Biomedical Engineering Reference
In-Depth Information
of stock solutions. In the laboratories, these solutions are usually prepared
manually, which is very time consuming and imprecise, and it requires large
volumes of liquid. The digital microfluidic biochip design described in the
previous section provides a hardware platform that allows automated solu-
tion preparation based on on-chip droplet transportation and mixing. In this
section, we present an efficient solution-preparation algorithm to generate a
preparation plan that lists the intermediate mixing steps needed to generate
the target solutions with the required concentrations. It determines the type,
concentration, and the number of dispensed droplets of stock solutions. The
derived preparation plan is then programmed into a microcontroller that
controls the biochip platform to carry out the solution-preparation process.
6.2.1 efficient Solution-Preparation Planning Algorithm
For a given bioassay, we refer to the set of solutions to be prepared as
target
solutions
. In this section, we present an efficient algorithm for preparation of
the target solutions.
6.2.1.1 Concentration Manipulation Using Mixing and Dispensing
Suppose that we have to prepare a target solution of reagent A with a concentra-
t ion of 0.2 M. Assu me t hat we on ly have a st ick solut ion of A wit h a concent rat ion
0.4 M. Therefore, we have to dispense droplets from the stock-solution reservoir
and dilute them appropriately by mixing with equal volumes of a diluent in a
reservoir. Note that, for better mixing, the reservoir must be filled to its capacity.
The dilution of one droplet must be followed by the dispensing of a droplet of
0.2 M concentration of A from the large mixed droplet in that reservoir. By vary-
ing the number of droplets routed from the stock solution reservoir to the mix-
ing reservoir, stock solution droplets can be diluted to different concentrations.
However, since the digital microfluidic biochip can only handle discrete
droplets, the number of droplets routed from the solution reservoirs can
only be integers. As a result, the stock solution droplets cannot be diluted to
an arbitrary concentration using one iteration of the mixing and dispensing
operation. Instead, only a set of discrete concentrations are feasible.
For this example, assuming that the dilution is performed in a reservoir
whose capacity is four times the volume of a unit droplet, we can only dis-
pense 1, 2, or 3 droplets from the stock solution whose concentration is 0.4 M.
The concentrations of the dispensed droplet after the dilution can only be
0.1 M, 0.2 M, or 0.3 M. Here we define the difference between the outcome
concentrations caused by dispensing one more (or less) droplet into the mixing
reservoir as
modulation resolution
.
