Feature Article

DESIGN

Current lab-on-a-chip systems

Effective and reliable diagnosis is an essential tool in medical practice. The convergence of new technologies such as biology, materials science, information technology, molecular chemistry and microelectronics has been revolutionising developments in diagnostics. New knowledge and the ability to manipulate at the nanoscale are also beginning to have a significant impact.

The term lab-on-a-chip (LOC) is usually applied to devices that integrate a variety of functions onto a substrate such as glass, ceramic or polymer. Typically, these devices are a few square centimetres in surface area and may be of a similar size and shape to a traditional standardised microscope slide, but other designs and sizes also exist. LOC systems can include a combination of a number of the following components.

• A microfluidic system. This would include features such as the means for introducing the sample to be analysed, handled and separated together with systems for any necessary reagents; subsystems for diluting or pretreating the sample and possible procedures such as cell lysis; channels for electrophoresis or other separation procedures; microvalves; micropumps; microbioreactor chambers or cell arrays with associated nutrient and waste removal systems; capillaries; heating or cooling areas; and purging systems.
• A detection or measurement system. This may be electronically based, for example, conductance, impedance, field-effect, electrochemical or transducers; magnetic detection; pH measurement; optically based such as light-scattering, fluorescence, multiphoton imaging or photothermal; acoustic such as standing surface acoustic waves and ultrasound; mechanical, for example, determination of the mass of cells using microcantilevers; temperature measurement; chemical detection systems; or cell counting systems.
• An electronic system employed for signal capture.
• Associated software or bioinformatics systems to process output from the chip.

Benefits of LOC approaches

LOC approaches offer significant advantages over conventional methods. These include

• the need for only tiny sample quantities, sometimes as low as just a few picolitres of analyte or a single cell
• extremely low consumption of reagents
• low power consumption
• a rapid response time because of the small distances required for diffusion or transport
• high surface to volume ratios
• the ability to control analytical conditions and precision closely
• the possibility of performing many different analyses in parallel
• the ability to carefully control chip fabrication, thereby facilitating consistency, accuracy and reliability of measurement
• the types of chips that may be mass produced and therefore low cost and disposable
• improvement of safety in many cases because of the need for much lower quantities of potentially infectious analytes and hazardous reagents
• massive reductions in the size of ancillary equipment that is needed.

LOC techniques can already be applied to a wide variety of applications including pharmaceutical screening and testing, toxicological studies, medical laboratory analyses such as blood and disease screening, measurement of metabolic and physiological status, the detection of individual cells, biomolecules or markers, immunoassays, deoxyribonucleic acid (DNA) testing and genomic studies.

Applying nanotechnology to improve LOC functionality

Many of the features of LOC systems are already at the microscale and further physical scaling down may present significant technical challenges. Entities such as individual cells and proteins would be too large to pass through or interact with certain features if the size of those features was reduced too far; and more complex physical and chemical effects may be manifested at the nanoscale, thus resulting in measurement difficulties. There are, however, many ways that systems can provide greatly extended functionality. These include

• nanopores for electrophoretic DNA sequencing, strands of DNA may be driven electrophoretically through solid state nanopores 1–2 nm in diameter to facilitate ultrafast DNA sequencing
• nanocontoured surfaces such as nanopits, nanobumps, nanogrooves on surfaces or scaffolds to facilitate cell adhesion, nonadhesion, growth or movement
• other physical nanoscale features such as nanonozzles or functionalised surfaces (see an example below of the use of other physical features)
• nanoscale actuators, for example, for switches and pumps
• nanowires fabricated from silica or polymers, carbon nanotubes and gold nanorods employed as components of nanoscale biosensors
• other novel nanoscale biosensors based on ion channels, protein imprinting and nanopatterning, direct affinity sensing, resonating nanocantilevers or nanomembranes
• other nanoscale detection systems based on electrical, for example, field-effect, conductance or impedance; magnetic; electrochemical; optical such as quantum dot, light scattering; or acoustic principles.
• micro/nanoscale features capable of biomimicking natural tissue, and cellular or subcellular features to more realistically model in vivo metabolic or physiological conditions or stresses; or that can facilitate important cell-to-cell signalling in vitro, for example, between different chambers containing different cell types in a micro bioreactor
• the use of nanoscale manipulation techniques such as optical tweezers
• nanoscale features supporting high density arrays, for example, for proteomic studies.

