New frontiers in point of care diagnotics - trends in genetics, biosensors and microfluidics
Almost precisely a decade ago, the US National Institutes of Health remarked that point-of-care (POC) testing might offer a paradigm shift towards predictive and pre-emptive medicine.
Recent advances in areas such as genetics testing, biosensors and microfluidics continue to enthuse proponents of such scenarios.
However, several challenges still need to be addressed, along the way.
Quicker, better and cheaper
POC testing, which simply means diagnostic tests are done near the patient rather than a clinical laboratory, provides diagnostic information to physicians and/or patients in near-real time. Samples do not need to be transported, or results collected. Short turnaround times are accompanied by high sensitivity and a sample-to-answer format, as well as reduced costs to the health service.
Unlike many other medical innovations, the push of POC technology has been accompanied by a pull from users. Patients find POCs convenient and empowering. Many POCs allow them to monitor their health and medical status at home. Alongside the growing availability of medical information on the Internet, and other enabling technologies such as telemedicine, POCs also mark the coming of age of personalized medicine.
Classifying POC tests
POC tests can be broken down in terms of size/disposability and complexity. At one end are small handheld tests, above all for glucose, and lateral flow strips which determine cardiac markers and infectious pathogens, or confirm pregnancy.
In recent decades, strip technology has been coupled to meter-type readers, typified by the now-widely used glucose meter. Due to their compact nature, such POC tests are often specialized and limited in overall functionality. However, some can be quite sophisticated. New POC tests for early detection of rheumatoid arthritis, for example, require only a single drop of whole blood, urine or saliva, and can be performed and interpreted by a general physician within minutes.
On the other side of the equation are laboratory instruments, which have been steadily reduced in size and complexity. Recent launches include small immunology or hematology analysers. These POC tests provide higher calibration sensitivity and quality control and are used for more complex diagnostic procedures. Such devices have been accompanied by increasing levels of automation, which translates into increased speed. However, it also leads sometimes to challenges in training users.
Three key technologies driving the POC market currently consist of genetic tests, biosensors and microfluidics. Combinations of biosensors and microfluidics have recently been developing at an especially dramatic pace.
Traditionally, genetic testing involved DNA analysis to detect genotypes of interest, either for clinical purposes or related to an inheritable disease. However, results took days or weeks, limiting the applicability of genetic testing in a POC setting.
Emergence of molecular genetics
In recent years, molecular genetics has emerged as one of the most exciting frontiers for POC testing. It detects DNA and RNA-level abnormalities that provoke and fuel most diseases. As a result, it offers precise diagnosis, determines the susceptibility of a patient to a specific disease and assesses his or her response to therapy. Molecular diagnostics can also establish a patient’s prognosis over time far more scientifically than what is often no more than a physician’s informed guess.
One of the first POC gene tests was US biotech firm Cepheid’s GeneXpert, developed to detect the chromosome translocation associated with chronic myeloid leukemia. The small benchtop device provided results in less than two hours, with minimal manual labour involved.
Several companies have been developing tests to analyse genetic polymorphisms which influence the effectiveness of drugs. For instance, Spartan from Canada has developed a one-hour test to analyse CYP2C19, the cytochrome P450 enzyme that activates the antiplatelet inhibitor clopidigrel. Different alleles of the CYP2C19 gene can impair the enzyme’s ability to metabolize the drug, leading to major adverse reactions. Others are developing quick turnaround tests (below 20 minutes), for instance, to detect polymorphisms associated with warfarin response, in order to guide dosage.
These developments focus on analysing very specific targets, with clinical decisions based on a handful of expected results. POC testing in such contexts evidently saves time and permits faster patient care.
Gene sequencing: challenges and breakthroughs
The case is different when the POC effort involves sequencing a gene or a whole genome. This is largely because the interpretation of (otherwise-quick) results are still time consuming and need trained experts.
In spite of this, some innovators are confident about the opportunity for handhelds in genomic sequencing. MinION is a 90 gm handheld device, and is seen by its developer Oxford Nanopore as a first step to ‘anything, anywhere’ sequencing. MinION, which has been used in UK hospitals and in West Africa during the Ebola outbreak, performs nanopore-based sequencing within just a few hours.
There is much more, however, that remains to be smoothed out. MinION shows a high error rate compared to existing next generation sequencing (NGS) platforms and it is impractical for use with larger genomes.
As these kinds of POC genomic technologies continue to develop, other enabling innovations are also likely to make an impact. For example, some researchers have harnessed mobile phone technology for gene variation analysis and DNA sequencing. Its implications in a POC setting would clearly be massive.
As mentioned above, another technology driving POC diagnostics consists of biosensors.
Biosensors are biological materials, closely associated with a transducer to detect the presence of specific compounds.
