Roberto S. Benson

Use of radiation in biomaterials science

Radiation is widely used in the biomaterials science for surface modification, sterilization and to improve bulk properties. Radiation is also used to design of biochips, and in situ photopolymerizable of bioadhesives. The energy sources most commonly used in the irradiation of biomaterials are high-energy electrons, gamma radiation, ultraviolet (UV) and visible light. Surface modification involves placement of selective chemical moieties on the surface of a material by chemical reactions to improve biointeraction for cell adhesion and proliferation, hemocompatibility and water absorption. The exposure of a polymer to radiation, especially ionizing radiation, can lead to chain scission or crosslinking with changes in bulk and surface properties. Sterilization by irradiation is designed to inactivate most pathogens from the surface of biomedical devices. An overview of the use of gamma and UV radiation to improve surface tissue compatibility, bulk properties and surface properties for wear resistance, formation of hydrogels and curing dental sealants and bone adhesives is presented. Gamma and vacuum ultraviolet (VUV) irradiated ultrahigh molecular weight polyethylene (UHMWPE) exhibit improvement in surface modulus and hardness. The surface modulus and hardness of UHMWPE showed a dependence on type of radiation, dosage and processing. VUV surface modified e-PTFE vascular grafts exhibit increases in hydrophilicity and improvement towards adhesion of fibrin glue.

Cell proliferation, viability, and in vitro differentiation of equine mesenchymal stem cells seeded on bacterial cellulose hydrogel scaffolds.

The culture of multipotent mesenchymal stem cells on natural biopolymers holds great promise for treatments of connective tissue disorders such as osteoarthritis. The safety and performance of such therapies relies on the systematic in vitro evaluation of the developed stem cell-biomaterial constructs prior to in vivo implantation. This study evaluates bacterial cellulose (BC), a biocompatible natural polymer, as a scaffold for equine-derived bone marrow mesenchymal stem cells (EqMSCs) for application in bone and cartilage tissue engineering. An equine model was chosen due to similarities in size, load and types of joint injuries suffered by horses and humans. Lyophilized and critical point dried BC hydrogel scaffolds were characterized using scanning electron microscopy (SEM) to confirm nanostructure morphology which demonstrated that critical point drying induces fibre bundling unlike lyophilisation. EqMSCs positively expressed the undifferentiated pluripotent mesenchymal stem cell surface markers CD44 and CD90. The BC scaffolds were shown to be cytocompatible, supporting cellular adhesion and proliferation, and allowed for osteogenic and chondrogenic differentiation of EqMSCs. The cells seeded on the BC hydrogel were shown to be viable and metabolically active. These findings demonstrate that the combination of a BC hydrogel and EqMSCs are promising constructs for musculoskeletal tissue engineering applications.

Effect of gamma irradiation on ethylene–octene copolymers produced by constrained geometry catalyst

Abstract Elastomeric ethylene–octene copolymers of differing molecular weights and comonomer content were exposed to 25 kGy of gamma radiation from 60 Co source in an inert atmosphere. Tests were performed to evaluate the changes in the chemical, physical, and mechanical properties. All properties showed a dependence on both composition and molecular weight. The effect of gamma irradiation is most noticeable in the mechanical properties such as the tensile modulus and elongation to break. The combination of the low octene-1 content and high molecular weight leads to higher post irradiation tensile modulus. FTIR spectroscopy studies indicate that formation of unsaturated chemical moities after irradiation of samples with same molecular weight occurs more readily in the lower octene-1 content copolymers. While at constant composition there is a preference for the higher molecular weight copolymers. The low molecular weight and low octene content copolymer showed some evidence of polymer chain scission as the dominant effect.