One important aspect of developments of this type are that they are highly multidisciplinary in nature with a strong convergence of bio-logical, physical, chemical, engineering, materials science and informatics approaches, each often involving a nanotechnological aspect.

New LOC systems

An example of the use of other physical features to provide extended functionality is a cell array currently under development in the ToxDrop FP6 Project, Highly Parallel Cell Culture in Nanodrops.¹ This employs a glass surface containing individual hydrophilically and hydrophobically functionalised areas and 800 drops per slide. Each individual drop contains 100 cells of a selected type that may be cultured for up to five days. As many as 50 parameters can be measured individually for each cell and the system is ultimately expected to be used for high throughput toxicity testing.

Another example developed by a group at Arizona University, USA, together with other collaborators, employs nanoscale superhydrophobic surfaces.² Onto these go individual droplets in which magnetic or paramagnetic nanoparticles can be readily manipulated, split and mixed in various ways using magnetofluidics with minimal surface tension effects.

Drivers of further development

There are a number of factors that are driving further advancement of LOC technology and the increased application of nanotechnology in this field. These include opportunities for LOC based systems to contribute in an important way in the following applications.

Novel pharmaceuticals. In the rapid screening of novel pharmaceutical molecules for activity and safety, for example, from combinatorial or antibody fragment libraries.

Emerging health risks. For rapid and highly specific testing of health risks such as prion diseases, nosocomial infections and severe acute respiratory syndrome.

Developing world. For tackling growing health problems in the developing world, for example, monitoring HIV/AIDS, tuberculosis and other large-scale killer diseases. The ability to engineer testing into highly portable and “smart” systems that are usable by workers, who do not necessarily have specialist training, would be extremely valuable in areas that do not have adequate laboratory facilities.

Replacing tests. The replacement of traditional and sometimes unreliable or inappropriate animal testing by effective alternatives, in many cases using human-derived cells and tissues, for many types of toxicity testing. These include the large volume of regulatory testing foreseen under the amended European Cosmetics Directive and the Registration, Evaluation, Authorisation and Restriction of Chemicals Regulation (REACH), whereby thirty thousand chemicals will require testing under REACH.

Testing nanoparticles. The safety testing of novel nanoparticles and other nanomaterials. Materials with nanoscale dimensions do not necessarily have the same properties as the same materials in bulk or particle size and shape, and a number of other characteristics also affect the materials’ potential toxicity. There are already many thousands of new nanomaterials that require characterisation and toxicity testing prior to being used on a large scale.

Prevalent Western diseases. Screening for problems such as cardiovascular disease, cancer and diabetes, which are increasingly prevalent in a Western society that is rapidly ageing and placing high pressure on available health care resources. Diagnosis and treatment at an early stage would prove highly cost-effective, particularly if available in point-of-care devices that are usable outside the laboratory in doctors’offices or patients’ homes.

The future for LOC technologies

Some challenges still remain such as how to understand and overcome novel effects associated with further miniaturisation of features, how to standardise components to work between different LOC systems, and how to integrate widespread application of LOC systems into general health care management. However, the progress made in the past 15 years and the developments already underway suggest a healthy future for LOC technologies.

Richard Moore is Manager, Nanomedicine and Life Sciences, at The Institute of Nanotechnology, Suite 5/9 Scion House, Stirling University Innovation Park, Stirling FK9 4NF, UK tel. +44 1786 458 020 e-mail: richard.moore@nano.org.uk, www.nano.org.uk www.nanomednet.org