Author: Ruairi James Mackenzie Edited by: Burcu Anil Kirmizitas
For years, microfluidics technology has promised a revolution in biological and biomedical research. Opinion leaders predicted that microfluidics would allow analysis of biological structures at an unprecedented level of efficiency and accuracy and would change how everyday research was conducted. Whilst that paradigm shift has not yet occurred, microfluidics has proved its use in biological research. What is microfluidics? What roles can it fulfil at the bench, and what obstacles remain in the path of a technology that seems to offer so much?
Microfluidics, at its core, is the study of fluids at a microlitre volume or below, flowing through spaces that are micrometres or less in size. The basic microfluidic devices used in biological research consist of miniscule microchannels etched onto a base material. The miniaturisation of electronics was a key influence in the development of microfluidics technology, and, as with microelectronic chips, the original materials used in microfluidics were silicon and glass, although these have now been replaced by less rigid substrates such as poly(dimethylsiloxane) (PDMS) that work better alongside biological samples. These etched channels combine to form a system where tiny volumes of liquid can be channelled and mixed.
The tiny dimensions used in microfluidic analysis produce interesting physical quirks. The flow of the liquids will be largely laminar, meaning that two parallel liquid flows will largely not mix. This facilitates prediction of liquid flow and makes control of the sample environment far easier. In addition, reaction times are faster and reagent consumption is far lower than for conventional macroscale reactions.
Microfluidic analysis allows high-throughput analysis of samples with minute volumes, using assays with streamlined experimental procedure. This marries well to microbiological studies, which use complex assays and very small samples.
What are the different applications of microfluidics in biological research? The miniaturisation and streamlining of complex biological assays has long been a goal of microfluidic research. The term “lab-on-a-chip” refers to the integration of several lab-based assays onto a microfluidic chip. DNA amplification and detection using Polymerase Chain Reaction (PCR) is one widely-used biological technique that has benefitted from the use of microfluidic technology. Usually, PCR requires several shifts in temperature, called thermal cycling, that facilitate the separation and recoupling of DNA strands. These changes in temperature take far less time in the small space of a microfluidic chip, which means microfluidic devices perform PCR in a fraction of the normal time; for example, in 2014, Farrar and Wittwer were able to perform 35 cycles of PCR, which normally take a few minutes each, in just 14.7 seconds using a customised microfluidic device. Assay specific devices, such as heaters to change sample temperature, can be integrated onto the chip, meaning the entire assay can be conducted using a single device.
Microfluidic miniaturisation has been applied to a huge range of macroscale assays, including ELISA, chemotaxis assays and cell cultures. As for PCR, these assays often exploit the intrinsic properties of analysis at minute dimensions to improve the techniques beyond what could be accomplished in traditional assays. Control of culture environment for individual cells is inexact in a petri-dish, but can be tightly controlled in a microfluidic culture chip. These innovations can lead to analyses that would simply not be possible in a larger setting and move beyond simply miniaturising larger experiments.
The pharmaceutical industry has been plagued in recent years by spiralling costs, forcing it to develop new techniques to reduce overheads. Microfluidic innovation is playing a role in these changes; the promising “organ-on-a-chip” technology seeks to reduce cost and increase efficiency in drug development as “lab-on-a-chip” assays have done in microbiology. Microfluidic chips have been designed to replicate in vivo organ structures, increasing the therapeutic relevance of findings produced by model studies. The complex microenvironments experienced by cells in the human body are replicated to a degree of accuracy not achievable with other in vitro techniques; the “artery-on-a-chip” developed by Axel Gunther and colleagues at the University of Toronto places a segment of artery into a loading chamber in the centre of a chip. From there, the artery can be kept at a constant pressure and temperature and perfused with drugs to allow pharmacological analysis.
Microfluidic applications in biological research are not confined to chips. Flow cytometry is an analytical technique that allows observation of single cells. To isolate single cells from a complex mix of cell types, a sample is introduced into a tube where two streams of “sheath fluid” are present. The two sheath streams form into a parallel laminar flow, and the sample is forced into a single file cell stream in a process called hydrodynamic focusing.
Microfluidics can improve on existing techniques, produce new analytical possibilities, and has potential both on and off chip. Why, therefore, has it not become ubiquitous in research labs worldwide?
Knowledge transfer between the engineers developing microfluidic technology and the biologist end-users is certainly a limiting factor. A review by Sackmann and colleagues in 2014 identified that 85% of recent microfluidic publications were in engineering journals. Increasing interaction between these groups of scientists will surely speed up the adoption process. Simplifying the design further will also be helpful – current technologies still often use external pumping equipment that would be difficult for anyone not well versed in fluid physics to understand. Nevertheless, the field is simplifying; the cheap production costs and versatility of microfluidic technology means it can be easily adapted to create diagnostic tests for clinicians in developing countries using paper as a substrate in place of PDMS. Simple assays using these paper-based devices have already been developed that can detect glucose and protein in urine. The exciting advances in medical microfluidics will be explored in a future article.
Microfluidics has immense potential, and whilst the slow rate of uptake may be frustrating for researchers in the field, the potential rewards of developing this technology means they should not be deterred.