Andreas A. Poot

Preparation and evaluation of molecularly-defined collagen-elastin-glycosaminoglycan scaffolds for tissue engineering.

Extracellular matrix components are valuable building blocks for the preparation of biomaterials involved in tissue engineering, especially if their biological, chemical and physical characteristics can be controlled. In this study, isolated type I collagen fibrils, elastin fibres and chondroitin sulphate (CS) were used for the preparation of molecularly-defined collagen-elastin-glycosaminoglycan scaffolds. A total of 12 different scaffolds were prepared with four different ratios of collagen and elastin (1:9, 1:1, 9:1 and 1:0), with and without chemical crosslinking, and with and without CS. Collagen was essential to fabricate coherent, porous scaffolds. Electron microscopy showed that collagen and elastin physically interacted with each other and that elastin fibres were enveloped by collagen. By carbodiimide-crosslinking, amine groups were coupled to carboxylic groups and CS could be incorporated. More CS could be bound to collagen scaffolds (10%) than to collagen-elastin scaffolds (2.4-8.5% depending on the ratio). The attachment of CS increased the water-binding capacity to up to 65%. Scaffolds with a higher collagen content had a higher tensile strength whereas addition of elastin increased elasticity. Scaffolds were cytocompatible as was established using human myoblast and fibroblast culture systems. It is concluded that molecularly-defined composite scaffolds can be composed from individual, purified, extracellular matrix components. Data are important in the design and application of tailor-made biomaterials for tissue engineering.

Copolymers of trimethylene carbonate and ε-caprolactone for porous nerve guides: Synthesis and properties

Copolymers of trimethylene carbonate and ε-caprolactone were synthesized and characterized with the aim of assessing their potential in the development of a flexible and slowly degrading artificial nerve guide for the bridging of large nerve defects. The effect of the monomer ratio on the physical properties of the polymers and its influence on the processability of the materials was investigated. Under the applied polymerization conditions (130°C, 3 days using stannous octoate as a catalyst) high molecular weight polymers (Mn above 93 000) were obtained. All copolymers had glass transition temperatures below room temperature. At trimethylene carbonate contents higher than 25 mol% no crystallinity was detected. A decrease in crystallinity resulted in the loss of strength and decrease in toughness, as well as in an increased polymer wettability. Amorphous poly(trimethylene carbonate), however, showed excellent ultimate mechanical properties due to strain-induced crystallization (Tm = 36°C). Low crystallinity copolymers could be processed into dimensionally stable porous structures by means of immersion precipitation and by combination of this technique with the use of porosifying agents. Porous membranes of poly(trimethylene carbonate) could be prepared when blended with small amounts of high molecular weight poly(ethylene oxide). Poly(trimethylene carbonate) and poly(trimethylene carbonate-co-ε-caprolactone) copolymers with high ε-caprolactone content possess good physical properties and are processable into porous structures. These materials are most suitable for the preparation of porous artificial nerve guides.

Preparation of degradable porous structures based on 1,3-trimethylene carbonate and D,L-lactide (co)polymers for heart tissue engineering

Biodegradable porous scaffolds for heart tissue engineering were prepared from amorphous elastomeric (co)polymers of 1,3-trimethylene carbonate (TMC) and D,L-lactide (DLLA). Leaching of salt from compression-molded polymer-salt composites allowed the preparation of highly porous structures in a reproducible fashion. By adjusting the salt particle size and the polymer-to-particle weight ratio in the polymer-salt composite preparation the pore size and porosity of the scaffolds could be precisely controlled. The thermal properties of the polymers used for scaffold preparation had a strong effect on the morphology, mechanical properties and dimensional stability of the scaffolds under physiological conditions. Interconnected highly porous structures (porosity, 94%; average pore size, 100 microm) based on a TMC-DLLA copolymer (19:81, mol%) had suitable mechanical properties and displayed adequate cell-material interactions to serve as scaffolds for cardiac cells. This copolymer is noncytotoxic and allows the adhesion and proliferation of cardiomyocytes. During incubation in phosphate-buffered saline at 37 degrees C, these scaffolds were dimensionally stable and the number average molecular weight (Mn) of the polymer decreased gradually from 2.0 x 10(5) to 0.3 x 10(5) in a period up to 4 months. The first signs of mass loss (5%) were detected after 4 months of incubation. The degradation behavior of the porous structures was similar to that of nonporous films with similar composition and can be described by autocatalyzed bulk hydrolysis.

