Surface work

Technology has come a long way since the introduction of artificial hips, and today surface modification of materials and devices is becoming increasingly important in a large and growing range of healthcare applications, such as orthopaedic implants.

Developers of medical technologies need to juggle several crucial demands. Enhancing biocompatibility, promoting adhesion, improving wear resistance, preventing corrosion, controlling the chemical and electrical insulation/conducting properties, and generating anti-microbial and anti-bacterial surfaces all provide opportunities for surface modification and coatings to play a major role.

Big advances have been made in developing new surface-modification techniques, and as someone closely involved in pioneering electronic beam (e-beam) applications I predict many more to come.

E-beams are already used for sterilisation and to melt and weld metals. One novel e-beam materials-processing technology offers several surface functionalities. Here a beam is scanned rapidly over a substrate surface to displace material in a controlled manner.

The result is a textured surface comprising an array of protrusions above the original surface and a corresponding array of intrusions or cavities in the substrate. This generates tailor-made, complex surface structures in a range of materials in a few seconds/cm', which could be particularly useful for bone ingrowth surfaces on orthopaedic implants.

The ability to engineer a surface is critical to the bonding or joining of dissimilar materials, such as titanium and bone tissue in an orthopaedic hip implant.

The complex surface structures made possible by e-beams can enhance the performance of orthopaedic implants in several ways. The ideal surface comprises micro and macro-scale porous textures with 're-entrant' features to provide bone ingrowth for long-term implant stability.

Using interconnected pores improves the vascularity of the new bone. Also, robust high-aspect ratio features help fix the implant in the post-operative period before bone growth stabilises the implant. Stiffness can be tailored to match surrounding material.

In contrast to other manufacturing routes, features are sculpted from the original material, greatly reducing the risk of loose particles causing excessive wear of the replacement joint.

One problem with commercially available hip implants made primarily from titanium or cobalt chrome is they tend to be stiffer than bone. This creates a joint mismatch, leading to resorption of the bone, followed by a loosening of the implant. As an alternative, fibre-reinforced composite materials can be tailored to match characteristics of the bone and designed to vary across an implant to account for inherent stress. Composite materials are also being used to improve prosthetic limbs.

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have developed a technology that can be used to enhance the joint between metal and composite parts. This would be between a composite prosthetic, and metal fittings and fixtures, where spikes in the metal penetrate between the fibres resulting in a more-damage resistant joint.

The technology, called Surfi-Sculpt, deflects an electron beam to create hundreds of surface features per second in metal and non-metallic substrates. Joints enhanced with this technology are being examined for automotive and aerospace applications and have the potential to offer similar benefits to the medical profession.

In 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 offers new opportunities for lasers, particularly for 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 — essential for many medical applications. Although excimer lasers have shown potential in modification of polymer surfaces for cell adhesion, further work is required to enhance and optimise their effects.