By: D.G. Waugh and J. Lawrence

Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire, LE11 3TU, UK

Over the past 10 years it has been shown that polymers have the potential to be used in a number of applications such as biomedical and MEMS/NEMS. In general, it is seen that most polymer materials possess poor adhesion characteristics and, as a direct result of this, tend to be difficult to wet, having inferior bond qualities. Numerous researchers have endeavoured to surface modify the surface of polymers to improve upon the adhesion and wettability characteristics. This has especially been seen throughout biomedical applications in which polymeric biomaterials have adequate bulk properties but lack sufficient surface properties in terms of sufficient cell response.

A number of techniques have been implemented to achieve surface modifications such as surface coatings, plasma and lithography. An alternative method offering flexibility, accuracy and negligible bulk modification is that of laser surface patterning. Utilizing lasers in this manner can be seen to a very attractive method for varying surface properties as they also allow for relatively clean processing emphasizing the reduced need for post-processing.

The study of wettability has been a major focal point for many researchers and has been applied to a wide range of applications such as biomedical, coating technologies and adhesion. As the interface between a solid and liquid can be very complex it is necessary to account for the wetting regime that takes place. Two of the most common wetting regimes is that of Wenzel and Cassie-Baxter which are explained in Figure 1.

In some cases it has been seen that it may be possible for a hydrophilic surface to give rise to some form of mixed state wetting regime owed to the roughness and topographical pattern on the surface. Leading on from this, it can be seen that it would be advantageous to devise a repeatable technique which would allow material surfaces to be modified in order to optimize that surface for the required application.

This study has demonstrated that a relatively inexpensive, low power, 10 W cw CO₂ laser holds the ability to significantly modify the surface of PMMA on the micron scale. In terms of wettability it was seen that, irrespective of an increase in surface roughness and surface oxygen content, the contact angles differed over the patterns induced using the laser system.

From the results obtained, there does not appear to be a clear parameter which appears to be dominating in order to explain the differing contact angles observed. However, it is possible to see that different wetting regimes may arise because of the variation in surface topographies between the samples. That is Wenzel, Cassie-Baxter or even an intermediate mixed state wetting regime may be taking place. Different regimes from sample to sample would explain the varying values of contact angle observed. As a result, more work is required to determine the contributing factors which establish the contact angle taking place.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

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