Proteins involved in the Vroman effect during exposure of human blood plasma to glass and polyethylene

The amounts of fibrinogen adsorbed to glass from various human blood plasmas have been measured as a function of time. The plasmas were 11 single donor plasmas, pooled plasma, a single donor high molecular weight kininogen (HMWK)-deficient plasma and HMWK-deficient plasma, which had been reconstituted with HMWK. For adsorption times between 1 min and 1 h more fibrinogen adsorbed from HMWK-deficient plasma compared with the amounts of fibrinogen which adsorbed from the other plasmas. This result supports the conclusion of several authors that HMWK is involved in the displacement of fibrinogen, initially adsorbed from normal human plasma to glass. Glass surfaces, pre-exposed to solutions of plasma and subsequently exposed to 1:1 diluted plasma, gives rise to a relatively high adsorption of HMWK which is independent of the plasma concentration of the precoating solution. The results indicate that HMWK from 1:1 diluted plasma is involved in the displacement of proteins from glass surfaces which had been pre-exposed to solutions with a low plasma concentration. Experiments with polyethylene as a substrate reveal that high density lipoprotein (HDL) from 1:1 diluted plasma is involved in the displacement of proteins from polyethylene surfaces which had been pre-exposed to solutions with a low plasma concentration. Moreover, evidence is presented that substantial amounts of albumin and fibrinogen, adsorbed from 1:1000 diluted plasma to glass and polyethylene, are displaced from the surfaces of these materials by proteins from 1:1 diluted plasma different from HMWK and HDL.

Flexible and elastic porous poly(trimethylene carbonate) structures for use in vascular tissue engineering

Biocompatible and elastic porous tubular structures based on poly(1,3-trimethylene carbonate), PTMC, were developed as scaffolds for tissue engineering of small-diameter blood vessels. High-molecular-weight PTMC (M(n) = 4.37 x 10(5)) was cross-linked by gamma-irradiation in an inert nitrogen atmosphere. The resulting networks (50-70% gel content) were elastic and creep resistant. The PTMC materials were highly biocompatible as determined by cell adhesion and proliferation studies using various relevant cell types (human umbilical vein endothelial cells (HUVECs), smooth muscle cells (SMCs) and mesenchymal stem cells (MSCs)). Dimensionally stable tubular scaffolds with an interconnected pore network were prepared by particulate leaching. Different cross-linked porous PTMC specimens with average pore sizes ranging between 55 and 116 microm, and porosities ranging from 59% to 83% were prepared. These scaffolds were highly compliant and flexible, with high elongations at break. Furthermore, their resistance to creep was excellent and under cyclic loading conditions (20 deformation cycles to 30% elongation) no permanent deformation occurred. Seeding of SMCs into the wall of the tubular structures was done by carefully perfusing cell suspensions with syringes from the lumen through the wall. The cells were then cultured for 7 days. Upon proliferation of the SMCs, the formed blood vessel constructs had excellent mechanical properties. Their radial tensile strengths had increased from 0.23 to 0.78 MPa, which is close to those of natural blood vessels.

Deposition of cellular fibronectin and desorption of human serum albumin during adhesion and spreading of human endothelial cells on polymers

