Surface analysis techniques are important tools to use in the verification of surface cleanliness and medical device functionality. How these techniques can be employed and some example applications are described.
Achieving and maintaining surface cleanliness is a critical issue in many sectors of the industry. Incorrectly applied, surface treatments or contaminants can cause poor product performance or even complete failure. But when the product in question is intrinsic to the ongoing health of a human being, there is no scope for error. Indeed, with issues such as the ongoing fight against MRSA* rarely out of the news agenda, surface treatment for medical devices has never been under closer scrutiny.
Some outside the industry think the solution is simple: just ensure that any functional or cleanliness treatments are applied to medical equipment/devices regularly and accurately. The problem with this, however, is that unless a failure is reported back to the supplier it is hard to determine if the required conditions have been achieved. A validation system is needed that can prove accuracy before a problem occurs and it is here that surface analysis techniques can prove invaluable.
Capabilities of the techniques
Surface analysis techniques study the extreme outer layer of a material for imperfections in topography or molecular content that could directly affect that material’s performance or, perhaps more importantly, the response it elicits from the biological recipient/host. Three of the most widely used surface analysis techniques are described below.
Secondary Ion Mass Spectrometry (SIMS). The surface is bombarded with a primary beam of energetic particles, normally ions. This results in the emission of a range of secondary particles, including positively and negatively charged ions. By analysing the resulting mass spectra, a detailed elemental and chemical breakdown can be achieved. SIMS using mass analysis by the Time-of-Flight method (ToF-SIMS) involves the use of a primary ion beam to produce packets of primary ions. These gently impact on the surface and generate packets of secondary ions at a well-defined point in time. Secondary ions of different mass will have different velocities. Their mass-dependent flight times through the analyser to the time-sensitive detector are measured to produce a spectrum. This mass spectrum can be interpreted to determine what complex molecular species are present, such as drugs, contaminants, lubricants or residual cleaning agents.
X-ray Photoelectron Spectroscopy. This technique provides quantitative compositional information from the top 10 atomic layers of a surface by irradiating it with a beam of monochromatic soft X-rays. The chemical state of elements found can also be quantified using this method. This is particularly useful if the objective of analysis is to validate whether or not a particular treatment used on a material’s surface has, or will, result in a desired chemical effect, for example, sterilisation or passivation.
3D Noncontact Profiling. This technique utilises white light interferometry to provide measurement of height variation in the sample. This provides a detailed map of the outer nanometres of the surface of a medical device and can also provide details relating to the thickness and distribution of treatments.
Sterilisation and cleaning. As mentioned, sterilisation validation is one example of what surface analysis techniques can achieve. Testing the effectiveness of cleaning fluids/procedures at removing contaminants is another crucial use.
Orthopaedic implants. Numerous factors contribute to an implant’s success, but two of the most important are cleanliness and host reaction. Surface analysis techniques can be used to confirm, for example, the presence of any amino acids that may be present after sterilising treatment and therefore confirm or deny the absolute cleanliness of the implant prior to insertion. The actual surface topo-graphy of the implant can be charted to provide additional information to supplement the chemical data. Accurate physical characterisation can help ensure that any treatments are distributed evenly. This can be crucial to the implant’s overall effectiveness, parti-cularly if it is treated with coatings such as hydroxyapatite that encourage the recipient body to accept metallic implants and also help to improve biological growth around them.
Cardiovascular stents. Instead of being treated with a coating that actively encourages biological growth, cardiovascular stents are often treated with coatings that help prevent excessive tissue growth, allowing maximum blood flow through the arteries and valves. Again, it is the behaviour of a material’s extreme surface that dictates whether or not this desired effect takes place. The chemical composition of the stent’s outer layer needs to be receptive, that is, not contain any contaminants that may prevent the coating from lasting or working. In addition, the coating itself must be accurately applied and at a uniform thickness throughout. Without the use of surface analysis techniques, it is extremely difficult to study these aspects of medical devices without the examination itself influencing the outcome.
These example applications demonstrate why the medical device sector has paid such close attention to the development of surface analysis techniques. This interest is likely to grow. Medical professionals, whether device manufacturers or practitioners, are under more pressure than ever to deliver reports and tangible test results on cleanliness procedures and product performance. Surface analysis techniques are likely to provide an increasingly valuable validation resource.
*Meticillin resistant Staphylococcus aureus (MRSA).