Wednesday July 2, 2008 PDMS, UVO and Contact Angle

Using the 10:1 pre-polymer/curing agent ratio, we prepared several vials of PDMS and poured them into the glass slide molds Chelsea has; we made slabs of PDMS. We cured the PDMS for 20 minutes in the 110C oven, then cut around the edges of the mold with a razor blade to de-mold the solid PDMS slab. The "good" side of the slab is the side that was down inside the mold (uh-oh!), because the top as it sits in the mold will have a meniscus (bad). So, for future reference, it is important to place the de-molded slab down-side facing up on the glass slide. The slab is stored in a Petri dish to minimize dust and dirt. We then expose these slabs to UVO cleaning. The UVO cleaner has its on/off switch on a power strip to the left of the instrument. It needs to be turned on 5 minutes prior to use to get to temperature. The UVO cleaner exposes the samples to free oxygen radicals and UV light; this causes oxygen to replace the methyl groups on the polydimethylsiloxane (PDMS), making the surface more hydrophilic. Eventually, an H attaches to the O making an OH group. We expose samples for 5, 10, 20 and 30 minutes.

After this has been accomplished, we use the Zeiss inverted microscope to examine Jennifer's PDMS sample, made from Doug's mold, and take pictures of the hex holes to use in calculating the volume of the wells. We waste an amazing amount of time trying to draw the hex array to label which holes we are measuring; Chelsea is amused by this as this step is totally unnecessary. I have a gorgeous useless drawing of a hex array in my lab notebook now though. We develop a system for labeling the hex arrays and we photograph three circles from each hex on the PDMS sample. The hex with the smallest holes is in the upper left corner, and that is array A1 and we sample 3 holes in that array: a, b, and c. The horizontal rows of hexes have numbers, and the vertical columns of hexes have capital letter designations. We can now easily locate the hex for each photograph. The images we take are saved in the Users folder, RET subfolder in the desktop.

In the afternoon we go to the Carter lab on the fourth floor and learn how to use the contact angle measuring device, the VCA Optima made by AST products, pictured above. The procedure for using this instrument may be found on the side margin of this blog. The contact angle measurement gives us a quantified measurement of the hydrophobic/hydrophilic nature of material. We are testing the static contact angle today to quantify the effect of UVO exposure of PDMS. We observe a relationship between UVO exposure and contact angle measurements: the greater the UVO exposure, the more hydrophilic the PDMS surface (makes sense. more O present on PDMS surface) and the lower the contact angle. We need to test more samples with different amounts of UVO exposure so that we can construct a graph of our results. Samples tested today are 0 , 5,10,20, and 30 minutes of UVO exposure. We will make samples with 45, 60, 75, and 90 minutes of UVO exposure next week to test.