Laser treatment of materials offer advantages over both chemical and other physical methods. They enable precise modification of certain surfaces that are difficult to treat with conventional chemical methods. The resulting modified surfaces are free from contaminants. Most importantly, the bulk properties of the material remain intact. The development of laser-assisted modification of polymer surfaces is a rapidly growing and developing field that has gained considerable interest among scientists in the past decade.

A substantial number of experiments and studies have been carried out by researchers in using lasers to alter the surface property of polymers. Carbon dioxide (CO 2) laser and excimer laser have been found to be able to modify the surface and produce micro- and nano-patterning on surfaces of thin polymer films such as polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET).

The design and fabrication of thin membrane-like matrices are currently being investigated by a number of research groups in the field of biomedical sciences. Laserskin TM, a hyaluronan-based membrane of 20 m m thickness with laser-drilled micro-perforations is used for cell cultivation. However, the relatively weak mechanical properties of hyaluronan base materials render them difficult to handle. To overcome this problem, the research group led by Professor Teoh Swee Hin from Centre For Biomedical Materials Applications and Technology (BIOMAT) successfully developed biaxially stretched poly( e -caprolactone) (PCL) film as a mechanically sound ultra-thin template for membrane tissue engineering. To date, PCL films between 1 and 10 m m thick have been successfully fabricated.

In order to mimic the microholes of Laserskin TM, which act as passages for cells to migrate between top and bottom surfaces of the film, BIOMAT developed perforated PCL membranes using a needle-operated robot. However, the flaps (Figure 1) produced at the sites of needle punching tend to close when placed in a culture solution. Therefore to address this problem, laser processing is being developed in collaboration with Dr Hong Minghui from the ECE/DSI Joint Laser Lab. A long pulse Nd:YAG laser and an ultra-short pulse femtosecond laser have been used to perform the micro-drilling of PCL films. Clean and neat micro-holes of diameter from 5 to 100 m m wide can be successfully achieved (Figure 2).

Figure 1: Needle perforated PCL film showing undesired flaps.	Figure 2: Clean and neat laser micro-drilling on PCL film by (a) Nd: YAG laser and (b) Femosecond pulse laser.

Both lasers make entirely different mechanisms in material removal. The Nd:YAG laser uses the light wavelength of 355 nm and a pulse duration of 7 ns. The photo-thermal interaction between laser light and material results in heating, melting and eventually vaporizing the material, thereby producing the micro-holes. While the ultra-short pulse femtosecond laser has the light wavelength of 800 nm and a pulse duration of 100 fs. This ultra-short pulse duration means the interaction time between the laser pulse and material is very short, and hence symptom of melting is kept at a minimum. Material pull-out by the intense electric field generated by the laser fluence is the main force in the micro-drilling. The different laser parameters such as laser fluence, pulse repetition rate, scanning speed and focal length can be controlled to give the desired micro-perforation specifications.

Both lasers show capability of fine micro-drillings with uniform hole dimensions and good repeatability, and prove to be a vital micro-processing method for thin biodegradable films in membrane tissue engineering.