Biocompatible polymer materials and composites are used for many different purposes (catheters, artificial heart components, dentistry products, vision etc.). One important field is the use of custom designed surfaces to be used as vision implants. It is estimated that half of Americans at the age of 75 and above have developed cataracts [1].

This condition can be cured by surgically replacing the original eye-lens with an artificial lens, known as an intra-ocular lens (IOL). One of the favorable materials in IOL manufacturing is hydrophilic acrylic – a soft, biocompatible polymer. Typically, curved surfaces are manufactured by applying abrasive grinding method using CNC machines. In addition, 2.5D objects/surfaces can be manufactured by means of laser micromachining, however, due to the light-matter interaction mechanisms, the surface of the micromachined objects appears rough (>1µm Ra) therefore is not usable for optical applications. These surfaces can be polished via mechanical methods, however, the process can take a few days [2], which makes it economically challenging. To speed up this process, alternative ways to polish transparent polymers are being looked at.

 The aim of this study is the investigation of polishing capabilities of rough (>1µm Ra) hydrophilic acrylic surfaces using bursts of femtosecond laser (“Carbide”, Light Conversion Ltd) pulses. To start off, femtosecond laser pulses were divided into burst packets of 2, 5, 10, 25 pulses. The temporal separation between the pulses in the burst was fixed 400 ps. By changing shape of the envelope of the burst, different configurations of sub-pulse amplitudes were obtained (from an increasing burst to decreasing). The next step was the preparation of samples, where a surface area of (10x10) mm was ablated to a depth of 0.5 mm. As expected, the surface after the ablation procedure was rough (> 1 µm Ra) and as a result non-transparent. Afterwards, surface polishing experiments were conducted using a galvo-scanner based scanning/focusing system. By changing the average laser power, the scanning velocity of the beam on the surface of the sample, the line scanning pitch and the number of sub-pulses within the burst it was possible to find a regime where the surface roughness can be minimized to  88 nm Ra (the initial value of ~ 1 µm). The produced surface resembles a transparent appearance (see fig. 1b). It was determined that using bursts of femtosecond laser pulses the surface quality can be increased by ~10-fold compared to using the conventional femtosecond regime.