This review of surface modification technologies focuses on the use of power beams, that is, lasers and electron beams, to modify polymer surfaces. A novel electron-beam materials processing technique is also described that offers scope for generating new types of surface functionality on metals used to construct stents and implants.
Surface modification of materials and devices is important in many health-care applications. Enhancing biocompatibility, promoting adhesion, improving wear resistance, preventing corrosion, providing hydrophilicity, hydrophobicity and electrical insulation/conducting properties, and generating antimicrobial and antibacterial surfaces are examples of where surface modification and coatings can play major roles. Considerable advances have been made in developing new surface modification techniques. A review of the most promising ones has recently been published.1,2 This article examines the use of lasers and electron beam (e-beam).
Laser surface modification
In the area of manufacturing, lasers are used for drilling, cutting, joining, hardening, surface modification and micromachining. The development of new laser sources, advanced laser optics and control systems offer new opportunities for lasers particularly in modifying surfaces of polymers. For example, the compact and energy efficient diode and fibre lasers enable higher process efficiencies and are relatively easy to integrate into manufacturing lines. New types of beam-forming optics and beam-monitoring systems allow the process to be controlled and conducted with high precision and accuracy, which are essential for many medical applications.
Lasers, particularly ultraviolet, femtosecond and picosecond pulsed lasers, offer great potential for surface modification. Illuminating many substances (in solid, liquid or gaseous form) with high-energy photons can cause a change to their chemical nature or structural form. The process relies on short, intense bursts of light to create a rapid rise in pressure to break chemical bonds or to induce photochemical reactions, generating new functional groups at the polymeric material surface. In a confined volume, the bond breaking increases the local particle number density (that is, pressure). The rapid rise in pressure is released as a shock wave that ejects material fragments as gases and particulates at high speed. The process takes place with little excess heat transferred to the surrounding material and as a result can be used to great effect in polymers. Processing of material with excimer lasers is usually most efficient when performed using an appropriate mask inserted into the beam. Images (usually reduced) of the mask patterns are then relayed via a lens onto the work piece. The resolution and size of the image required determines the complexity of the imaging optical system.
Figure 2: SEM of fibroblasts on 7-µm pillars with three pulses (1.5 µm deep).
Excimer laser ablation has been used to produce surface microtextures on polycarbonate and polyetherimide.3 Square features of 7, 25 and 50 µm were produced with depths of 0.5–2.5 µm. Scanning electron and confocal micrographs from typical laser treated surfaces with fibroblast cultures are shown in Figures 1 and 2. A 50-µm grid (Figure 1) with five pulses allowed the cells to spread. The 7-µm grid with three pulses (Figure 2) caused the cells to spread and elongate. The results obtained from fibroblasts demonstrated that the textured polymer surfaces showed good cytocompatibility. It was concluded that further texturing and edge effects may lead to an increase in adhesion of the neutrophils on polymer surfaces.
The cellular recognition of microtextures produced by argon fluoride (ArF) excimer laser ablation of polyetheretherketone (PEEK) has also been investigated.4,5 Full characterisation of surfaces, topographically and chemically, before and after ablation was performed. Surface grooves were produced on a medical grade of PEEK with an ArF excimer laser (λ = 193 nm). Nickel masks, produced by electroforming and features of 20, 40, 60 and 100 μm were used to produce the grooves. The process was performed above the ablation thres-hold at a fluence of 200 mJ/cm2 in air. For contact masking various depths were obtained by varying the number of scans (one, two and four scans).
Surface characterisation was conducted using a range of analytical techniques. Preliminary cellular interaction with the projection imaging ablated surfaces was investigated. MG63 osteoblast-like cells were seeded onto the PEEK following sterilisation in an autoclave at 124°C for 45 min. After 48 hours the samples were dehydrated using a graded series of ethanol and fixed with 2% paraformaldehyde for FEG-SEM observation.
Microtopography was successfully introduced onto the PEEK surface using laser ablation. The ablated regions of the surface were thought to be more hydrophobic, which may be the result of a change in surface chemistry or an increase in surface roughness. Cell alignment was observed on the (unablated) surface ridges, although it was not clear if preferential attachment to these ridges was due to changes in surface topography or surface chemistry. It was concluded that microtopography generated by laser ablation of PEEK was shown to control osteoblast-like cell orientation with preferential cell attachment to the surface ridges.
Although excimer lasers have shown some potential in modification of polymer surfaces for cell adhesion, further work will be required to enhance and optimise their effects.
