Biomedical Engineering Reference
In-Depth Information
which sensors can provide early warning. Real-time response is also neces-
sary for surgery and neonatal clinical diagnostics. Finally, biological samples
are sensitive to the environment and to temperature variations, and it is dif-
ficult to maintain an optimal clinical or laboratory environment on chip. To
ensure the integrity of assay results, it is therefore desirable to minimize the
time that samples spend on-chip before assay results are obtained. Increased
throughput also improves operational reliability. Long assay durations imply
that high actuation voltages need to be maintained on some electrodes,
which accelerate insulator degradation and dielectric breakdown, reducing
the number of assays that can be performed on a chip during its lifetime.
1.3 Protein Crystallization
This topic considers protein crystallization as an example of target appli-
cation for optimized chip design. Proteins play a key role in all biologi-
cal processes. The specific biological function of a protein is determined by
the three-dimensional (3-D) arrangement of the constituent amino acids.
Therefore, their structure needs to be understood for effective protein engi-
neering, bioseparations, rational drug design, controlled drug delivery, as
well as for the design of novel enzyme substrates, activators, and inhibitors.
A widely used method to study the 3-D structure of proteins is to crystallize
them and determine the structure using x-ray diffraction [40].
(See Figure 1.5.)
Studies have been reported in the literature that help one gain a funda-
mental understanding of the mechanism of crystallization [41], but, owing
to the complexity and the number of parameters involved in the problem, it
may take years before the process is understood well enough to have practi-
cal value. However, structural biologists need immediate information about
the structure of proteins; hence, empirical methods are widely employed for
crystallization. For example, an empirical approach typically used, among
others, is a 2-D coarse sampling that involves systematic variation of salt
concentration versus pH [41].
Protein crystallization is a multiparametric process that involves the steps
of nucleation and growth, where molecules are brought into a thermo-
dynamically unstable and a supersaturated state. In order to “hit” upon the
correct parameters for the crystallization of proteins, typically, a very large
number of experiments (10
3
to 10
4
) are required, which leads to the consump-
tion of large protein volumes.
Efforts are under way to reduce the consumption of proteins, by miniatur-
izing the crystallization setup. Screening for protein crystallization includes
many repetitive and reproducible pipetting operations. To ease this man-
ual and time-consuming task, several automatic methods have been intro-
duced. In 1990, Chayen et al. introduced a microbatch method in which only
