Keizo Makuuchi

Radiation formation of hydrogels for biomedical purposes. Some remarks and comments

Abstract Formation of hydrogels by means of radiation technique is described and some differences connected with irradiation in solid state and solution are pointed out. Structures of primary macroeradicals of some hydrophillic polymers are given. Examples of applications of hydrogels as drug delivery systems, implants, injectable systems, stimuli-responsive systems, hybrid-type organs as well as general requirements for such systems are reviewed. Hydrogel wound dressings produced by radiation technology and marketed in the Central Europe are described.

Electron beam crosslinked PEO and PEO/PVA hydrogels for wound dressing

Abstract In order to prepare polyethylene oxide (PEO) hydrogel for wound dressing, different molecular weight PEO and PEO/poly(vinyl alcohol), PVA blend hydrogels were obtained with electron beam irradiation. Gel formation of PEO in aqueous solution was saturated at 40 kGy and the achieved gel fraction was 60–70%. The PEO hydrogel obtained was very fragile, hence PVA was added at 10–30% to give toughness to the PEO hydrogel. The PEO/PVA hydrogel blend showed satisfactory properties for wound dressing. To evaluate the healing effect of PEO/PVA hydrogel blend for dressing, the hydrogel covered a wound formed on the back of marmots. Healing under the wet environment of the hydrogel dressing had some advantages compared with that of gauze dressing which gives a dry environment: (1) the healing rate is faster, (2) easier to change the dressing, i.e. the hydrogel can be peeled off without any damage to the regenerated surface, and (3) no dressing material remains on the wound.

High-melt-strength polypropylene with electron beam irradiation in the presence of polyfunctional monomers

High-melt-strength polypropylene (PP) was achieved with irradiation by an electron beam generated from an accelerator in the presence of polyfunctional monomers (PFM). Among 16 PFMs, the relatively shorter molecular chain bifunctional monomers such as 1,4-butanediol diacrylate (BDDA) and 1,6-hexanediol diacrylate (HDDA) were the most effective for enhancing the melt strength of PP. The concentration and dose of the HDDA to obtain the high melt strength PP in irradiation under nitrogen gas atmosphere were 1.5 mmol/100 g PP and 1 kGy, respectively. DSC measurement and dynamic mechanical analysis showed that the thermal behavior of the high-melt-strength PP was different from that of the original PP. Crystallinity and crystallization temperature during cooling after heating were lower and higher in high melt strength PP than original PP, respectively. Elongational viscosity at 180°C of the high-melt-strength PP showed a remarkable increase at a certain elongational time with constant strain rate, demonstrating the typical property of high-melt-strength samples. This implies that a few higher molecular chains of PP, formed by intermolecular combination of its chain by HDDA in irradiation, give higher melt strength to induce entanglement of molecular chains.

Growth-promotion of plants with depolymerized alginates by irradiation

Abstract Alginate has been degraded by gamma-ray irradiation from a Co-60 source in liquid state (aqueous solution) and in solid state (powder form). The irradiated alginate with a molecular weight less than 10 4 shows a strong effect on the growth-promotion of rice and peanut. Low concentration of degraded alginate from 4% solution irradiated at 100 kGy is effective for the growth-promotion of plants and the suitable concentrations are ca 50 ppm for rice and ca 100 ppm for peanut.

Kappa-carrageenan–polyethylene oxide hydrogel blends prepared by gamma irradiation

Abstract A composite hydrogel system from kappa-carrageenan (KC) and polyethylene oxide was prepared by γ irradiation. KC was blended with low (LPEO) and medium (MPEO) molecular weight PEO having a concentration of 5% (w/w) at a ratio of 1:1. The properties of the blended hydrogels were evaluated in terms of gel fraction, swelling behavior and gel strength. KC/MPEO hydrogel system has a gel content of 75% at a dose of 10 kGy while 60% gel content was noted for KC/LPEO at the same dose. Water gain of more 10,000% was exhibited by both systems investigated. Gel strength of KC/MPEO was higher compared to KC/LPEO hydrogel. KC/MPEO hydrogel retained 88% (364 N) of its gel strength even after autoclave sterilization.