E-beam surface modification
E-beams are already used for sterilisation and to melt and weld metals. A novel e-beam materials processing technology has been developed that offers a variety of surface functionalities.6 In this technique, an e-beam is deflected rapidly over a substrate surface to displace material in a controlled manner. The result is a textured surface consisting of an array of protrusions above the original surface and a corresponding array of intrusions or cavities in the substrate. The process can be used to generate tailor-made, complex surface structures in a range of materials in a few seconds/cm2.
Principles of the process
E-beam texturing7 is the precursor to this novel materials processing technology. In that process, the e-beam melts and starts to vaporise the substrate material; the vapour pressure then causes molten material to be expelled to the periphery of the hole. Electromagnetic coils are used to focus the e-beam and then deflect it over the material and give a rapid and controlled process. The surface may have re-entrant features and typical processing speeds are 500–5000 holes per second.
Figure 3: A diagram showing the movement of the e-beam to build up a sculpted surface.
This texturing process has now been advanced with a novel sculpting technology.8 In this latest development once a molten pool of material has been created, the beam is translated sideways. The combined effects of vapour pressure and surface tension allow the material from the hole to be piled up behind the beam (Figure 3). By repeating this process many times at the same or overlapping sites, protrusions up to several milli-metres high may be grown, and each will be accompanied by one or more corresponding intrusions or holes. A series of protrusions can be built up simultaneously across a substrate.
With careful control of the e-beam process parameters (including, beam accelerating potential, beam current and focus), the design of a unique pattern, and precisely defined deflection movements, it is possible to create a variety of different surfaces. These include high aspect ratio spikes, burr-free holes, blades, channels, swirls and networks. Within any pattern, the size, shape, angle of incidence and distribution of the features can all be varied to produce customised surfaces. Currently, protrusions ranging in dimensions from tens of microns to several millimetres have been successfully made. This process has the flexibility to create a variety of structures, many of which may be impossible to produce using any other processing route. It is performed under vacuum, thereby avoiding surface contamination. The technology has many potential applications, some of which are described below.
Stents are now widely used with minimally invasive surgery to open and support the walls of the coronary artery. In recent years the trend has been to use drug-eluting stents in which the stents are coated with a slow release polymer coating that contain drugs designed to prevent restenosis, the re-narrowing of an artery after it has been treated with angioplasty or stenting.
Drug eluting stents have recently received some bad press, following reports that they may increase the likelihood of late thrombosis (blood clotting) compared with bare metal stents.9 It has been suggested that this may be attributed to an allergic reaction to the polymer coating of the stent. The sculpting technology offers the possibility of creating shaped cavities that could be filled with the drug for controlled release. This system would reduce the surface area of coating in contact with the body at any one time and hence reduce the chance of an adverse reaction occurring.
Figure 4: “Fin” like features in stainless steel.
Developments to take the technique down to the micron level are on-going. Processing at this scale has been made possible through the design of a high-brightness e-beam system and this opens up possibilities for microengineering products such as stents. To date, holes of 10 μm diameter have been drilled in metallic foils and Figure 4 shows sample features of the order of 10s of μm in stainless steel.
The complex surface structures made possible by this technology have the potential to enhance the perform-ance of orthopaedic implants in a number of ways. It is thought that the ideal surface could consist of micro- and macro-scale porous textures with re-entrant features to provide osseointegration for long-term implant stability. Use will also be made of interconnected pores to improve the vascularity of the new bone. Furthermore, robust high aspect ratio features may provide fixation of the implant during the postoperative period before bone growth stabilises the implant. The stiffness of the complex structures produced may be tailored to better match the surrounding material. Early demonstrator surfaces are shown in Figure 5 which will require further optimisation. In contrast to other manufacturing routes, features are sculpted from the original material with this technique. Therefore the risk of loose particles causing excessive wear of the replacement joint is greatly reduced.
Figure 5: Potential for novel surface treatment of orthopaedic implants.
Commercially available hip implants are primarily manufactured from titanium or cobalt chrome. These materials have stiffness an order of magnitude higher than that of bone, which may create a mismatch at the joint, leading to resorption of the bone followed by loosening of the implant. Fibre reinforced composite materials allow the material properties to be tailored to match those of the surrounding bone and be designed to vary across an implant to take account of the inherent stress state. For these reasons, the potential of fibre reinforced composite materials is being evaluated for this application. The ease of moulding and lightweight nature of composite materials is also being exploited in the design of improved prosthetic limbs.
Often composite materials still require the use of metal fittings and fixtures, which involves the challenge of joining composites to metal. Typically, this is achieved through the use of overmoulding processes or by adhesively bonding the two parts. When these processes fail, they often do so in a catastrophic manner and especially when subject to adverse environmental conditions. The novel sculpting technique has shown promise in improving the properties of composite to metal joints.10 The metallic component is processed with an e-beam to give the required surface profile prior to being enmeshed with the fibrous structure of the composite material, in a Velcro-like manner.
