Porous polyurethane tubes as vascular graft

Ultrasensitive Touch Sensor for Simultaneous Tactile and Slip Sensing

Touch is a general term to describe mechanical stimuli. It is extremely difficult to develop touch sensors that can detect different modes of contact forces due to their low sensitivity and data decoupling. Simultaneously conducting tactile and slip sensing presents significant challenges for the design, structure, and performance of sensors. In this work, a highly sensitive sandwich-structured sensor is achieved by exploiting the porosity and compressive modulus of the sensor's functional layer materials. The sensor shows an ultra-high sensitivity of 1167 kPa-1 and a low-pressure detection limit of 1.34 Pa due to its considerably low compression modulus of 23.8 Pa. Due to this ultra-high sensitivity, coupled with spectral analysis, it allows for dual-mode detection of both tactile and slip sensations simultaneously. This novel fabrication strategy and signal analysis method provides a new direction for the development of tactile/slip sensors.

A composite scaffold fabricated with an acellular matrix and biodegradable polyurethane for the in vivo regeneration of pig bile duct defects

Bile duct regeneration is urgently needed to restore the normal function of the damaged biliary system. In this study, an artificial bile duct (ABD) was fabricated for extrahepatic bile duct regeneration based on biodegradable polyurethane (BPU) and ureter acellular matrix (UAM) to endow it with favorable biocompatibility and eliminate bile leakage during in vivo bile duct regeneration. The mechanical properties, in vitro simulation of bile flow and cytocompatibility of BPU-UAM ABD were evaluated in vitro, and surgical implantation in the biliary defect site in minipigs was implemented to reveal the in vivo degradation of BPU-UAM and regeneration of the new bile duct. The results indicated that BPU-UAM ABD with a mechanical strength of 11.9 MPa has excellent cytocompatibility to support 3T3 fibroblast survival and proliferation in extraction medium and on the scaffolds. The in vivo implantation of BPU-UAM ABD revealed the change of collagen content throughout the new bile duct regeneration. Biliary epithelial cells were observed at day 70, and continuous biliary epithelial layer formation was observed after 100 days of implantation. Altogether, the BPU-UAM ABD fabricated in this study possesses excellent properties for application study in the regeneration of bile duct. STATEMENT OF SIGNIFICANCE: Extrahepatic bile duct defects carry considerable morbidity and mortality because they are the only pathway for bile to go down into the intestinal tract. At present, no artificial bile duct can promote biliary regeneration. In this study, BPU-UAM ABD was built based on biodegradable polyurethane and ureter acellular matrix to form a continuous compact layer of polyurethane in the internal wall of UAM and avoid bile leakage and experimental failure during in vivo implantation. Our work verified the effectiveness of the synthesized biodegradable polyurethane emulsion-modified urethral acellular matrix in bile regeneration and continuous biliary epithelial layer formation. This study provided a new approach for the curing of bile duct defects and inducing new bile tissue formation.

A New Polyurethane/Heparin Vascular Graft for Small-Caliber Vein Repair

Small-caliber (1.2 mm inner diameter) vein grafts, made from a mixture of heparin and polyurethane with superior compliance, excellent antithrombogenicity and biocompatibility, have been developed. Eighteen rabbits were used; 12 for the heparin containing grafts and the other six were pure polyurethane grafts as controls. Each graft segment (2 cm in length) was implanted into the femoral veins using a newly developed anastomosis method. Sodium heparin was given before surgery, but no anticoagulant was used thereafter. All the rabbits lived during the whole experimental period of 1 year. Histological analyses of vessels retrieved 2, 4, 8, 12 and 24 weeks after implantation revealed regeneration of endothelial-like cells (in 2 weeks), elastin-like tissues (in 8 weeks), and neoadventitia-like layers (in 12 weeks). The patency rate for the heparin containing grafts was 100%, but was only 83.3% in the no heparin controls. These results indicate that “ideal” small diameter blood vessels can be synthesized and used directly without cellularization before implantation. By the properly selecting scaffold materials, a native vein can repair itself spontaneously to certain degree.