More insight into the mechanism of adhesion of human endothelial cells (HEC) on to polymeric surfaces may lead to the development of improved small-diameter vascular grafts. HEC suspended in 20% human serum-containing culture medium adhere and spread well on moderately water-wettable polymers such as tissue culture polystyrene (TCPS). Earlier it was demonstrated that during adhesion and spreading of HEC on TCPS, cellular fibronectin is deposited on to this surface. It was postulated that fibronectin deposition is accompanied by desorption of adsorbed serum proteins, e.g. human serum albumin (HSA). The amounts of adsorbed (cellular) fibronectin and HSA on TCPS surfaces pretreated for 1 h with solutions of human serum (ranging from 0.01%–20%), were determined after incubation of these surfaces for 6 h with HEC in culture medium and after incubation with culture medium without cells. Protein adsorption was determined by means of a two-step enzyme-immunoassay (EIA). HEC adhesion and spreading on TCPS resulted in a significant deposition of fibronectin irrespective of the serum concentration in the solution used for the pretreatment of TCPS. The deposition of cellular fibronectin on to TCPS, pretreated with human serum, was accompanied by displacement of adsorbed HSA. Desorption of HSA from TCPS was only detectable with the EIA at serum concentrations ranging from 0.01%–1%. Using131-l-labelled HSA as tracer protein; it could, however, be demonstrated that HSA was also displaced from TCPS, pretreated with solutions of higher serum concentrations. Pretreatment of the hydrophobic vascular graft material PET (poly(ethylene terephthalate); Dacron) and of FEP (fluoroethylenepropylene copolymer; a Teflon-like polymer) with a solution containing 20% human serum resulted in a reduced adhesion of HEC compared to uncoated surfaces. We suggest that this may be caused by a poor displacement of adsorbed serum proteins from these hydrophobic surfaces by cellular fibronectin. This may explain why HEC normally fail to adhere on to prosthetic surfaces.

Flexible and elastic scaffolds for cartilage tissue engineering prepared by stereolithography using poly(trimethylene carbonate)-based resins

The aim of this study is to investigate the applicability of flexible and elastic poly(trimethylene carbonate) (PTMC) structures prepared by stereolithography as scaffolds for cartilage tissue engineering. A three-armed methacrylated PTMC macromer with a molecular weight of 3100 g mol(-1) is used to build designed scaffolds with a pore diameter of 350 ± 12 μm and a porosity of 54.0 ± 2.2%. Upon seeding of bovine chondrocytes in the scaffolds, the cells adhere and spread on the PTMC surface. After culturing for 6 weeks, also cells with a round morphology are present, indicative of the differentiated chondrocyte phenotype. Sulphated glycosaminoglycans and fibrillar collagens are deposited by the cells. During culturing for 6 weeks, the compression moduli of the constructs increases 50% to approximately 100 kPa.

Tissue engineering of small-diameter vascular grafts: A literature review

For the treatment of cardiovascular disease, functional arterial blood vessel prostheses with an inner diameter less than 6 mm are needed. This article gives an overview of the preparation of such vascular grafts by means of tissue engineering.

Delivery systems for the treatment of degenerated intervertebral discs

The intervertebral disc (IVD) is the most avascular and acellular tissue in the body and therefore prone to degeneration. During IVD degeneration, the balance between anabolic and catabolic processes in the disc is deregulated, amongst others leading to alteration of extracellular matrix production, abnormal enzyme activities and production of pro-inflammatory substances like cytokines. The established treatment strategy for IVD degeneration consists of physiotherapy, pain medication by drug therapy and if necessary surgery. This approach, however, has shown limited success. Alternative strategies to increase and prolong the effects of bioactive agents and to reverse the process of IVD degeneration include the use of delivery systems for drugs, proteins, cells and genes. In view of the specific anatomy and physiology of the IVD and depending on the strategy of the therapy, different delivery systems have been developed which are reviewed in this article.

Platelet deposition in a capillary perfusion model: quantitative and morphological aspects

The capillary perfusion model according to Cazenave and co-workers was characterized by investigating the effects of protein precoating, perfusion time and shear rate on platelet deposition using 111Indium labelling of human platelets and scanning electron microscopy (SEM). Compared with uncoated polyethylene, platelet deposition was increased after precoating with purified human von Willebrand factor, fibrinogen or fibronectin, and decreased by preadsorbed immunoglobulin G, albumin or whole plasma. Platelet aggregates were observed on immunoglobulin G-coated polyethylene, whereas all other surfaces showed single adherent platelets. Complete platelet spreading was only observed after precoating with fibronectin. The quantitative data concerning platelet deposition were evaluated by using the convective-diffusion theory. Our results indicate the applicability of this perfusion model for the in vitro testing of biomaterials.