Figure 6: A tube processed with a travelling e-beam sculpted pattern.
Components of almost any geometry may be processed, including cylindrical shapes (Figure 6). The spikes in the metal penetrate between the fibres resulting in a more damage resistant joint. These joints are currently being examined for automotive and aerospace applications and would offer similar benefits to the medical industry.
Promise has been shown in the use of this technology to promote adhesion between a substrate and a coating. The re-entrant features provide improvements in mechanical interlocking with adjoining parts and the protrusions help to distribute the stresses more evenly across a joint interface. The flexibility of the process may also be exploited to tailor-make a surface. For example, protrusions may be aligned in the direction of maximum stress or the density of features may be altered to distribute the stress uniformly throughout a component. The fact that the process is performed under vacuum and consequently produces a “clean” surface, offers advantages for bonding applications. Figure 7 shows a titanium surface coated by a thermally sprayed alumina coating. Although in this case, the coating follows the profile of the surface to some extent, there was evidence that the coating was preferentially filling the cavities. It is expected that a finer scale surface profile would allow a smoother surface to be created.
Figure 7: An alumina coated titanium surface.
Direct surface modification techniques that can consistently and effectively modify surfaces at a low or reasonable cost and can be adapted for small and large surfaces and complicated geometry are becoming more important. The emergence of a new generation of low power lasers with unique properties has increased the possibility of their use in many new exciting applications. Excimer lasers offer great potential for surface modification, particularly of polymers.
In addition, the medical industry has a range of demands for the surfaces of implants and stents to enhance their performance. Advances in e-beam technology have resulted in the development of a novel surface processing technique that is capable of addressing these needs by rapidly producing bespoke surface features in a clean environment. It is being investigated for a range of medical applications.
1. P.V. Vadgama, “Surfaces and Interfaces for Biomaterials” ed. P.V. Vadgama, Woodhead Publishing Ltd, ISBN 1 85573 930 5, 2005.
2. S.M. Tavakoli, “Surface Modification of Polymers to Enhance Biocompatibility,” Chapter 26, 719–741, Surfaces and Interfaces for Biomaterials, ed. P.V. Vadgama, Woodhead Publishing Ltd, 2005.
3. J.A. Hunt et al., “Laser surface Modification of Polymers to Improve Biocompatibility,” 12th European Conference on Biomaterials, Porto, Portugal, 10–13 September 1995.
4. V.I. Corfield et al., “Surface Modification of Peek To Enhance Biocompatibility,” presentation T109, 17th European Society for Biomaterials Conference, Barcelona, Spain, 11–14 September 2002.
5. V.I. Corfield et al., “ArF laser Ablation of PEEK to Introduce Microtopography and Control Cell Interaction,” 7th World Biomaterials Conference, 17–21 May 2004.
6. Surfi-Sculpt, International Patent Publication Number WO 2004/028731 A1, “Workpiece Structure Modification,” Applicant: The Welding Institute, Inventors: B.G.I. Dance and E.J.C. Kellar.
7. International Patent Publication Number WO 2002/094497 A3, “Modulated Surface Modification,” Applicant: The Welding Institute, Inventor: B.G.I. Dance.
8. A.L. Buxton and B.G.I. Dance, “Surfi-Sculpt, Revolutionary Surface Processing With an Electron Beam,” Proceedings, 4th International Surface Engineering Congress, 1–3 August 2005 (ISBN: 0-87170-835-3).
9. Stenting newsletter pp 20–21, 29 September 2006.
The authors acknowledge the work of Symmetry Medical (www.symmetrymedical.com) in the field of orthopaedic implants and are grateful for the contribution it has made to this article. The work of Dr Vicki Corfield, and the support of Dr Ruth Cameron and Professor Bill Bonfield at Cambridge University, and Dr Rachel Williams and Dr John Hunt at Liverpool University on laser surface modification are also greatly appreciated.
Professor Mehdi Tavakoli* is a Consultant and Technology Manager, Advanced Materials and Processes Group at TWI Ltd, e-mail: firstname.lastname@example.org
Anita Buxton is Senior Project Leader, Electron Beam Processes Group
Ian Jones is Senior Project Leader
Bruce Dance is Research Metallurgist, Electron Beam Processes Group all at TWI Ltd, Granta Park, Great Abington, Cambridge CB21 6AL, UK, Tel. +44 1223 899 000, www.twi.co.uk
* To whom all correspondence should be addressed.