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Enhancing Polymer Surface Degradation

29/02/2012


Electron-beam irradiation can be used on medical devices to achieve controlled degradation of bioresorbable polymers. Here, a group of academic and industry researchers shows its effects on the surface properties of poly-l-lactide.

The investigation
The mechanism of degradation of bioresorbable polymers can be classified as either surface erosion or bulk erosion. For applications such as tissue fixation, drug delivery and regenerative medicine, surface erosion would be the more desirable mode of degradation, whereby the rate of degradation of polymer bonds is faster than the rate at which water can diffuse into the matrix. Thus, the device will degrade from surface to core.

An important attribute of bioresorbable polymers is the ability to tailor both their mechanical properties and their degradation kinetics, which has led to their use in the above applications.1 Bioresorbable polymers have generated significant expectation as materials for achieving improved osteosynthesis. A central challenge for these materials remains the assurance of consistent and predictable in vivo degradation. Moreover, the associated "acid burst effect,” which can occur during implantation, also needs to be addressed.2 A way in which to refine the control of polymer degradation is to use electron-beam (e-beam) radiation.

Previous work illustrated the potential of using e-beam radiation at energies of <1.5 MeV to influence polymer properties in a depth-dependent manner that leads to the initiation of pseudo surface erosion in vitro.3 Moreover, the precise manipulation of e-beam process conditions allowed for the treatment of bioresorbable polymers on a scale relevant to medical devices such as anterior cruciate ligament interference screws. By using lower energies and higher doses, it is envisaged that the critical need for an early initiation of surface degradation/erosion in a controlled manner will be addressed.

The potential of e-beam radiation to simultaneously affect the surface properties of bioresorbable polymers such as poly-l-lactide (PLLA) is an area of investigation. 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. 












                                                                        Bioreabsorbable polymer wire for stitches              Bioreabsorbable polymer for screws and pins


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
  • 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.

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 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 degradation. The nature of this degradation in terms of surface erosion can be related to the energy used to deliver the surface specific dose.

As expected, mechanical failure of the sample was found to be dependent on energy and dose of e-beam irradiation with cracks initiating from the irradiated edge and propagating towards the sample centre. Moreover, the extent to which mechanical degradation was observed is synonymous with the energy being delivered by the beam.

XPS analysis of irradiated and non-irradiated PLLA sample surfaces indicated the presence of all expected elements, namely oxygen and carbon. The calculated O1s/C1s ratios are increased for all irradiated polymer samples compared with the non-irradiated controls. Raman data showed all spectral lines characteristic of the polymers studied (Figure 1). Spectral data collected for irradiated PLLA samples showed subtle differences in the size and resolution of peaks associated with νC-COO stretching vibrations indicative of changes in PLLA chain length. Accelerated degradation studies showed increasing mass loss with increased e-beam energy and surface dose. SEM examination of degraded surfaces revealed increased levels of surface cracking for irradiated surfaces compared with the non-irradiated control samples.       


                                 Figure 1: Calculated O1s/C1s ratios for all sample types


Conclusion

E-beam irradiation is a major underpinning technology that can be used to achieve predictable and controlled degradation of bioresorbable polymers. Furthermore, this can then allow degradation to proceed in a manner occurring from the outside of the device towards the centre, engendering early stage mass-loss, maintenance of internal mechanical strength and ultimately the provision of optimum conditions for tissue healing.

This work has illustrated the potential of e-beam technology in achieving a depth-dependent degradation rate and ultimately improved bioresorbable medical devices.

References
1. A.P. Gupta and V. Kumar, "New Emerging Trends in Synthetic Biodegradable Polymers – Polylactide: A Critique,” European Polymer Journal, 43, 10, 4053–4074 (2007).  
2. W. Heidemann et al., "Degradation of Poly(D,L)lactide Implants With or Without Addition of Calciumphosphates In Vivo,” Biomaterials, 22, 2371–2381 (2001).
3. M-L Cairns et al., "Through-Thickness Control of Polymer Bioresoption via Electron Beam Irradiation,” Acta Biomaterialia, 7, 2, 548–557 (2011).



The academic investigators from the left are:
Professor John Orr and Dr Fraser Buchanan of the School of Mechanical and Aerospace Engineering, Queen’s University Belfast;Dr Glen Dickson of School of Medicine, Dentistry and Biomedical Sciences, Queen’s University Belfast and Dr Marie-Louise Cairns of the School of Mechanical and Aerospace Engineering, Queen’s University Belfast, Belfast UK.





The industry collaborators are:
David Farrar, Technology Manager Biomaterials at Smith & Nephew, and



Arthur Dumba, UK Quality Manager at Isotron, Moray Road, Elgin Industrial Estate, Swindon SN2 8XS, UK, tel. +44 (0)8456 889 977,
e-mail: arthur.dumba@isotron.com,
www.isotron.com





   

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