Cryoprotective agents (CPAs) are used during cryopreservation of cells in order to
protect them from freezing injuries and increase the viability post thaw. Dimethyl
sulphoxide (DMSO) is the most commonly used and in this investigation we aim to
understand the feasibility of using a two stream microfluidic device to introduce a 10%
(v/v) solution of DMSO into a cell suspension. DMSO will change the osmolarity of the
solution and if preservation solutions containing DMSO are not added appropriately, cells
may respond adversely. We have developed a microfluidic device that can be used to add
cryopreservation solutions in a gradual fashion and avoid cell damage.
The two-stream microfluidic channel is operated in two different flow configurations, one
with a DMSO rich heavy donor stream on the top of the cell stream and the other with the
vice-versa arrangement, for a range of Reynolds numbers (0.7 < Re < 10) The effects of
different initial donor stream concentrations (1% - 15%) and flow rate fractions (0.23 –
0.77), on the transport of CPA molecules from one stream to another, is experimentally
modeled for these configurations. For the former flow configuration, convective motion
is produced (along the depth of channel) due to the influence of gravity resulting in high
outlet cell stream concentrations even for very low residence times within the channel.
This behavior is directly contrasting to the observations in the alternate configuration
where-in mass transport is dominated by diffusion.
Prior research on laminar flows in micro channels typically neglect effects of gravity and
even if accounted, assumes the lack of other significant physical effects of viscosity or
diffusion. The extent of mixing of two miscible fluids of different densities (DMSO and
PBS) is characterized using Atwood number (At) as a modeling parameter. We have determined a range of At (1.5x10-3 < At < 3x10-3) where in transition occurs from
gravity driven advection to molecular diffusion based mixing in the channel, for low
residence times in the channel (Re > 2) and the highest effectiveness of mixing for a flow
rate fraction of q f = 0.77.
The ability to recover cells flowing through the device is critical so we monitored cell
recovery and cell motion for specific operating conditions of Pe = 2000, 4000, an initial
donor stream concentration of 15% (v/v) and a flow rate fraction q f = 0.5. It was
determined from the study that the effects of gravity is very important and needs to be
accounted in both the flow configurations in the channel. It has been found that it is most
beneficial to operate the device at flow conditions when equilibrium DMSO concentration is achieved at the outlet of the device. This equilibrium is attained for a
faster cell stream processing rate in the flow configuration with the heavier donor stream
on the top, keeping all other operating conditions to be exactly the same. The recovery of
the cells from the device has been found out to be appreciably high for the tested
operating conditions mentioned above, if both the streams are collected at the outlet.
University of Minnesota M.S. thesis. December 2010. Major: Mechanical Engineering. Advisor: Prof. Allison Hubel. 1 computer file (PDF); ix, 70 pages, appendices A-D.
Chandran, Rohini Bala.
Introduction of DMSO into cell suspensions using a two-stream microfluidic device..
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