The engineers at TTP, one of Europe’s leading independent product and technology development companies faced this challenge when they set out to develop new technology that would enable diagnostic assays to be performed at the point of care (POC)—i.e., in doctor’s surgeries or at the patient’s bedside. POC diagnostic testing systems typically enable rapid, low-cost, and de-skilled diagnostic testing in medical centres for, say, a range of infectious diseases and thus compete with off-site clinical laboratories.
Having developed several POC instruments for clients, TTP decided to develop a simple sample processing and testing engine that can be customised to suit a range of clients’ needs. Taking advantage of its culture of freedom to pursue internal developments as well as its expertise in product development, TTP developed Puckdx, a flexible, low-cost solution for translating diagnostic assays to a simple, easy-to-use desktop device. But the system may also have other sample processing and fluid mixing applications besides diagnostics.
Much of Puckdx’s design and functionality was inspired by watching and replicating what biologists in the lab do with hand-held pipettors – but in a very simple way. Ease of use was a must, to allow medical staff without laboratory qualifications to run medical tests. Beyond that, TTP emphasised flexibility, since platforms that can run several types of diagnostic tests in the rapidly expanding POC diagnostic market are likely to have the edge. And, all things being equal, it was felt that the lowest cost system has the potential to become the market leader.
Many POC systems comprise a consumable cartridge and an easy-to-use instrument small enough to fit on a worktop in a doctor’s surgery. The Puckdx cartridge is slightly larger than a coffee pod and has pre-filled foil-sealed chambers to store fluids, in particular reagents for diagnostic assays, but potentially also other ingredients for other applications. The biological sample to be tested, or other material to be processed, is introduced into the cartridge, which is then inserted into the Puckdx instrument about the size of a small coffee machine (75 x 150 x 200mm).
And that’s where the engineering simplicity comes in: A central shaft is raised to mechanically and pneumatically engage with a movable pipette, the only moving part within the cartridge. The coupling allows the instrument to rotate the pipette about the central axis and then lower it to collect fluids from different chambers in the cartridge base. As it engages each chamber, a spike pierces the foil to prevent air locks as fluid is aspirated or dispensed.
A fixed dip tube in each of the cone-shaped fluid chambers means that the pipette can reach the bottom of the tapered V-shaped floor to effectively extract fluids. A precision syringe pump, connected to the pipette via the coupling, provides the means to aspirate and dispense fluids from the chambers with the same precision as a hand-held laboratory pipettor.
This architecture allows fluids to be transferred, mixed and filtered as part of any sample preparation process. The design based on a rotating central shaft achieves a compact, simple, and thus reliable, emulation of the linear liquid transfer movements usually carried out at the lab bench.
From there on, Puckdx supports a range of chemistries and applications. Some diagnostic applications, PCR for example, require the use of coated ferrous beads to extract and purify DNA for analysis. The DNA sticks to the ferrous beads, and a mechanism in Puckdx raises two magnets to capture the beads on a side wall whilst the waste fluid is removed via the pipette. Clean wash buffer can then be added, the magnet removed, and the beads suspended to help purify the DNA. This wash process can be repeated.
Other systems use one magnet to capture the beads, but Puck utilises two to create a strong magnetic field on the side walls of the chamber. This also results in a neutral ‘particle free’ plane along the dip tube axis and at the lowest point in the chamber, so that the waste fluid can be removed without the risk of losing precious sample material, thus realising high yielding steps with a very simple mechanical design. The instrument can also raise other mechanisms under the cartridge, such as heaters, sonication probes and shakers.
Once the sample material has been purified, it can be pumped into a small reaction chamber for further chemical steps. For PCR-based diagnostic applications, the challenges of rapidly heating and cooling microlitres of fluid while at the same time optically interrogating the sample were overcome by building a microfluidic chamber that is thin enough to be thermally reactive but thick enough to allow excitation of fluorescent biomarkers via LEDs to one side. This detection chamber is formed as part of the carousel plastic moulding, meaning that no additional parts are required. Puckdx can currently complete 40 PCR cycles in around 10 mins. An even quicker design, using a thin film heater, is being developed and should reduce this to 3 minutes.
The simplicity of Puckdx translates into low cost. To realise the very low manufacturing cost target of $1 for the cartridge, tight moulding tolerances and critical dimensions had to be avoided. With only one moving part, the Puckdx system is inherently dependable and relies on a self-centring movement of the robotic pipette arm as it’s lowered to form the fluid connection with the dip tube. Performance tests have shown the joint is capable of withstanding pressures of >1Bar (14.5 psi).
The original design has been developed for the diagnostics sector, specifically POC testing, but thanks to flexibility and low cost one can envisage other applications that require precise, on-site fluid mixing or manipulation. Some chemists, for example, are exploring the possibility of using the robotic nature of Puckdx as an inexpensive research tool, where every test requires a subtle change to processing parameters, such as fluid ratios.