  Flexible microfluidics have found extensive utility in the biological and
biomedical fields. A leading substrate material for compliant devices is
polydimethylsiloxane (PDMS). Despite its many advantages, PDMS is inherently
hydrophobic and consequently its use in passive (pumpless) microfluidics
becomes problematic. To this end, many physical and chemical modifications have
been introduced to render PDMS hydrophilic, ranging from amphiphilic molecule
additions to surface plasma treatments. However, when transitioning from lab
benchtop to realized medical devices, these modifications must exhibit
long-term stability. Unfortunately, these modifications are often presented but
their mechanisms and long-term stability are not studied in detail. We have
investigated an array of PDMS modifications, utilizing contact angle goniometry
to study surface energy over a 30-day evolution study. Samples were stored in
air and water, and Fourier Transform Infrared-Attenuated Total Reflectance
(FTIR-ATR) analysis was used to confirm surface functional group uniformity. We
have identified preferred modification techniques for long-lasting PDMS devices
and characterized often overlooked material stability.