Triggerable Degradation of Polyurethanes for Tissue Engineering Applications

Tissue engineered and bioactive scaffolds with different degradation rates are required for the regeneration of diverse tissues/organs. To optimize tissue regeneration in different tissues, it is desirable that the degradation rate of scaffolds can be manipulated to comply with various stages of tissue regeneration. Unfortunately, the degradation of most degradable polymers relies solely on passive controlled degradation mechanisms. To overcome this challenge, we report a new family of reduction-sensitive biodegradable elastomeric polyurethanes containing various amounts of disulfide bonds (PU-SS), in which degradation can be initiated and accelerated with the supplement of a biological product: antioxidant-glutathione (GSH). The polyurethanes can be processed into films and electrospun fibrous scaffolds. Synthesized materials exhibited robust mechanical properties and high elasticity. Accelerated degradation of the materials was observed in the presence of GSH, and the rate of such degradation depends on the amount of disulfide present in the polymer backbone. The polymers and their degradation products exhibited no apparent cell toxicity while the electrospun scaffolds supported fibroblast growth in vitro. The in vivo subcutaneous implantation model showed that the polymers prompt minimal inflammatory responses, and as anticipated, the polymer with the higher disulfide bond amount had faster degradation in vivo. This new family of polyurethanes offers tremendous potential for directed scaffold degradation to promote maximal tissue regeneration.

Morphology, mechanical properties, and shape memory effects of poly(lactic acid)/ thermoplastic polyurethane blend scaffolds prepared by thermally induced phase separation

Novel blended scaffolds combining biobased polylactic acid (PLA) and thermoplastic polyurethane (TPU) were fabricated by thermally induced phase separation (TIPS) using two different solvents. Pure PLA and TPU polymer scaffolds using 1,4-dioxane as the sole solvent exhibited typical ladder-like structures, while blended PLA/TPU scaffolds using the same solvent showed a more uniform microstructure. When de-ionized water was added to the solution as a non-solvent, scaffolds with the mixed solvent showed more open cells and greater interconnectivity. In compression tests, it was found that specimens, including pure PLA, TPU, and blended scaffolds with the mixed solvent, showed a higher compressive modulus than their counterparts that used dioxane as the single solvent. Dynamic mechanical analysis (DMA) was employed to characterize the shape memory properties of the scaffolds. DMA indicated that the shape fixing ratio was highest in the PLA scaffolds, while the shape recovery ratio of the TPU scaffolds was the greatest among the specimens. More interestingly, when the mixed solvent was used, the shape memory property of the blended scaffolds displayed a similar deformation curve to the TPU scaffold. This was due to the presence of the TPU phase and similarity in structure between PLA/TPU and TPU scaffolds when mixed solvent was used. In the degradation test, the blended scaffolds showed a balanced degradation behavior in-between the more easily degraded PLA and the more stable TPU in the phosphate-buffered saline (PBS), and the addition of water to the systems accelerated the degradation process of the specimens. Cell culture results showed that all of the scaffolds had good biocompatibility.

Mechanical properties of cellulose: chitosan blends for potential use as a coronary artery bypass graft

The development of intimal hyperplasia is the major cause of failure of both autologous saphenous vein and synthetic coronary artery bypass grafts. This is partially due to graft-host vessel compliance mismatch. Cellulose and chitosan (CELL:CHIT) are both biocompatible, nontoxic, and naturally occurring biopolymers that have been used extensively for biomedical applications. Elastic properties of membranes made of CELL:CHIT blends with different ratios between each polymer were determined using uniaxial tests and the ratio that yielded the less stiff membrane was chosen to prepare a small diameter hollow tube. The presence of chitosan had a favorable impact on the elasticity of the membranes, where the CELL:CHIT 5:5 ratio showed the lowest Young's modulus. Small diameter tubular constructs were fabricated using this optimal CELL:CHIT ratio and the compliance was determined on samples with different wall thickness and internal diameter. The compliance of the hollow tube with inner diameter of 4 mm and wall thickness of 1.2 mm was found to be 5.91%/mmHg×10(-2), which is higher than those of Dacron, expanded polytetrafluorethylene, and saphenous vein, but very close to that of human coronary artery. Burst strength tests revealed that the tubes can withstand at least 300 mmHg. Finally, the tubes showed satisfactory cell attachment property when myofibroblast cells adhered and proliferated on the lumen of the samples.