VUV modification promotes endothelial cell proliferation on PTFE vascular grafts

Abstract Small diameter (⩽6 mm ID ) synthetic vascular grafts, used as lower-limb vessel replacements in patients without suitable autologous saphenous veins, have a failure rate of 53% after 4 yr. Graft failure is due to thrombosis and intimal hyperplasia, an increase in smooth muscle cells in the lumen of the vessel which leads to progressive closing and ultimate occlusion of the vessel. In an effort to increase patency rates of synthetic grafts, investigators have seeded vascular grafts with endothelial cells prior to implantation in an attempt to control both thrombosis and smooth muscle proliferation. This technique has been successful for the development of an endothelial monolayer in animal trials, but has met with limited success in humans. The hydrophobicity, low surface energy, and weak electrical charge of expanded polytetrafluoroethylene (ePTFE) provides conditions which are not optimal for endothelial cell attachment. The purpose of this study is to evaluate the effect of vacuum ultraviolet (VUV) modification of ePTFE on endothelial cell adhesion and proliferation. Pieces of ePTFE graft material were exposed to 10, 20 or 40 W VUV radiation for 10, 20 or 40 min using a UV excimer lamp. Prior to cell adhesion and proliferation experiments, the grafts pieces were autoclaved and cut into pledgets. Half of the pledgets were precoated with fibronectin ( 20 μg/ml ). Cell adhesion was measured by seeding 3H-thymidine labeled human umbilical vein endothelial cells (HUVEC) onto the pledgets for 60 min. The pledgets were then washed and the remaining radioactivity assayed using scintillation counting. For the cell proliferation experiments, pledgets were seeded with unlabeled HUVEC which were allowed to adhere to the graft material for 18 h. The cells were then exposed to 3H-thymidine ( 1 μCi/ml ) for approximately 48 h and then washed to remove any unincorporated 3H-thymidine. Incorporation of 3H-thymidine was measured using scintillation counting. Four replicate samples each, with and without fibronectin, were evaluated for each power and exposure time for both the adhesion and proliferation experiments. VUV modification had no effect on cell adhesion for all power levels studied. In addition, it appears that cell adhesion is independent of the presence of fibronectin. Cell proliferation, on the other hand, is augmented by modification, especially in the presence of fibronectin. These results suggest that VUV modification may provide a better surface for endothelial cell colonization of synthetic vascular grafts.

ESR investigations on irradiated polystyrene

Abstract Electron spin resonance investigations on the free radicals generated in polystyrene by gamma irradiation, in the temperature range 300–460 K, are reported. Assuming the competition between first and second order recombination processes, the isothermal decay of free radicals has been accurately fitted. It is proved that both the first and the second order isothermal reaction rates are well described by a Vogel–Fulcher–Tamman–Arrhenius equation.

Study of the relationship between crack tip strain and crack propagation in polyurethane films using micro-FTi.r.

Abstract In the present study, a series of polyether-urethane-ureas (PEUU) were selected for investigation of crack propagation behaviour under dynamic loading conditions. These model polyurethanes were synthesized by two-stage polymerization. The hard segments were composed of 4,4′-diphenyl methane diisocyanate (MDI) and ethylene diamine (EDA). The soft segments were polyglycols having different chemical structures and number average molecular weights of 1000 and 2000. Monitoring of the variation in molecular orientation at the crack tip region was accomplished using polarized FT i.r. microscopy. Molecular orientation of the four major functional groups, NH, CH-, C=O, and C=C representing the domain, matrix, and interface region were measured as a function of strain for uncracked samples using the i.r.-dichroism technique. NH- and C=O functional groups present in the urea and correlated with the hard domains behaviour, exhibit a generalized orientation function-strain curve which was characterized by three regions. Region 1 was associated with an initial decrease in the orientation function at low strains followed by region 2, which is the minimum obtainable orientation, and region 3 a subsequent increase in the orientation function with an increase in strain. The molecular orientation was used to determine the real strain at the crack tip. The strains at the crack tip for the pure (PEUU) were between 4 to 7 times higher than the applied strain. It was observed that higher soft segment molecular weights correlated with a larger strain at the crack tip. For the same soft segment molecular weights, polypropylene glycol (PPG) based PEUU showed higher strains at the crack tip. Therefore, the strain at the crack tip depends on chemical structure and the molecular weight of the soft segment. According to the strain data and the generally accepted deformation theory for PEUU elastomers, in all PEUUs, crack propagation occurred after the individual hard segments separated and oriented along the stretching direction.

Biomimetic hydroxyapatite powder from a bacterial cellulose scaffold

In a persistent search to find affordable biomaterial sources, calcium deficient hydroxyapatite (CdHA) precipitated onto bacterial cellulose (BC) composites are excellent candidates. CdHA is resorbable in vivo and biomimetic to HA found in native bone due to its calcium deficiency. The shape of the starting BC scaffold limits BC/CdHA composite applications. In order to make greater use of these composites, this study aimed to investigate the feasibility in generating a CdHA powder from the original composite. The CdHA powder could be utilized at bone injury sites as stand-alone bone filler or as filler in an injectable system. This study selected thermal and enzymatic methods to investigate the effectiveness of BC removal from the original composite matrix. Scanning electron microscopy was used to examine the nanoscale hydroxyapatite rosettes. Energy dispersive x-ray spectroscopy was used to determine the calcium deficiency of hydroxyapatite. Thermogravimetric analysis was used to detect the presence of cellulose in the composites by mass. Fourier transform infrared spectroscopy was used to obtain chemical information of the degraded materials. Each degradation method successfully produced calcium deficient hydroxyapatite powders.