Radiation crosslinked poly(butylene succinate) foam and its biodegradation

The gel fraction (%) for poly(butylene succinate) (Bionolle) blended with blowing agents was found to be higher than the pure Bionolle due to the presence of other component that also underwent crosslinking. It was also found that the cell diameter of the Bionolle foam produced from irradiated Bionolle blend decreases as the gel fraction increases, because the crosslink density increases and thus it retards the bubble growth. Lower gel fraction, less than 5% was effective enough to give sufficient melt strength for production of foams. It was found that the biodegradability of Bionolle foam in the enzymatic degradation test and soil burial test was higher than that of crosslinked Bionolle (before foaming) because it contained many bubbles.

Protein adsorption characteristics of porous and tentacle anion-exchange membrane prepared by radiation-induced graft polymerization

Abstract A polymer chain containing a diethylamino group was grafted onto the pore surface of a porous hollow-fiber membrane by radiation-induced graft polymerization. Dependence of the protein binding capacity of the membrane on environmental parameters such as salt concentration, pH and temperature was investigated. Saturation capacity of protein bound onto the graft chain containing ion-exchange group was governed by the conformation of the graft chain and the intensity of ion-exchange interaction. The conformation of the graft chain was investigated based on the pore radius of the membrane estimated from the permeation flux of a buffer solution through the membrane. By sufficiently permeating a bovine serum albumin (BSA) solution within the concentration range of 0.2–10 mg-BSA/ml through the membrane, the BSA binding capacity was determined. With increasing salt concentration or pH of the protein buffer solution, the graft chain shrank and BSA binding capacity decreased. On the other hand, the BSA binding capacity slightly increased with increasing temperature, and the conformation of the graft chain was insensitive to temperature in the range from 278 to 303 K. The bound BSA could be quantitatively eluted by permeating a buffer solution containing 0.5 M NaCl, and no deterioration in the BSA binding capacity was observed during five cycles of adsorption, elution and conditioning.

Enzymatic degradation of radiation crosslinked poly(ε–caprolactone)

Abstract Poly( e –caprolactone) (PCL), crosslinked by gamma rays in a super cooled state, had high thermal stability, better transparency and heat shrink property. Biodegradation of such PCL was carried out using lipase AK enzyme in a phosphate buffer solution having a pH of 7.0. The enzymatic degradation was remarkably affected by reaction temperature. Preferable temperatures for enzymatic degradation were 50°C for uncrosslinked PCL and 55°C for crosslinked. The enzymatic degradation rate of crosslinked PCL is slower than that of the uncrosslinked due to network structure. However, it was confirmed that enzymatic degradation occurs even for high crosslink density of PCL such as 80% of gel content.

Heat resistance of radiation crosslinked poly(ε-caprolactone)

Polycaprolactone (PCL) was gamma-irradiated at different phases such as solid state at 30 to 55°C, molten state, and supercooled state, under vacuum or air atmosphere, to elucidate its crosslinking behavior. Irradiation in the molten state (80°C) gave higher gel content compared to room temperature. The resulting gel, however, contains many voids due to the gas evolved during irradiation. Conversely, irradiation of PCL in the supercooled state led to the highest gel content among the three irradiation conditions and it was free of voids. Based on these findings, to evaluate heat resistibility of crosslinked PCL prepared by irradiation in supercooled state, the crosslinked PCL was hot pressed at 200°C to form a film. Unirradiated PCL melted at 60°C. The film prepared from 160 kGy irradiated PCL (crosslinked sample) under an applied load of 0.667 MPa, at a temperature of 110°C did not break even after 3 h. At a temperature of 120°C, the film has a tensile strength of 3 MPa. Furthermore, the film extended by hot pressing is transparent and has high heat shrinkability.

Crosslinking of poly(ε-caprolactone) by radiation technique and its biodegradability

Abstract Poly( e -caprolactone), PCL, (melting temperature, 60°C) was gamma-irradiated in the solid state at 30–55°C, the molten state, and the supercooled state (irradiation at 45–55°C after melting, 80°C) under vacuum to improve its heat resistance. Irradiation of PCL in the supercooled state led to the highest gel content and this polymer has high heat resistance. On the other hand, relatively smaller doses such as 15 and 30 kGy were effective to improve processability of PCL by formation of branch structure during irradiation.