Search PubMed⌕ Search

Biomedical subjects

P J Dijkstra

Publications and source records attributed to P J Dijkstra.

At least 19 recordsLinked to original sources
PubMed2006

First steps towards tissue engineering of small-diameter blood vessels: preparation of flat scaffolds of collagen and elastin by means of freeze drying.

Porous scaffolds composed of collagen or collagen and elastin were prepared by freeze drying at temperatures between -18 and -196 degrees C. All scaffolds had a porosity of 90-98% and a homogeneous distribution of pores. Freeze drying at -18 degrees C afforded collagen and collagen/elastin matrices with average pore sizes of 340 and 130 mum, respectively. After 20 successive cycles up to 10% of strain, collagen/elastin dense films had a total degree of strain recovery of 70% +/- 5%, which was higher than that of collagen films (42% +/- 6%). Crosslinking of collagen/elastin matrices either in water or ethanol/water (40% v/v) was carried out using a carbodiimide (N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride, EDC) in combination with a succinimide (N-hydroxysuccinimide, NHS) in the presence or absence of a diamine (J230) or by reaction with butanediol diglycidylether (BDGE), followed by EDC/NHS. Crosslinking with EDC/NHS or EDC/NHS/J230 resulted in matrices with increased stiffness as compared to noncrosslinked matrices, whereas sequential crosslinking with the diglycidylether and EDC/NHS yielded very brittle scaffolds. Ethanol/water was the preferred solvent in the crosslinking process because of its ability to preserve the open porous structure during crosslinking. Smooth muscle cells were seeded on the (crosslinked) scaffolds and could be expanded during 14 days of culturing.

Biomechanical Phenomena↗
PubMed2005

Electrospinning of collagen and elastin for tissue engineering applications.

Meshes of collagen and/or elastin were successfully prepared by means of electrospinning from aqueous solutions. Flow rate, applied electric field, collecting distance and composition of the starting solutions determined the morphology of the obtained fibres. Addition of PEO (M(w)=8 x 10(6)) and NaCl was always necessary to spin continuous and homogeneous fibres. Spinning a mixture of collagen and elastin resulted in fibres in which the single components could not be distinguished by SEM. Increasing the elastin content determined an increase in fibres diameters from 220 to 600 nm. The voltage necessary for a continuous production of fibres was dependent on the composition of the starting solution, but always between 10 and 25 kV. Under these conditions, non-woven meshes could be formed and a partial orientation of the fibres constituting the mesh was obtained by using a rotating tubular mandrel as collector. Collagen/elastin (1:1) meshes were stabilized by crosslinking with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS). This treatment afforded materials with a high thermal stability (T(d)=79 degrees C) without altering their original morphology. Upon crosslinking PEO and NaCl were fully leached out. Smooth muscle cells grew as a confluent layer on top of the crosslinked meshes after 14 d of culture.

Animals↗
PubMed2001

Tissue reactions to epoxy-crosslinked porcine heart valves post-treated with detergents or a dicarboxylic acid.

Calcification limits the long-term durability of xenograft glutaraldehyde (GA)-crosslinked heart valves. Previously, a study in rats showed that epoxy-crosslinked heart valves reduced lymphocyte reactions to the same extent as the GA-crosslinked control and induced a similar foreign-body response and calcification reaction. The present study was aimed at reducing the occurrence of calcification of epoxy-crosslinked tissue. Two modifications were carried out and their influence on cellular reactions and the extent of calcification after 8 weeks' implantation in weanling rats was evaluated. First, epoxy-crosslinked valves were post-treated with two detergents to remove cellular elements, phospholipids and small soluble proteins, known to act as nucleation sites for calcification. The second approach was to study the effect of the impaired balance between negatively and positively charged amino acids by an additional crosslinking step with a dicarboxylic acid. The detergent treatment resulted in a washed-out appearance of especially the cusp tissue. With the dicarboxylic acid, both the cusps and the walls had a limited washed-out appearance. The wall also demonstrated some detachment of the subendothelium. After implantation, both detergent and dicarboxylic acid post-treatment histologically resulted in reduced calcification at the edges of cusps and walls. However, total amounts of calcification, measured by atomic emission spectroscopy, were not significantly reduced. Data concerning the presence of lymphocytes varied slightly, but were in the same range as the GA-crosslinked control, i.e., clearly reduced compared with a noncrosslinked control. It is concluded that both the double detergent and the dicarboxylic acid post-treatment of epoxy-crosslinked heart valve tissue do not represent a sound alternative in the fabrication of heart valve bioprostheses.