Quantitative grafting of peptide onto the nontoxic biodegradable waterborne polyurethanes to fabricate peptide modified scaffold for soft tissue engineering

Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) peptide has frequently been used in the biomedical materials to enhance adhesion and proliferation of cells. In this work, we modified the nontoxic biodegradable waterborne polyurethanes (WBPU) with GRGDSP peptide and fabricated 3-D porous scaffold with the modified WBPU to investigate the effect of the immobilized GRGDSP peptide on human umbilical vein endothelial cells (HUVECs) adhesion and proliferation. A facile and reliable approach was first developed to quantitative grafting of GRGDSP onto the WBPU molecular backbone using ethylene glycol diglycidyl ether (EX810) as a connector. Then 3-D porous WBPU scaffolds with various GRGDSP content were fabricated by freeze-drying the emulsion. In both of the HUVECs adhesion and proliferation tests, enhanced cell performance was observed on the GRGDSP grafted scaffolds compared with the unmodified scaffolds and the tissue culture plate (TCP). The adhesion rate and proliferation rate increased with the increase of GRGDSP content in the scaffold and reached a maximum with peptide concentration of 0.85 μmol/g based on the weight of the polyurethanes. These results illustrate the necessity of the effective control of the GRGDSP content in the modified WBPU and support the potential utility of these 3-D porous modified WBPU scaffolds in the soft tissue engineering to guide cell adhesion, proliferation and tissue regeneration.

Elastase-sensitive elastomeric scaffolds with variable anisotropy for soft tissue engineering

PURPOSE

To develop elastase-sensitive polyurethane scaffolds that would be applicable to the engineering of mechanically active soft tissues.

METHODS

A polyurethane containing an elastase-sensitive peptide sequence was processed into scaffolds by thermally induced phase separation. Processing conditions were manipulated to alter scaffold properties and anisotropy. The scaffold's mechanical properties, degradation, and cytocompatibility using muscle-derived stem cells were characterized. Scaffold in vivo degradation was evaluated by subcutaneous implantation.

RESULTS

When heat transfer was multidirectional, scaffolds had randomly oriented pores. Imposition of a heat transfer gradient resulted in oriented pores. Both scaffolds were flexible and relatively strong with mechanical properties dependent upon fabrication conditions such as solvent type, polymer concentration and quenching temperature. Oriented scaffolds exhibited anisotropic mechanical properties with greater tensile strength in the orientation direction. These scaffolds also supported muscle-derived stem cell growth more effectively than random scaffolds. The scaffolds expressed over 40% weight loss after 56 days in elastase containing buffer. Elastase-sensitive scaffolds were complete degraded after 8 weeks subcutaneous implantation in rats, markedly faster than similar polyurethanes that did not contain the peptide sequence.

CONCLUSION

The elastase-sensitive polyurethane scaffolds showed promise for application in soft tissue engineering where controlling scaffold mechanical properties and pore architecture are desirable.

