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Expanding possibilities

A point of care (POC) diagnostic technology has been developed that delivers results within five minutes, works with whole blood, yet matches the sensitivity of the latest laboratory tests, say its developers. It is expected to considerably expand the scope for POC immunoassay testing, which has traditionally produced insufficiently quantitative results and suffered from poor sensitivity. The technology measures the concentration of an analyte by using piezoelectric film to generate a voltage proportional to its rate of binding to an antibody on the film surface.

Piezofilm technology

POC tests using piezofilm technology can produce results comparable with those obtained by laboratory analysis. They have a dynamic range of 1000–10,000, a detection limit of 15 pg/mL, and can generate a result from a whole blood sample of 30 µL in volume. In addition, they can report a result in 5–10 minutes because the test automatically detects when sufficient data has been collected, rather than waiting for an endpoint to the reaction, as in the case with lateral-flow tests.

Piezofilm detection takes advantage of the piezoelectric effect, whereby applying mechanical or temperature stress to piezoelectric materials causes them to produce an electric charge. The technology uses the electric charge generated when a piezofilm is deformed to measure the rate of binding between an antibody or antigen mounted on the surface of a piezofilm layer, and the target analyte in a whole blood sample. The piezofilm sensor consists of a capillary channel with reporter labels dried onto its surface. These reporter labels have antibodies specific to the target analyte adsorbed on their surface. On one side of the channel is an antibody or antigen bound to the surface of a piezoelectric material, in this case a polyvinylidene fluoride (PVDF) film. This consists of a plastic polymer sheet coated with a thin electrically conductive and optically transparent indium tin oxide layer.

The cartridge also acts as the blood collection device. As the sample is drawn into the cartridge, it resuspends the dried reporter label, which binds to the target analyte (see Figure 1). This then binds to the antibodies mounted on the piezofilm to form a “sandwich” consisting of the reporter label, the target analyte and the antibodies on the piezofilm. Whilst this binding is occurring, a bright light emitting diode (LED) is flashed onto the film. The light is flashed because the piezofilm only responds to changes in temperature; it is not affected by the relatively constant ambient temperature surrounding the film. The reporter label absorbs light emitted by the LED. This light is converted to heat, which is dissipated in the immediate vicinity of the label. Reporter molecules bound to the piezofilm as part of the “sandwich” dissipate their heat directly into the piezofilm causing a piezoelectric response. This generates a voltage increasing at a rate that is directly proportional to the rate of binding of the target analyte to the surface and thus its concentration in the blood sample (see Figure 2). The piezofilm sensor is highly sensitive and is directly responsible for the level of performance achieved by this technology. Crucially, although reporter molecules that are not bound to the surface absorb light, the heat is dissipated into the blood sample and does not generate a kinetic voltage in the piezofilm. This means that it is possible to distinguish between the bound and unbound label without the need for washing and separation. By selecting LED wavelengths that have minimal absorption by red cells, it is possible to detect analytes in whole blood. In addition, because the antibodies are bound onto a thin piezofilm, the whole system can be packaged into a small, self contained device such as a disposable strip or cartridge, which makes it desirable for POC use. Making accurate, reliable and cheap measurements of electrical outputs directly from a test strip is a routine part of other POC test equipment such as glucose sensors.

Applications

A TSH test based on this new technology is currently being commercialised. TSH assays do not require any additional tests, which means a POC test for thyroid function can be developed that can standalone and act as a proof-of-concept for piezofilm technology. Determinations are the largest volume immunoassay tests performed in laboratories and there is a major unmet need for a POC test. This is because thyroid dysfunction affects nearly 20 million people in the United States alone, but 80% of cases go undiagnosed. Amongst the remaining 20%, almost half do not have their TSH controlled within the normal range. POC testing for thyroid dysfunction can potentially increase the number of diagnoses and reduce complications from untreated hypothyroidism. In addition, recently published studies show that treatment should also be offered to patients with subclinical hyperthyroidism because they have a significantly higher risk o f cardiovascular problems than those whose thyroid is functioning normally.¹ More frequent testing and tighter control of TSH should aid compliance and the adjustment of patients’ drug doses.

The thyroid-stimulating hormone (TSH) test will have a detection limit of 0.075 µIU/mL (International Units), to allow screening for hyperthyroid disease or thyroxine overdose.

The device itself will consist of a disposable cartridge with multiple detection windows, each with their own capture antibody and adjacent LED, to allow multi-analyte sensing or to act as test controls. When the test strip is due to be read, it is inserted into a compact reader with touch screen and communications port for data download. It will record patient, test and user identification, the time and date of the test, and cartridge batch number and expiry date. The technology is easy to use because it requires no sample handling and the reader is operated with a single on/off button, which means the test can be Clinical Laboratory Improvement Amendment (CLIA) waived² for many applications.

Further potential

Piezofilm technology for POC testing gives higher sensitivity and wider dynamic range results in a shorter time. Although it is initially being demonstrated and commercialised for TSH testing, the technology can be applied to other POC applications such as detecting the acute disease markers required in accident and emergency and intensive care unit settings, and to other types of testing such as ribonucleic acid and deoxyribonucleic acid analysis.

Neil Butler is Chief Executive Officer, Vivacta Ltd, 100 Guillat Avenue, Kent Science Park, Sittingbourne ME9 8GU, UK, tel. +44 1795 419 400, e-mail: nb@vivacta.com, www.vivacta.com.