Animals↗
PubMed2001

Partially carboxymethylated feather keratins. 2. Thermal and mechanical properties of films.

Free cysteine thiol groups of keratin extracted from chicken feathers were partially carboxymethylated with iodoacetic acid (25-76% cysteine modification). Stable dispersions were used for the preparation of films by solution casting. Glycerol was used as a plasticizer (0.05-0.47 g/g of keratin), and films were stored at a constant relative humidity (20, 30, 50, 70, or 90%). The degree of crystallinity in the films was higher when more cysteine residues were carboxymethylated. The films displayed an optimum in mechanical properties at approximately 50% cysteine carboxymethylation. The tensile strength at this optimum was 25 MPa, the E modulus, 350 MPa, and the elongation at break, 50%. Probably, this optimum was the result of both a decreasing amount of disulfide bonds and an increasing degree of crystallinity for higher degrees of cysteine modification. The influences of a higher amount of glycerol and of different storage conditions on the mechanical properties of films from keratin with a defined degree of cysteine modification were also investigated.

Animals↗
PubMed2000

The kinetics of 1,4-butanediol diglycidyl ether crosslinking of dermal sheep collagen.

Dermal sheep collagen was crosslinked with 1,4-butanediol diglycidyl ether (BDDGE) or modified with glycidyl isopropyl ether (PGE). The reduction in amine groups as a function of time was followed to study the overall reaction kinetics of collagen with either BDDGE or PGE. Linearization of the experimental data resulted in a reaction order of 2 with respect to the amine groups in the PGE masking reaction, whereas a reaction order of 2.5 was obtained in the BDDGE crosslinking reaction. The reaction orders were independent of the pH in the range of 8.5-10.5 and the reagent concentration (1-4 wt %). The reaction order with respect to epoxide groups was equal to 1 for both reagents. As expected, the reaction rate was favored by a higher reagent concentration and a higher solution pH. Because the BDDGE crosslinking reaction occurs via two distinct reaction steps, the content of pendant epoxide groups in the collagen matrix was determined by treating the collagen with either O-phosphoryl ethanolamine or lysine methyl ester. The increase in either phosphor or primary amine groups was related to the content of pendant groups. Crosslinking at pH 9.0 resulted in a low reaction rate but in a high crosslink efficacy, especially after prolonged reaction times. A maximum concentration of pendant epoxide groups was detected after 50 h. Reaction at pH 10.0 was faster, but a lower crosslinking efficacy was obtained. At pH 10.0, the ratio between pendant epoxide groups and crosslinks was almost equal to 1 during the course of the crosslinking reaction.

Animals↗
PubMed2000

Microspheres for protein delivery prepared from amphiphilic multiblock copolymers. 2. Modulation of release rate.

Amphiphilic multiblock copolymers, based on hydrophilic poly(ethylene glycol) (PEG) blocks and hydrophobic poly(butylene terephthalate) (PBT) blocks were used as matrix material for protein-loaded microspheres. The efficiency of lysozyme entrapment by a double emulsion method was found to depend on the swelling behavior of the polymers in water and decreased from 100% for polymers with a degree of swelling of less than 1.8 to 11% for PEG-PBT copolymers with a degree of swelling of 3.6. The particle size could be controlled by varying the concentration of the polymer solution used in the microsphere preparation. An increase in the polymer concentration resulted in a proportional increase in the particle size. The in vitro release profiles of the encapsulated model protein lysozyme could be precisely tailored by variation of the copolymer composition and the size of the microspheres. Both a slow continuous release of lysozyme, and a fast release which was completed within a few days could be obtained. The release behavior, attributed to a combination of diffusion and polymer degradation, could be described by a previously developed model.

Algorithms↗
PubMed2000

Microspheres for protein delivery prepared from amphiphilic multiblock copolymers. 1. Influence of preparation techniques on particle characteristics and protein delivery.