A biodegradable vascularizing membrane: a feasibility study

Regenerative medicine and in vivo biosensor applications require the formation of mature vascular networks for long-term success. This study investigated whether biodegradable porous membranes could induce the formation of a vascularized fibrous capsule and, if so, the effect of degradation kinetics on neovascularization. Poly(l-lactic acid) (PLLA) and poly(dl-lactic-co-glycolic) acid (PLGA) membranes were created by a solvent casting/salt leaching method. Specifically, PLLA, PLGA 75:25 and PLGA 50:50 polymers were used to vary degradation kinetics. The membranes were designed to have an average 60mum pore diameter, as this pore size has been shown to be optimal for inducing blood vessel formation around nondegradable polymer materials. Membrane samples were imaged by scanning electron microscopy at several time points during in vitro degradation to assess any changes in pore structure. The in vivo performance of the membranes was assessed in Sprague-Dawley rats by measuring vascularization within the fibrous capsule that forms adjacent to implants. The vascular density within 100microm of the membranes was compared with that seen in normal tissue, and to that surrounding the commercially available vascularizing membrane TheraCyte. The hemoglobin content of tissue containing the membranes was measured by four-dimensional elastic light scattering as a novel method to assess tissue perfusion. Results from this study show that slow-degrading membranes induce greater amounts of neovascularization and a thinner fibrous capsule relative to fast degrading membranes. These results may be due both to an initially increased number of macrophages surrounding the slower degrading membranes and to the maintenance of their initial pore structure.

The mechanical properties of infrainguinal vascular bypass grafts: their role in influencing patency

When autologous vein is unavailable, prosthetic graft materials, particularly expanded polytetrafluoroethylene are used for peripheral arterial revascularisation. Poor long term patency of prosthetic materials is due to distal anastomotic intimal hyperplasia. Intimal hyperplasia is directly linked to shear stress abnormalities at the vessel wall. Compliance and calibre mismatch between native vessel and graft, as well as anastomotic line stress concentration contribute towards unnatural wall shear stress. High porosity reduces graft compliance by causing fibrovascular infiltration, whereas low porosity discourages the development of an endothelial lining and hence effective antithrombogenicity. Therefore, consideration of mechanical properties is necessary in graft development. Current research into synthetic vascular grafts concentrates on simulating the mechanical properties of native arteries and tissue engineering aims to construct a new biological arterial conduit.

Treatment of type II endoleaks with a novel polyurethane thrombogenic foam: Induction of endoleak thrombosis and elimination of intra-aneurysmal pressure in the canine model

OBJECTIVE

The clinical significance and treatment of retrograde collateral arterial perfusion of abdominal aortic aneurysms after endovascular repair (type II endoleak) have not been completely characterized. A canine abdominal aortic aneurysm model of type II endoleak with an implanted pressure transducer was used to evaluate the use of polyurethane foam to induce thrombosis of type II endoleaks. The effect on endoleak patency, intra-aneurysmal pressure, and thrombus histology was studied.

METHODS

Prosthetic aneurysms with an intraluminal, solid-state, strain-gauge pressure transducer were created in the infrarenal aorta of 14 mongrel dogs. Aneurysm side-branch vessels were reimplanted into the prosthetic aneurysm of 10 animals by using a Carrel patch. Type II (retrograde) endoleaks were created by excluding the aneurysm from antegrade perfusion with an impermeable stent graft. Thrombosis of the type II endoleak was induced by implantation of polyurethane foam into the prosthetic aneurysm sac of four animals. Six animals with type II endoleaks were not treated. In four control animals, no collateral side branches were reimplanted, and therefore no endoleak was created. Intra-aneurysmal and systemic pressures were measured daily for 60 to 90 days after the implantation of the stent graft. Endoleak patency and flow were assessed during surgery and at the time of death by using angiographic imaging and duplex ultrasonography. Histologic analysis of the intra-aneurysmal thrombus was also performed.

