged 1 microm h-1, twice as fast as without Pre-Ca. No precipitation was observed on untreated titanium with Pre-Ca up to day 14 of immersion in the SCS.

Calcium Hydroxide↗

PubMed1996

Characterization of silane-treated hydroxyapatite powders for use as filler in biodegradable composites.

Hydroxyapatite (HA) powder was treated with different silane adhesion promoters, to optimize its performance as a filler in polymer composites. The silane coupling agents investigated possessed vinyl, methacryloxy, primary amine, secondary amine, and diamine functionality. The different coatings were evaluated with respect to their influence on ionic exchange. X-ray photoelectron spectroscopy revealed the presence of a few monolayers thin silane films on HA powder. Silane coupling agents were able to bond chemically on the HA surface because a thin coating remained after washing of the powder with water. The water stability of this bond was evaluated by successive extractions and was judged limited, especially in the case of the hydrophilic aminosilanes. Zeta-potential measurements indicated the "transparency" of the coatings for ionic transport, that was corroborated by two in vitro dissolution studies, in Gomori's Tris-maleate buffer, and in simulated body fluid. However, aminosilane coatings could delay the release of calcium and phosphate ions during the first 2 days of immersion of treated HA powder in Gomori's buffer.

Biocompatible Materials↗