Authors: Martin Day, Carolyn Ruddell, Richard Swainson, Steve Thomas, Patrick Ward and Ian Williamson – Carclo Diagnostic Solutions Ltd.
Introduction
Today’s trend towards Point-of-Care (PoC) and Near-Patient testing is being driven by the need for laboratory-level accuracy in a rapid timeframe, all at a low cost. Most of the PoC tests currently on, or approaching, the market are conducted using a disposable test cartridge which requires reading using a portable electronic instrument. Whilst consolidation may occur at some stage in the future, the sheer quantity of programmes under development supports the somewhat unrealistic future vision of a health clinic awash with small diagnostic instruments.
This article describes the development of a novel disposable device utilising Capillary Agglutination Technology (CAT). The device incorporates sample handling, microfluidics and electronics – all combined in a device which reads the sample and provides a permanent record of the test result.
Intellectual Property
Following the formation of a dedicated team to manage Carclo Diagnostic Solution’s novel microfluidic technology, several innovative steps have been made to support the development of a truly integrated diagnostic test platform. The team’s approach is focused on developing enabling Intellectual Property which can be mixed and matched, as appropriate, to deliver a range of PoC device strategies supporting individual customer requirements.
Applications have been submitted for Patents covering techniques to manage patient sample volume, reagent application, fluid control and surface modification. Carclo Diagnostic Solutions sees these enabling technologies as key to supporting the development of an integrated device.
Capillary Agglutination Technology
Development of Capillary Agglutination Technology (CAT) has been underway for several years, initially at Platform Diagnostics Ltd in Liverpool and subsequently in collaboration with British Biocell International. Early in 2011, Carclo formed its own dedicated team of experienced assay development specialists and established new laboratory facilities at I-TAC Bio, STFC Daresbury Laboratory, to undertake a new innovative development programme working closely with other Carclo teams.
The CAT microfluidic assay system relies on the use of biochemical reagents contained within a small disposable device to generate a change in fluid viscosity proportional to the amount of analyte present in a biological sample of interest. In CAT, a reagent combines with an analyte in a sample of body fluid (blood, urine, etc) and forms an agglutinate of larger molecules. The agglutinate acts to slow down the flow of the sample in micro-capillary channels formed by precise moulding of a plastic “chip”. A comparator channel is used as a reference, enabling adjustment for differences in sample viscosity and allowing improved accuracy of the test result. The generated time/distance measurements provide a simplified end-point, thereby reducing complexity and costs, compared with other systems that rely on signal intensity detection and quantitation. Reagents from an existing D-dimer haemagglutination test (SimpliRed) were originally used to demonstrate the feasibility of the system.
CAT avoids infringement of other patents and is protected by patents covering different aspects of the technology.
A new programme is now underway, developing prototype assays for a range of different analytes, in conjunction with the latest developments in device hardware from Carclo. By developing prototype tests in different areas (haematology, haemostasis, infection and inflammation, etc.), the company aims to demonstrate the flexibility and applicability of the system. Several significant technical challenges have been conquered, including the design of bespoke reagentry and the development of new methodologies for the different types of tests. The results of ongoing feasibility studies are extremely encouraging at this stage.
Disposable plastic chip
The technology of plastic injection moulding is ideally suited to the manufacture of low cost disposable medical devices. The process is usually rapid and capable of producing complex geometries. Moreover the materials are generally low cost.
Capillary tracks can be moulded into the basic plastic component using fine features in the injection mould tool. Capillary channels allow very small quantities of reagents to be used and mean that only small samples of body fluids are required in order to perform the test and achieve a result. These features require careful design input to ensure they are robust and can withstand the high pressures repeatedly experienced by the mould tool during the moulding process.
Tooling strategies have been developed to support the fabrication of fine capillary features utilising substrates capable of being high-speed machined along with post-fabrication surface treatments allowing a high level of wear resistance at a microscopic level to be applied to the mould tool surface. These developments allow Carclo to continue to push the boundaries of fine feature machining and resulting fine capillary geometry. The result is a moulded chip incorporating capillary channels which handles all the liquids.
However the plastics commonly used for manufacturing diagnostic consumables [polystyrene (PS), polypropylene (PP), polycarbonate (PC), etc], are usually hydrophobic in nature. This means that liquids do not readily flow on the surface. During development, it was determined that modification of the material surface would be required.
Technologies for the surface modification of plastics include flaming, corona, plasma and the use of surfactants. Each has its advantages and disadvantages, see table, and was considered. Eventually, it was decided to use a surfactant.
Prototype mould tooling was built at each phase of the programme to allow development work to continue. The capillary channels require sealing on the open side of the channel to ensure that the biohazard represented by blood or urine is contained and fluid flow is maximised. Following research into adhesives and other joining methods, it was decided to apply a plastic film to the chip using laser welding. This process is ideally suited to such technology as it is highly accurate, clean and rapid.
Electronics
The aim of the electronics development was to develop an electronic point of care device with on-board processing that is sufficiently low cost to allow the device to be disposable. This is in contrast to existing technologies where passive point of care test devices are then introduced into a ‘base station’ style reader which has the full electronic capability to interpret the results of the test and display meaningful data to the user.
To fit in with the initial CAT platform, the electronics development has initially focused on detecting the flow of fluid in capillary channels. In the first instance this utilised the conductivity of the blood as a means for the detection of fluid flow but this technique can lead to electrolysis of the fluids involved and great care has to be taken to avoid any form of electrochemical corrosion.
Recent developments in the implementation of capacitive sensing technologies onto low cost microcontrollers led to a modified approach. Instead of introducing electrical conductors into the test capillary and potentially disturbing the fluid flow, development work led to sealing the channel with a polymer membrane and placing sensing electrodes on the opposing side. Fluid flowing over these electrodes will alter the dielectric environment and result in a measurable change in the capacitance of the electrodes. This is similar to the way that the touch screen on an Apple iPhone detects the presence of a fingertip.
The current technology is being further developed to allow both simple digital detection of fluid flow (ie is fluid present or not?) and also more complicated analogue sensing – such as the exact position of fluid along a channel or the speed that the fluid is travelling at any one time. Other electronic enhancements are also being integrated such as localised heating areas and optical detection of biochemical phenomena.
All of the electronics development is being integrated using Conductive Inkjet Technology’s digital printed circuit technology to allow both reduced development times and low cost production.
A novel Write Once Display development is in the process of being optimised to extend the display life. This will enable the test results to be permanently stored in the patient’s records.
Device integration
Laser-welding is a fast, clean and accurate technology for joining plastics. Carclo Diagnostic Solutions has completed the development of laser welding processes to join the capillary plastic chip, a plastic film to seal the channels and a film with microcircuitry.
The result of the assay can be output as a digital signal and read as text or digits on a low cost display integrated with the disposable chip. The result is a very low cost device which both conducts the test and provides the result. It is anticipated that the finished device will cost in the region of $1.00 US.
The current phase of product development also covers the integration of an optical read system within the device to measure colour change or fluorescence following a chemical reaction between the reagent and patient sample.
The current hardware platforms are focused on enabling technologies supporting assays for whole-blood, coagulation, blood-typing and urine PoC testing.
Concluding remarks
Carclo Diagnostic Solutions has brought together the technologies of injection moulding/toolmaking of precise features, chemical assay development, surface treatment, laser welding and micro-electronics and is offering potential partners the opportunity to license its novel, enabling technology for Point-of-Care testing.