Stimulus-active polymer actuators for next-generation microfluidic devices
Appl. Phys. A
Stimulus-active polymer actuators for next-generation microfluidic devices
Wolfgang Hilber 0
Wolfgang Hilber 0
0 Institute for Microelectronics and Microsensors, Johannes Kepler University Linz , Linz , Austria
Microfluidic devices have not yet evolved into commercial off-the-shelf products. Although highly integrated microfluidic structures, also known as lab-on-a-chip (LOC) and micrototal-analysis-system (lTAS) devices, have consistently been predicted to revolutionize biomedical assays and chemical synthesis, they have not entered the market as expected. Studies have identified a lack of standardization and integration as the main obstacles to commercial breakthrough. Soft microfluidics, the utilization of a broad spectrum of soft materials (i.e., polymers) for realization of microfluidic components, will make a significant contribution to the proclaimed growth of the LOC market. Recent advances in polymer science developing novel stimulus-active soft-matter materials may further increase the popularity and spreading of soft microfluidics. Stimulus-active polymers and composite materials change shape or exert mechanical force on surrounding fluids in response to electric, magnetic, light, thermal, or water/solvent stimuli. Specifically devised actuators based on these materials may have the potential to facilitate integration significantly and hence increase the operational advantage for the end-user while retaining costeffectiveness and ease of fabrication. This review gives an overview of available actuation concepts that are based on functional polymers and points out promising concepts and trends that may have the potential to promote the commercial success of microfluidics.
1 Introduction
The field of microfluidics comprises research into and
development of miniaturized systems for the handling,
treatment, metering, and analysis of small amounts of
liquids or gases [
1
]. The corresponding lab-on-a-chip
(LOC) and micrototal-analysis-system (lTAS) devices
[
2
] have repeatedly been predicted to revolutionize fluid
analysis due to their potential to replace bulky and
costintensive bench-top equipment and the associated manual
handling of large amounts of biological and chemical
reagents. However, despite significant research activities
in this highly multidisciplinary field over more than two
decades, only few concepts have ultimately reached the
level of commercialization. Studies addressing this issue
[
3, 4
] identified the lack of standardization and
integration as the main barriers to acceptance by the end-user
and thus to commercial breakthrough: After acquiring
one of the few commercially available microfluidic
products, the operator may face difficulties in connecting
the microfluidic device to ancillary hardware, such as
external supplies, valves, pumps and other microfluidic
components [5]. In contrast to assessments in the early
stages [
6
], which predicted silicon-based microsystem
technology as the most promising approach for
microfluidic applications, soft-matter-based and hybrid
solutions have become more significant [
7
]. However,
polydimethylsiloxane (PDMS), the most popular polymer
for realizing microfluidic components at the laboratory
scale, is usually avoided by the manufacturing sector
mainly due to difficulties with processing and technology
at the industrial scale. Compared to other commercially
utilized standard polymers such as polycarbonate (PC)
and poly(methyl methacrylate) (PMMA), PDMS is a
relatively expensive material whose large-scale
production requires enormous effort if the level of
quality and reliability of the final products expected by
the potential end-user is to be reached. Recent
developments in the field of functional polymers—that is,
softmatter materials which respond to an external, mainly
physical, stimulus—may be essential in contributing to
the foreseen commercial success of microfluidic
concepts. Most of these functional polymers do not respond
to a single stimulus, but rather to a set of physical
stimuli. This review intends to introduce a clear structure
to the class of functional polymers used for microfluidic
actuation and to bridge a gap in the scientific literature.
First, a generic classification of microfluidic actuation
concepts, the basic deformation modes of the functional
materials, and the material classes according to physical
stimuli are introduced (Sect. 2). In the following sections,
a variety of functional materials are presented, starting
with concepts based on PDMS, which is in pure state
barely responsive to external stimuli, but becomes
responsive when combined with other materials in the
form of layered structures or composites (Sect. 3). Next,
the multifaceted material class of polymer hydrogels
(PHs) is discussed; this class of polymers can be made
responsive to almost any conceivable physical stimulus,
ranging from temperature to pH value, light, water and
solvent concentration, electric and magn (...truncated)