RESULTS

Intra-aneurysmal pressure values are indexed to systemic pressure and are represented as a percentage of the simultaneously obtained systemic pressure, which has a value of 1.0. All six animals with untreated type II endoleaks maintained patency of the endoleak and side-branch arteries throughout the study period. Compared with control aneurysms that had no endoleak, animals with patent type II endoleaks exhibited significantly higher intra-aneurysmal pressurization (systolic pressure: patent type II endoleak, 0.702 +/- 0.283; control, 0.172 +/- 0.091; P < .001; mean pressure: endoleak, 0.784 +/- 0.229; control, 0.137 +/- 0.102; P < .001; pulse pressure: endoleak, 0.406 +/- 0.248; control, 0.098 +/- 0.077; P < .001; P < .001 for comparison for all groups by analysis of variance). Treatment of the type II endoleak with polyurethane foam induced thrombosis of the endoleak and feeding side-branch arteries in all four animals with type II endoleaks. This resulted in intra-aneurysmal pressures statistically indistinguishable from the controls (systolic pressure, 0.183 +/- 0.08; mean pressure, 0.142 +/- 0.09; pulse pressure, 0.054 +/- 0.04; not significant). Angiography and histology documented persistent patency up to the time of death (mean, 64 days) for untreated type II endoleaks and confirmed thrombosis of polyurethane foam-treated endoleaks in all cases.

CONCLUSIONS

Untreated type II endoleaks were associated with intra-aneurysmal pressures that were 70% to 80% of systemic pressure. Treatment with polyurethane foam resulted in a reduction of intra-aneurysmal pressure to a level that was indistinguishable from control aneurysms that had no endoleak.

CLINICAL RELEVANCE

Endovascular repair of abdominal aortic aneurysms is dependent on the successful exclusion of the aneurysm from arterial circulation. Type II endoleaks originate from retrograde flow into the aneurysm sac. This study demonstrates the use of polyurethane foam to induce thrombosis in a canine model of a type II endoleak, thereby reducing intra-aneurysmal pressure to levels similar to levels in animals without endoleaks. This approach may be a strategy for future treatment of type II endoleaks.

Preparation and characterization of highly porous, biodegradable polyurethane scaffolds for soft tissue applications

In the engineering of soft tissues, scaffolds with high elastance and strength coupled with controllable biodegradable properties are necessary. To fulfill such design criteria we have previously synthesized two kinds of biodegradable polyurethaneureas, namely poly(ester urethane)urea (PEUU) and poly(ether ester urethane)urea (PEEUU) from polycaprolactone, polycaprolactone-b-polyethylene glycol-b-polycaprolactone, 1,4-diisocyanatobutane and putrescine. PEUU and PEEUU were further fabricated into scaffolds by thermally induced phase separation using dimethyl sulfoxide (DMSO) as a solvent. The effect of polymer solution concentration, quenching temperature and polymer type on pore morphology and porosity was investigated. Scaffolds were obtained with open and interconnected pores having sizes ranging from several mum to more than 150 microm and porosities of 80-97%. By changing the polymer solution concentration or quenching temperature, scaffolds with random or oriented tubular pores could be obtained. The PEUU scaffolds were flexible with breaking strains of 214% and higher, and tensile strengths of approximately 1.0 MPa, whereas the PEEUU scaffolds generally had lower strengths and breaking strains. Scaffold degradation in aqueous buffer was related to the porosity and polymer hydrophilicity. Smooth muscle cells were filtration seeded in the scaffolds and it was shown that both scaffolds supported cell adhesion and growth, with smooth muscle cells growing more extensively in the PEEUU scaffold. These biodegradable and flexible scaffolds demonstrate potential for future application as cell scaffolds in cardiovascular tissue engineering or other soft tissue applications.

Pore size, tissue ingrowth, and endothelialization of small-diameter microporous polyurethane vascular prostheses

