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Development of microfluidics has focused on carrying out chemical synthesis and analysis
in ever-smaller volumes of solution. In most cases, flow systems are made of either quartz,
glass, or an easily moldable polymer such as polydimethylsiloxane (Whitesides 2006). As the
system shrinks, the ratio of surface area to volume increases. For studies of either free radical
chemistry or protein chemistry, this is undesirable. Proteins stick to surfaces, biofilms grow on
surfaces, and radicals annihilate on walls (Lewis et al. 2006). Thus, under those circumstances
where small amounts of reactants must be employed, typical microfluidic systems are incompatible
with the chemistry one wishes to study. We have developed an alternative approach. We
use ultrasonically levitated microliter drops as well mixed microreactors. Depending on whether
capillaries (to form the drop) and electrochemical sensors are in contact with the drop or whether
there are no contacting solids, the ratio of solid surface area to volume is low or zero. The only
interface seen by reactants is a liquid/air interface (or, more generally, liquid/gas, as any gas may
be used to support the drop). While drop levitation has been reported since at least the 1940's,
we are the second group to carry out enzyme reactions in levitated drops, (Weis; Nardozzi 2005)
and have fabricated the lowest power levitator in the literature (Field; Scheeline 2007). The low
consumption aspects of ordinary microfluidics combine with a contact-free determination cell
(the levitated drop) that ensures against cross-contamination, minimizes the likelihood of biofilm
formation, and is robust to changes in temperature and humidity (Lide 1992). We report kinetics
measurements in levitated drops and explain how outgrowths of these accomplishments will lead
to portable chemistry/biology laboratories well suited to detection of a wide range of chemical
and biological agents in the asymmetric battlefield environment.
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