The entrapment of lysozyme in amphiphilic multiblock copolymer microspheres by emulsification and subsequent solvent removal processes was studied. The copolymers are composed of hydrophilic poly(ethylene glycol) (PEG) blocks and hydrophobic poly(butylene terephthalate) (PBT) blocks. Direct solvent extraction from a water-in-oil (w/o) emulsion in ethanol or methanol did not result in the formation of microspheres, due to massive polymer precipitation caused by rapid solvent extraction in these non-solvents. In a second process, microspheres were first prepared by a water-in-oil-in-water (w/o/w) emulsion system with 4% poly(vinyl alcohol) (PVA) as stabilizer in the external phase, followed by extraction of the remaining solvent. As non-solvents ethanol, methanol and mixtures of methanol and water were employed. However, the use of alcohols in the extraction medium resulted in microspheres which gave an incomplete lysozyme release at a non-constant rate. Complete lysozyme release was obtained from microspheres prepared by an emulsification-solvent evaporation method in PBS containing poly(vinyl pyrrolidone) (PVP) or PVA as stabilizer. PVA was most effective in stabilizing the w/o/w emulsion. Perfectly spherical microspheres were produced, with high protein entrapment efficiencies. These microspheres released lysozyme at an almost constant rate for approximately 28 days. The reproducibility of the w/o/w emulsion process was demonstrated by comparing particle characteristics and release profiles of three batches, prepared under similar conditions.

Drug Compounding↗
PubMed2000

Control of protein delivery from amphiphilic poly(ether ester) multiblock copolymers by varying their water content using emulsification techniques.

Protein-containing films and microspheres, based on poly(ethylene glycol)-poly(butylene terephthalate) (PEG-PBT) multiblock copolymers, were prepared from water-in-oil (w/o) emulsions. The properties of the matrices could be controlled by the water-to-polymer ratio (w/p) in the w/o emulsion. A linear increase in water uptake of the matrices was observed with increasing emulsion w/p. This could be explained by an increase in the number of dispersed water-rich domains in the polymer matrix. At low volume fraction of the dispersed phase (epsilon), lysozyme release was mainly dependent on the permeability of the swollen polymer bulk. Above a critical volume fraction (the percolation threshold epsilon(c)), the dispersed water-rich phase formed an interconnected network, which largely enhanced the permeability of the matrix. Determination of the permeability of PEG-PBT matrices for vitamin B(12) as a function of epsilon confirmed the formation of such an interconnected network. This interconnected network could be used to achieve controlled release of a large protein (bovine serum albumin, BSA) from PEG-PBT films and microspheres. Due to its hydrodynamic diameter, BSA was screened by the polymer network when epsilon was low. However above epsilon(c), the fraction released BSA increased with increasing volume fraction of the dispersed phase. A very rapid BSA release could be obtained, with the majority of the incorporated BSA released within 1 day, as well as a slow and continuous release, lasting for over 150 days. When BSA-containing microspheres were prepared with a volume fraction just below the percolation threshold, a delayed release was observed. This was attributed to the effect of polymer degradation on matrix permeability.

Drug Delivery Systems↗
PubMed2000

Zero-order release of lysozyme from poly(ethylene glycol)/poly(butylene terephthalate) matrices.

Protein release from a series of biodegradable poly(ether ester) multiblock copolymers, based on poly(ethylene glycol) (PEG) and poly(butylene terephthalate) (PBT) was investigated. Lysozyme-containing PEG/PBT films and microspheres were prepared using an emulsion technique. Proteins were effectively encapsulated and dense polymer matrices were formed. The swelling in water of PEG/PBT films reached equilibrium within 3 days. The degree of swelling increased with increasing PEG content and with increasing molecular weight of the PEG segment. The release rate of lysozyme from PEG/PBT films could be tailored very precisely by controlling the copolymer composition. Release rates increased with increasing PEG/PBT weight ratio and increasing molecular weight of the PEG segment. For films prepared from block copolymers with PEG blocks of 4000 g/mol, first-order lysozyme release was observed. For matrices prepared from polymers with PEG segments of 1000 and 600 g/mol, the lysozyme release profile followed near zero-order kinetics. A mathematical description of the release mechanism was developed which takes into account the effect of polymer hydrolytic degradation on solute diffusion. The model was found to be consistent with the experimental observations. Finally, determination of the activity of released protein showed that lysozyme was not damaged during the formulation, storage and release periods.