Small-diameter microporous polyurethane vascular prostheses with an average pore size of between 5 and 30mum at the outer surfaces and 30mum at the luminal surface were prepared. Thirty-two PU and 8 expanded polytetrafluoroethylene (ePTFE) prostheses were implanted into the abdominal aorta of rats for periods ranging from 1 to 8 weeks. Harvested prostheses were analysed histologically and morphologically. The progress of endothelial-like cells and the extent of infiltration of perigraft tissues were quantified. All of the prostheses showed fast growth of endothelial-like cells in the second week, with the PU prostheses having an external pore size of 30mum producing the highest rate. It was also during the second week that perigraft tissue grew most significantly into the prosthetic structure. This coincident may suggest the importance of rapid tissue regeneration for the early endothelial healing. The role of the ingrowth perigraft tissues is likely to support and stabilize the neointima. The thickening of neointima was mainly located at the vicinity of the proximal anastomoses of some of the PU prostheses and was unrelated with the extent of perigraft tissue infiltration. In the PU prostheses, a complete lining of endothelial-like cells was achieved by the end of 4 weeks. Expanded PTFE prostheses displayed smooth, thin intima, very limited tissue ingrowth, and incomplete coverage of endothelial-like cells.

Surface microarchitectural design in biomedical applications: in vivo analysis of tissue ingrowth in excimer laser-directed micropored scaffold for cardiovascular tissue engineering

A micropatterned microporous segmented polyurethane film (20 x 12 mm in size, 30 micrometer thick) with four regions was prepared by excimer laser microprocessing to provide an in vivo model of transmural tissue ingrowth in an open cell-structured scaffold specially designed for cardiovascular tissue engineering. Three microporous regions had the same circular micropores (30 micrometer diameter) but different pore density arrangements (percentage of total pore area against unit area was 0.3%, 1.1%, and 4.5%), and the other region remained nonporous. The covered stent, prepared by wrapping the regionally different density-microporous film on an expandable metallic stent (approximately 3.1 mm in diameter), was delivered to the luminal surface of canine common carotid arteries and placed after expansion of the stent to a diameter of approximately 8 mm using a balloon catheter. At 4 weeks of implantation, all the covered stents (n = 10) were patent. The luminal surfaces of the covered stents were almost confluently endothelialized both in nonporous and microporous regions. Histological examination showed that the neointimal wall was formed by tissue ingrowth from host through micropores (transmural) and anastomotic sites. Thrombus formation occurred frequently in the lowest density porous region and nonporous region. With an increase in pore density, the thickness of the neointimal wall decreased. This study demonstrated how the micropore density of implanted devices influences tissue ingrowth in arterial implantation.

A photochemical method for the surface modification of poly(etherurethanes) with phosphorylcholine-containing compounds to improve hemocompatibility

Phosphorylcholine groups attached to polymer surfaces are known to improve hemocompatibility. A photochemical method is presented to couple phosphorylcholine-containing aryl azides to poly(etherurethane) surfaces (PEUs). Two aryl azides that consist of a photoactivatable 4-azidobenzoyl group, a short spacer chain, and a phosphorylcholine endgroup were synthesized. The two compounds differ only in the type of spacer used: triethylene glycol for compound 1 and hexanediol for compound 2. These compounds were physically adsorbed to PEU surfaces. Upon UV irradiation, reactive intermediates are formed that react with nucleophilic groups on the polymer surface. The modified surfaces showed decreased underwater contact angles, indicating that hydrophilic phosphorylcholine groups are present at the surface. ESCA measurements showed the presence of phosphorus and positively charged nitrogen atoms in the outermost polymer layers (analyzed depth about 50 A), which is a strong indication of the presence of phosphorylcholine groups. Hemocompatibility in vitro was tested with thrombin generation assays and platelet adhesion tests. In thrombin generation assays the clotting time of platelet-rich plasma in contact with the polymer surface is determined. Clotting times were clearly prolonged for the modified surfaces. Surfaces modified with compound 2 showed slightly higher clotting times than those modified with compound 1. Repeated surface modification with compound 2 further increased the clotting time. For the tested surfaces an increase in the clotting time corresponds to an increase in the concentration of phosphorylcholine groups at the surface (as measured by ESCA and contact angle). Platelet adhesion studies with scanning electron microscopy demonstrated that fewer platelets (showing less activation) adhered to the modified surfaces than to the unmodified polyurethane.

