Med-Tech Innovation Surface modification
• chain scission where the polymer chains are broken apart. For bioresorbable polymers such as PLLA, PLG and PLDL, the dominant effect is chain scission, which reduces molecular weight and consequently leads to a change in the nature of the hydrolytic degradation of the polymer.
Bioreabsorbable polymer wire for stitches Bioreabsorbable polymer for screws and pins
Pictured from left: Professor John Orr, Dr Fraser Buchannan, Dr Glenn Dickson and Dr M.-L. Cairns, of Queen’s University Belfast
Primarily, a 1.5 MeV Dynamatron continuous DC e-beam was used, which allowed the energy of the beam to be varied. Varying the energy allows Isotron to control the penetration depth of the electrons emitted. Varying the dose delivered, controls the level of cross-linking or chain scission that occurs within the product. To be able to determine the dose delivered by the beam, tests were performed with dosimeters. The results indicated that using a current of 3.0 mA at varying voltages, the beam would be delivering approximately 3 kGy per second. From these data it is possible to extrapolate the dwell times for the sample under the beam. There is a moving device (a shutter) between the beam and the product. This is opened once the current and voltage are stable. The operation of the beam means voltage and current are on for a period of time before and after the shutter is open. Isotron irradiated the bioresorbable polymer samples
Understanding the role that changes in surface physicochemical properties play in influencing surface erosion and ensuing degradation may be critical in obtaining a degree of control over these phenomena. Therefore, e-beam technology has the potential to be an underpinning methodology for the control of medical device degradation and bioresorption.
The materials
The application of e-beam irradiation treatments was applied to a range of bioresorbable polymers including PLLA, L-lactide/glycolide co-polymer (PLG) and L-lactide/ DL-lactide co-polymer (PLDL). Polymer samples were prepared by compression moulding and cut to standard four-point bend samples prior to e-beam processing. Processing conditions were refined through a series of studies to an identified appropriate energy range of 0.5– 0.75 MeV. E-beam irradiation of samples was performed at Isotron.
E-beam irradiation E-beam radiation is a stream of highly energised electrons produced by an electron accelerator. E-beam works in a similar manner to gamma radiation, but has a number of advantages: • it is highly controllable • it has smaller penetration depth and therefore is more suited to small-scale medical devices
• it is more cost-effective for small batches. The effect of e-beam radiation on polymers has been well established. The two main outcomes of the irradiation of polymers are • cross-linking where the polymer chains are linked together
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using three beam energy levels: 0.5, 0.75 and 1.5 MeV. At each energy level doses of 150 kGy and 500 kGy were delivered. To avoid excessive temperature rises during the irradiation process, the delivery of 500 kGy was staggered. Individual doses of 150, 150, 100 and 100 kGy were delivered with a 2-minute cool down period in between. Previous studies confirmed the expected changes in bulk polymer properties such as molecular weight, mechanical integrity, crystallinity and thermal properties. To investigate the effect of e-beam radiation on bioresorbable surface properties, a variety of analytical techniques are used including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and atomic force microscopy (AFM). For confirmation of associated changes in degradation behaviour, accelerated hydrolytic degradation (70 °C) was also performed and samples monitored for mass loss and changes in surface topography via scanning electron microscope (SEM).
Results
E-beam irradiation causes the radiolytic degradation of bioresorbable polymers PLLA, PLG and PLDL through chain scission. This produces changes in the physical properties of the material, including molecular weight, morphology and mechanical strength. The extent to which the materials are affected depends on the surface dose delivered. By increasing the delivered surface dose, the effect on molecular weight of the bioresorbable polymers studied was enhanced. Furthermore, with control of the beam energy, the penetration depth of the beam in the polymers can be manipulated; lower energies produce a shallower penetration depth and therefore a more superficial effect on polymer properties. Controlling the penetration depth of the e-beam to within a near surface region has produced the effect of initiating pseudo surface erosion during hydrolytic
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