Drug Stability↗
PubMed2000

In vivo behavior of epoxy-crosslinked porcine heart valve cusps and walls.

Calcification limits the long-term durability of xenograft glutaraldehyde-crosslinked heart valves. In this study, epoxy-crosslinked porcine aortic valve tissue was evaluated after subcutaneous implantation in weanling rats. Non-crosslinked valves and valves crosslinked with glutaraldehyde or carbodiimide functioned as control. Epoxy-crosslinked valves had somewhat lower shrinkage temperatures than the crosslinked controls, and within the series also some macroscopic and microscopic differences were obvious. After 8 weeks implantation, cusps from non-crosslinked valves were not retrieved. The matching walls were more degraded than the epoxy- and control-crosslinked walls. This was observed from the higher cellular ingrowth with fibroblasts, macrophages, and giant cells. Furthermore, non-crosslinked walls showed highest numbers of lymphocytes, which were most obvious in the capsules. Epoxy- and control-crosslinked cusps and walls induced lower reactions. Calcification, measured by von Kossa-staining and by Ca-analysis, was always observed. Crosslinked cusps calcified more than walls. Of all wall samples, the non-crosslinked walls showed the highest calcification. It is concluded that epoxy-crosslinked valve tissue induced a foreign body and calcification reaction similar to the two crosslinked controls. Therefore, epoxy-crosslinking does not represent a solution for the calcification problem of heart valve bioprostheses.

Animals↗
PubMed2000

Amphiphilic poly(ether ester amide) multiblock copolymers as biodegradable matrices for the controlled release of proteins.

Amphiphilic poly(ether ester amide) (PEEA) multiblock copolymers were synthesized by polycondensation in the melt from hydrophilic poly(ethylene glycol) (PEG), 1,4-dihydroxybutane and short bisester-bisamide blocks. These amide blocks were prepared by reaction of 1,4-diaminobutane with dimethyl adipate in the melt. A range of multiblock copolymers were prepared, with PEG contents varying from 23-66 wt %. The intrinsic viscosity of the PEEA polymers varied from 0.58-0.78. Differential scanning calorimetry showed melting transitions for the PEG blocks and for the amide-ester blocks, suggesting a phase separated structure. Both the melting temperature and the crystallinity of the hard amide-ester segments decreased with increasing PEG content of the polymers. The equilibrium swelling ratio in phosphate buffered saline (PBS) increased with increasing amount of PEG in the polymers and varied from 1.7 to 3.7, whereas the polymer that contained 66 wt % PEG was soluble in PBS. During incubation of PEEA films in PBS, weight loss and a continuous decrease in the resulting inherent polymer viscosity was observed. The rate of degradation increased with increasing PEG content. The composition of the remaining matrices did not change during degradation. A preliminary investigation of the protein release characteristics of these PEEA copolymers showed that release of the model protein lysozyme was proportional to the square root of time. The release rate was found to increase with increasing degree of swelling of the polymers.

Biocompatible Materials↗
PubMed2000

Partially carboxymethylated feather keratins. 1. Properties in aqueous systems.

Feather keratins were extracted from chicken feathers with an aqueous solution of urea and 2-mercaptoethanol. The keratin solution obtained was dialyzed to remove the reagents. Upon dialysis, extensive protein aggregation occurred. To obtain stable solutions or dispersions in water, cysteine residues were modified prior to dialysis with iodoacetamide, iodoacetic acid, or bromosuccinic acid, thereby blocking free thiol groups and introducing hydrophilic groups. For the development of biodegradable materials with good mechanical properties from these biopolymers, disulfide bonds between the keratin molecules are needed. Therefore, cysteine residues were only partially modified by using different reagent/cysteine molar ratios. The reaction rate constants of iodoacetate with glutathione and 2-mercaptoethanol were successfully used to predict the degree of modification of keratin cysteine. It was shown that, for carboxymethylated keratin, fewer aggregates were formed for higher degrees of cysteine modification, while more protein was present as oligomers. Aggregates and oligomers were stabilized through intermolecular disulfide bonds.

Animals↗
PubMed1999

A controlled release system for proteins based on poly(ether ester) block-copolymers: polymer network characterization.