A biosensor system consists of a biospecific capture entity to detect the target molecule, a chemical interface to control the system function and a transducer for signal detection and measurement. Transducers can be electrochemical, optical, thermometric, magnetic or piezoelectric. Their aim is to produce an electronic signal proportional to an analyte or a group of analytes.
The biospecific capture entity (typically whole cells, enzymes, DNA/RNA strands, antibodies, antigens) is chosen according to the target analyte, while the chemical interface ensures the biospecific capture entity molecule is immobilized upon the relevant transducer.
One key requirement in a biosensor is selective bio-recognition for a target analyte, and the ability to maintain this selectivity in the presence of interference from other compounds. The selectivity depends on the ability of a bio-receptor to bind to the analyte. Bio-receptors are developed from biological origins (e.g. antibodies) or patterned after biological systems (such as peptides, surface- and molecularly-imprinted polymers).
The second requirement in a biosensor is sensitivity. This depends on a wide range of factors, such as the properties of the sensor material, the geometry of the sensing surface and resolution of the measurement system. One of the most important factors in this context is surface chemistry, used to immobilize the bio-recognition element on the sensing surface.
In the field of POC, there has for some time been considerable excitement about biomedical (or biological) microelectromechanical systems, known by their abbreviation BioMEMS.
BioMEMS are biosensors fabricated on a micro- or nano-scale, resulting in higher sensitivity, reduced detection time and increased reliability. Reagent volumes are also reduced due to the smaller size of BioMEMS, which increases their operational cost-effectiveness.
The miniaturization inherent to BioMEMS means greater portability, which is of course a cardinal requirement for POC applications.
Next-generation POC systems are expected to go beyond diagnostics to advance warning, by ‘learning’ about patients (including vital signs such as heart rate, oxygen saturation, changes in plasma profile etc.), and discovering problems in advance through the use of sophisticated algorithms. Such monitoring systems are likely to comprise different types of wearable or implantable biosensors, communicating via wireless or 4G links to their smartphones and onwards to a medical centre. Such systems would dramatically reduce response time and make testing available in environments where laboratory testing is simply not feasible.
Microfluidics, also known as lab-on-a-chip, miniaturize and integrate most of the functional modules used in central laboratories into a single chip. The technology is seen as a high-potential driver of POC diagnostics, not least in developing countries.
There are three principal families of POC microfluidic tests - lateral flow devices, desktop or handheld platforms and (emerging) molecular diagnostic systems. The systems range from zero-instrumented POC devices for the detection of pathogens to fully-instrumented equipment such as NGS sequencing and droplet-based microfluidics.
Microfluidic applications have grown at a dizzying speed, due to the inherent advantages and promises of the technology. These include the ability to manipulate very small volumes of liquids and perform all analytical steps in an automated format - from sample pretreatment, through reaction and separation to detection. Assay volumes are therefore reduced dramatically, while sample processing and readout are accelerated. Other salient features of microfluidics consist of parallel processing of samples with greater precision control, and versatility in formats for different detection schemes. These of course translate to greater sensitivity.
Key technology trends in the field of microfluidics, which have a direct bearing on POC use, include growing miniaturization, higher efficiency chemical reagents, accelerated sampling times as well as larger throughputs in synthesis and screening. As with BioMEMS biosensors, the advantages of microfluidics also consist of low device production costs and disposability,
Some researchers are looking at the commodification of microfluidics - for example, mass production by using inexpensive materials such as paper, plastic and threads, coupled to cost-effective manufacturing processes.
Paper has drawn the highest degree of attention, given that it is lightweight, biocompatible with assays and ecologically friendly. In terms of operation, paper microfluidics is seen as an innovative means to escape the limitations of external pumps and detection systems. Flow in paper is driven by simple capillary forces. Another major advantage of paper is its application in colorimetric tests for detection by the naked eye. Given the proliferation of smartphones equipped with high-resolution cameras, some experts view paper microfluidics becoming the tool of choice for POC diagnostics in developing countries.
Biosensor-microfluidics combinations: developing at a ‘violent’ pace
Efforts to merge biosensors with microfluidics have also been demonstrated since the mid-2000s. Progress has been encouraging. Last year, a University of Copenhagen research team, led by biotechnologist Alexander Jönsson and visiting Canadian scientist Josiane Lafleur, noted that the “marriage of highly sensitive biosensor designs with the versatility in sample handling and fluidic manipulation” offered by microfluidics promises to “yield powerful tools for analytical and, in particular, diagnostic applications.” Their article, ‘Recent advances in lab-on-a-chip for biosensing applications’, was published in the February 2016 issue of the journal ‘Biosensors and Bioelectronics’, and noted that areas where microfluidics and biosensors converged was “rapidly and almost violently developing.” Nevertheless, the authors also found there is still much more to be done, with the observation that “solutions where the full potentials are being exploited are still surprisingly rare.”