Controlled delivery of therapeutics from microporous membranes. I. Fabrication and characterization of microporous polyurethane membranes containing polymeric microspheres

This paper describes a process for the inclusion of polymer microspheres in microporous polyurethane tubes and membranes. These composites were fabricated via a spray, phase-inversion technique using Cardiothane 51, a medical grade polyurethane, and either spray-dried poly(D,L-lactide-co-glycolide 50:50) microspheres or commercially available fluorescent polystyrene-latex microspheres. Characterization of the polyurethane membranes was performed using Fouriertransform infrared spectroscopy, differential scanning calorimetry, dynamic mechanical analysis, hydraulic permeability testing, scanning electron microscopy, and visible and fluorescence light microscopy. The results indicated the feasibility of layering microspheres throughout the microporous membrane or wall of the microporous tube, and the potential of such composite structures for local delivery of bioactive substances.

Small caliber vascular grafts. Part II: Polyurethanes revisited

Polyurethanes are considered to be one of the most bio- and blood-compatible biomaterials known today. By intelligent utilization of principles governing the structure/property relationship of these polymers, one can generate systems which resemble, in principle, the physical-mechanical behavior of living tissue. Thus, it is not surprising that these materials played a major role in development of small caliber vascular grafts targeted for vascular access, peripheral and coronary artery bypass indications. Numerous technologies, often esoteric in nature, were and are utilized to generate porous, potentially multilayered conduits possessing some or many characteristics of natural blood vessels. Properties such as durability, elasticity, compliance, pulsatility, and propensity for healing became attainable via polyurethanes. Furthermore, additional surface and/or bulk modification via attachments of biologically active species such as anticoagulants, cell proliferation suppressants, anti-infective compounds or biorecognizable groups are possible due to reactive groups which are part of the polyurethane structure. These modifications are designed to control or mediate host acceptance and healing of the graft. Finally, a myriad of practical processing technologies are used to fabricate functional grafts. Among those, casting, electrostatic and wet spinning of fibers and monofilaments, extrusion, dip coating or spraying of mandrels with polymer/additive solutions are often coupled with chemical-potential-difference-driven coagulation and phase inversion leading to grafts feeling and often behaving like natural vessels. Historically, the first polyurethanes utilized were hydrolytically unstable polyester polyurethanes containing hydrolysis-prone polyester polyols as soft segments, followed by hydrolytically stable but oxidation sensitive polyether polyols based polyurethanes. Polyether-based polyurethanes and their clones containing silicone and other modifying polymeric intermediates represented significant progress. Many viable technologies were discovered and developed using polyether-based polyurethanes. Chronic in vivo instability observed on prolonged implantation became, however, a major roadblock. The path led finally to the use of hydrolytically and oxidatively stable polycarbonate polyols as the soft segment to generate biodurable materials with resistance to biodegradation adequate for vascular access or perhaps peripheral graft indications. This biodurability needs to be further increased in order to utilize the full potential of polyurethanes in development of patent small caliber graft. Modification of both the soft and hard segments needs to be considered in order to maximize biodurability of both basic building blocks of the polyurethane. This paper reviews the achievements, discusses trends, and offers the view of the future in this exciting area of material/device combination.

Surface modification of polymers for medical applications

Most of the conventional materials do not meet the demands required for both their surface and bulk properties when used as biomaterials. An effective approach for developing a clinically applicable biomaterial is to modify the surface of the material which already has excellent biofunctionality and bulk properties. This review article focuses on the surface modification of polymers by grafting techniques, which have long been known in polymer chemistry but are not yet widely applied to biomaterials. A grafted surface can be produced primarily either by graft polymerization of monomers or covalent coupling reaction of existing polymer molecules onto the substrate polymer surface. The major surface properties that should be modified include two kinds of biocompatibility. One is the surface property that elicits the least foreign-body reactions and the other is the cell- and tissue-bonding capability. In addition, physiologically active surfaces with, for instance, selective adsorbability may be required. Attempts to produce these biocompatible or biospecific surfaces by grafting techniques are briefly overviewed in this article.