The properties of a series of multiblock copolymers, based on hydrophilic poly(ethylene glycol) (PEG) and hydrophobic poly(butylene terephthalate) (PBT) blocks were investigated with respect to their application as a matrix for controlled release of proteins. The degree of swelling, Q, of the copolymers increased with increasing PEG content and with increasing molecular weight of the PEG segment. Within the composition range tested, Q varied from 1.26 for polymers with PEG segments of 600 g/mol and a PBT content of 60 weight.% up to 3.64 for polymers with PEG segments of 4000 g/mol and a PEG/PBT weight ratio of 80:20. Equilibrium stress (compression)-strain measurements were performed in order to estimate mesh sizes. The mesh size of the copolymers ranged from 38 to 93 A, which was experimentally confirmed by diffusion of vitamin B(12) (hydrodynamic diameter d(h)=16.6 A), lysozyme (d(h)=41 A) and bovine serum albumin (d(h)=72 A). The in vitro degradation of PEG/PBT copolymers with a PEG block length of 1000 g/mol and PEG/PBT weight ratios of 70:30, 60:40 and 40:60 was studied. Matrices with increasing PEG contents exhibited a faster weight loss in phosphate-buffered saline (pH 7.4) at 37 degrees C. Over a degradation period of 54 days, M(n) decreased by about 35-45%, while the composition of the matrices, determined by NMR, remained almost constant.

Animals↗
PubMed1999

Crosslinking and modification of dermal sheep collagen using 1, 4-butanediol diglycidyl ether.

Crosslinking of dermal sheep collagen (DSC) was accomplished using 1, 4-butanediol diglycidyl ether (BDDGE). At pH values > 8.0, epoxide groups of BDDGE will react with amine groups of collagen. The effects of BDDGE concentration, pH, time, and temperature were studied. Utilization of a 4-wt % BDDGE instead of 1-wt % resulted in a faster reaction. Whereas similar values of shrinkage temperature were obtained, fewer primary amine groups had reacted at a lower BDDGE concentration, which implies that the crosslinking reaction had a higher efficacy. An increase in pH from 8.5 to 10.5 resulted in a faster reaction but reduced crosslink efficacy. Furthermore, an increase in reaction temperature accelerated the reaction without changing the crosslink efficacy. Crosslinking under acidic conditions (pH < 6.0) evoked a reaction between epoxide groups and carboxylic acid groups of collagen. Additional studies showed that no oligomeric crosslinks could be formed. However, hydrolysis of the epoxide groups played a role in the crosslink mechanism especially under acidic reaction conditions. The macroscopic properties of these materials were dependent on the crosslinking method. Whereas a flexible and soft tissue was found if crosslinking was performed at pH < 6.0, a stiff sponge was obtained under alkaline conditions. Reaction of DSC with a monofunctional compound (glycidyl isopropyl ether) led to comparable trends in reaction rate and in similar macroscopical differences in materials as observed with BDDGE.

Animals↗
PubMed1999

Characterization and biocompatibility of epoxy-crosslinked dermal sheep collagens.

Dermal sheep collagen (DSC), which was crosslinked with 1, 4-butanediol diglycidyl ether (BD) by using four different conditions, was characterized and its biocompatibility was evaluated after subcutaneous implantation in rats. Crosslinking at pH 9.0 (BD90) or with successive epoxy and carbodiimide steps (BD45EN) resulted in a large increase in the shrinkage temperature (T(s)) in combination with a clear reduction in amines. Crosslinking at pH 4.5 (BD45) increased the T(s) of the material but hardly reduced the number of amines. Acylation (BD45HAc) showed the largest reduction in amines in combination with the lowest T(s). An evaluation of the implants showed that BD45, BD90, and BD45EN were biocompatible. A high influx of polymorphonuclear cells and macrophages was observed for BD45HAc, but this subsided at day 5. At week 6 the BD45 had completely degraded and BD45HAc was remarkably reduced in size, while BD45EN showed a clear size reduction of the outer DSC bundles; BD90 showed none of these features. This agreed with the observed degree of macrophage accumulation and giant cell formation. None of the materials calcified. For the purpose of soft tissue replacement, BD90 was defined as the material of choice because it combined biocompatibility, low cellular ingrowth, low biodegradation, and the absence of calcification with fibroblast ingrowth and new collagen formation.

Animals↗