VUV-light-induced deposited silica films

Abstract A novel technique to deposit dielectric films at room temperature is described. The deposition of the silica takes place inside a cylindrical glass chamber where a silent discharge is generated between two electrodes connected to a high voltage, high frequency AC source. The chamber contains two parallel glass tubes where the electrodes are located and is filled with argon or xenon at a pressure of 100 mbar. Under these conditions, it has been shown that high intensity VUV light is generated peaking at 126 nm for argon and at 172 nm for xenon. This VUV radiation seems to produce photoablation of the glass tubes that surround the electrodes. Upon operation of the lamp, polyimide, polypropylene and silicon wafer substrates lying at the bottom of the vessel became coated with silica. The films, identified using X-ray photoelectron spectroscopy (XPS), revealed that the silica is oxygen-deficient with a composition of SiO x where x is between 1.7 and 1.8. The deposition rate on silicon wafers was measured by ellipsometry. When Xe gas is used the deposition rate is much lower than when Ar is used. This result is consistent with a photoablation process since the energy of the photons generated in Ar peaks at 10 eV while those generated in Xe peaks at 7 eV. These energy values should be compared with the O–Si bond strength energy that is 8.3 eV. The morphology and structure of the films were examined by scanning and transmission electron microscopies. Deposition of carbonaceous films occurred when the glass tubes containing the electrodes were coated with carbon.

Dynamic mechanical studies of oriented p-oxybenzoate (POB)-poly(ethylene terephthalate) (PET) copolyester films

Abstract The effects of orientation on the dynamic mechanical properties of p- oxybenzoate poly(ethylene terephthalate) (POB/PET ( 60 40 )) copolyesters were studied using films prepared by extrusion drawing and by uniaxially drawing sections of compression moulded sheets. The extrusion drawn sample ED-1 showed a higher degree of orientation than uniaxially drawn samples U-2S and U-8S. The loss tangent curves for these oriented samples are characterized by a β-relaxation at 62°C and a high temperature (α′) relaxation. The position of the α′-relaxation peak is dependent on the degree of orientation. Samples U-8S (fH = 0.05), U-2S (H = 0.06) and ED-1 (H = 0.21), have their α′ relaxation peak at 135, 140 and 156°C, respectively. The dynamic storage modulus (E′) below 50°C increases with the degree of orientation. The highly oriented sample ED-1 maintains its high storage modulus (4 GPa) over a larger temperature range (25–125°C).

10 – Polymeric Biomaterials

Publisher Summary This chapter provides a brief overview of several medical applications that polymers have made seminal contributions to over the years. Currently, with the rapid growth in modern biology and the collaborative effort, cross-disciplines such as materials science, engineering, chemistry, biology, and medicine, polymeric biomaterials are now being fashioned into bioactive, biomimetic, and most importantly, with excellent biocompatibility. Examples of this newer generation of polymeric biomaterials are also included in this chapter. Applications of biomaterials in ophthalmology include contact lenses, intraocular lenses (IOLs), artificial orbital walls, artificial corneas, artificial lacrimal ducts, glaucoma filtration implants, viscoelastic replacements, drug delivery systems, scleral buckles, retinal tacks and adhesives, and ocular endotamponades. The first generation of polymeric contact lenses was made of poly(methyl methacrylate) (PMMA), a polymer commercially known as Plexiglas and is a classical example of hard or rigid lens material. PMMA can be prepared using bulk free-radical polymerization and lathed into lens shape. IOLs are commonly used to replace natural lenses and provide clear optical imaging for patients undergoing cataract surgery. The most widely used foldable IOL, AcrySof, is fabricated from a copolymer of phenylethyl acrylate and phenylethyl methacrylate with a crosslinking reagent and a UV-absorbing chromophore. Polymers have made significant impact on biomedical research and medical practice, and will continue to be the major workforce for biomaterials in the twenty-first century.