Magnotech: Philips’ magnetic biosensor platform designed for point-of-care testing

A new biosensor platform developed by Philips that uses magnetic nanoparticles to measure target molecules could bring complex in-vitro diagnostic tests out of the laboratory and into decentralized settings, including the patient’s bedside and at home. The speed, ease of use, robustness and accuracy of this new technology could address the requirements of critical care environments by potentially speeding up the diagnosis of life-threatening diseases. In addition, it could be suitable for monitoring chronically ill patients at home.

In-vitro diagnostic tests to detect molecular ‘biomarkers’ of disease in samples of body fluids such as blood or saliva are a valuable tool for diagnosing disease. As a general rule, the cost and complexity of these tests increases as the concentration of the target biomarker decreases. For example, measuring the relatively high concentrations of glucose in blood samples to assist in the management of diabetes has been reduced to a simple pinprick test that patients can perform on a daily basis at home. At the other end of the scale, measuring very low concentrations (picomolar levels) of blood protein biomarkers for the diagnosis of cardiovascular disease currently requires large sample volumes and a time-to-result of tens of minutes.

Scientists at Philips have now developed a new type of biosensor, based on magnetic nanoparticle technology, which can conveniently measure picomolar concentrations of specific proteins in blood or saliva in a matter of minutes. Integrated into a disposable biosensor cartridge that inserts into a hand-held analyzer, it may be capable of providing the speed, ease-of-use, robustness and accuracy required to move sensitive in-vitro diagnostic protein tests out of the laboratory and up to the patient’s bedside, doctor’s office, or ultimately even the home.

Figure 1: Philips’ new biosensor technology features a plastic disposable cartridge that automatically fills itself from a single drop of blood.

Magnetic labeling and activation

Existing laboratory-based blood protein assays typically involve a significant amount of fluid handling (for example, pipetting, reagent mixing, and centrifugation), resulting in relatively complex equipment setups. In addition, the volume of blood required often involves a skilled phlebotomist or nurse withdrawing a syringe-full of blood from the patient.

In contrast, Philips’ new biosensor utilizes a disposable cartridge that automatically fills itself from a single drop of blood (see Figure 1). Once filled, no other fluid movement is required. The entire assay process within the cartridge is performed by externally applying magnetic fields to control the movement of magnetic nanoparticles in the cartridge. The magnetic nanoparticles are preloaded into the cartridge during its manufacture and automatically disperse into the sample as the cartridge fills with blood. Coated with appropriate ligand molecules, they bind to target protein molecules in the sample blood (see Figure 2A). After a short time, typically around a minute, a large fraction of the target protein molecules end up being bound to the surface of the magnetic nanoparticles.

A small electromagnet situated beneath the cartridge then generates a magnetic field that attracts all the magnetic nanoparticles to the biosensor’s active surface, which is coated with ligand molecules that bind to a second binding site on the target protein. As a result of this magnetic attraction, the surface concentration of the target protein is significantly increased, which speeds up the binding process. The target protein molecules end up locked in a sandwich between the active surface on one side and attached nanoparticles on the other (see Figure 2B). This type of assay is therefore often referred to as a ‘sandwich assay’.

An electromagnet situated above the cartridge then generates a magnetic field that pulls unbound magnetic nanoparticles away from the active surface (see Figure 2C). In this way, a very fast and accurately controlled separation between bound and unbound magnetic nanoparticles is achieved, which replaces traditional washing steps. Because each magnetic nanoparticle that remains on the surface is bound there by a target protein molecule, the number of nanoparticles remaining at the surface is a measure of the target protein concentration in the blood sample.

In the final phase, the number of bound nanoparticles is measured using an optical technique based on frustrated total internal reflection. Illuminated at the correct angle, light hitting the underside of the sensor’s active surface is normally reflected without any loss in intensity (total internal reflection). However, when nanoparticles are bound to the opposite side of the surface they scatter and absorb the light, reducing the intensity of the reflected beam. These intensity variations in the reflected beam, which correspond to the number of bound nanoparticles, are detected by a CMOS image sensor similar to that used in a digital camera (see Figure 3).

Figure 3: The target protein molecules end up locked in a sandwich between the active surface on one side and attached nanoparticles on the other. The number of attached nanoparticles is measured using an optical technique based on frustrated total internal reflection.

System integration

The cartridge, which is constructed entirely from plastic components and has no moving parts or embedded electronics, is disposable. It plugs into a hand-held unit that contains the electromagnets, optical detection system, control electronics, software and the read-out display.

The active area of the biosensor is sufficiently large that it can be spotted with ligands for several different proteins, opening up the possibility of performing multiple assays in a single operation. In addition to the ‘sandwich assay’ described above, the technology can be adapted to perform other types of assay, such as ‘competition assays,’ which may be suitable for the detection of drugs-of-abuse and other small molecules in body fluid samples.

Philips has prototyped the system to this stage of integration and tests have shown that it is capable of detecting protein concentrations down to sub-picomolar levels.

Proof-of-concept

Philips has demonstrated proof-of-concept for its new biosensor technology in several biological assays, including the detection of cardiac troponin I (cTnI)[1,2], parathyroid hormone (PTH)[3] and several drugs-of-abuse molecules (amongst others, morphine)[1]. Cardiac troponin is a blood-borne protein that at elevated levels provides a useful biomarker for the diagnosis of myocardial infarction (heart attack). The parathyroid hormone assay was specifically chosen as a proof-of-concept because it is a challenging assay with low concentrations. The morphine assay represents the first application for the technology in drugs-of-abuse testing.

During these proof-of-concept tests, the biosensor’s magnetic field actuation was shown to speed up assays by a factor of more than 100 when compared to simply letting the nanoparticles diffuse to the sensor’s active surface. In the cTnI assay, picomolar concentrations were successfully detected in blood plasma in less than 5 minutes.

These results demonstrate that Philips’ new magnetic nanoparticle biosensor technology shows promise for meeting the speed, compactness, ease-of-use and small sample volume requirements of rapid diagnostic bedside tests.

[1] Rapid integrated biosensor for multiplexed immunoassays based on actuated magnetic nanoparticles, Lab on a Chip 9, p. 3504-3510, 2009.
[2] Rapid, finger-prick POC test for cardiac troponin with picomolar sensitivity using magnetic particle labels, presentation during the AACC Annual Meeting, July 27-31, 2008, Washington DC, USA.
[3] Sensitive and rapid immunoassay for parathyroid hormone using magnetic particle labels and magnetic actuation, Journal of Immunological Methods, 338 (1), p. 40-46, 2008.