MICROFLUIDIC FORMATION OF CELL-LADEN HYDROGEL MODULES FOR TISSUE ENGINEERING - Micro and Nanotechnologies in Engineering Stem Cells and Tissues - page 189

rates and channel dimensions. In addition, by combining the fabrication techniques
of hydrogel fibers and air valves, hydrogel fibers coded with varying chemical
compositions and topographies along the fibers are generated (Fig. 8.2h-j). 31 Using
this method, hydrogel fibers encapsulating multiple and spatially controlled cells can
be created for tissue engineering applications.
8.3 CELL ASSAY SYSTEMS USING MICROFLUIDIC DEVICES
Because of their size uniformity, the cell-laden hydrogel modules are manipulated
as monodisperse beads. Therefore, these modules can be handled, arrayed, and
retrieved using microfluidic bead-based assay systems for accurate analyses.
The characteristics of microfluidic devices for handling hydrogel modules and
their potential for use in analysis of cell biological systems are reviewed in this
section.
8.3.1 Microfluidic Devices for Handling Modules
Cell-based microfluidic devices can provide precise spatial and temporal control of
samples and reagents, which is difficult to achieve using 2D cell culture systems.
Moreover, they allow real-time monitoring and analysis of samples in order to obtain
insight into cell dynamics. Microarray systems especially have extensive applica-
tions for drug discovery and diagnostic and basic scientific studies.
8.3.1.1 Microwell Devices Microwell devices have a simple mechanism to trap
samples statically in an array. 32,33 They allow samples to sink into microwells and
hold a sample in each microwell. Microwell devices are fabricated accurately
using a variety of technologies such as photolithography. Microwells have the
advantage of having a massively parallel format ( > 10000) and allowing the
observation of all samples from the same direction. In addition, microwells are
suitable for trapping a single sample in almost all microwells by adjusting the well
diameters and depths. Therefore, microwell devices are used to screen samples for
drug kinetics studies.
8.3.1.2 Hydrodynamic Microarray Devices Hydrodynamic microarray devices
are commonly used for trapping samples in microfluidic systems. The following
are the advantages of hydrodynamic microarrays over microwell devices: hydro-
dynamic microarrays allow transporting of samples, immobilizing of samples for
convenient analysis, delivering reagents to samples with continuous observation, and
retrieving of selected samples.
The most common way to immobilize samples in microfluidic systems is to
arrange the side channels in a main channel. 34 When the diameter of the side channel
is sufficiently small, samples are trapped by suction because the sidestream from the
main channel runs samples into the side channel. The hydrodynamic systems are
capable of releasing samples as the sidestream is reversed. Also, using hydrodynamic
trapping holes, similar to the microwells, samples can be immobilized and retrieved