How Temperature Control Shapes Point-of-Care Diagnostic Design
When developing a point-of-care diagnostic, decisions about the instrument, the consumable, and the chemistry are more closely connected than is often recognised. Temperature (as a key reaction variable) is one example. Considering it alongside the rest of the system early can change what the product looks like by the time it reaches manufacturing.
Why temperature matters
Chemical reactions are driven by temperature (higher temperature, faster reactions). In diagnostics, changes in reaction temperature will affect the test result and how reliably it can be read. As temperature drifts, the result drifts too. This impacts all aspects of a device’s analytical performance.
Three approaches
In our view, there are three broad ways to manage temperature as a critical parameter: control temperature directly, holding the reaction at the level it needs; compensate for temperature, measuring temperature and correcting the biased result via an algorithm; or reduce how much time the reaction has to be affected by it, by making the reaction so fast that small changes in ambient temperature are not significant to the accuracy of the result.
Each comes with a different trade-off. Direct control is generally the most reliable route, because it removes the variable rather than working around it, but it usually costs more to build in (enhanced hardware required). Compensation is cheaper to add, but depends on how well the correction model matches the real behaviour of the reaction, which isn’t always straightforward, and still leads to a limited ambient operating range. Compensation also requires a two-dimensional algorithm that characterises the temperature effect across the anticipated concentration range (this is because of the non-linear reaction rate of most chemistry pathways). Minimising reaction time reduces the amount of time the reaction is affected by temperature, but only works if the chemistry allows it (don’t try this with Troponin!). This is good for some enzymatic assays and ISEs. None of the three is universally right. Which one fits depends on the assay, the budget, and how much Total Error the analyte can tolerate.
Where does the control belong?
Assume direct control is the preferred approach for a given product. That still leaves a design decision: does the heating element belong in the reusable instrument, or can it sit on the disposable itself (interesting!)?
Instrument-side heating is the conventional route, and a proven one, but it adds significant cost to the instrument (and will zap the battery of your portable device!). It also has the limitation that the heater is located a distance away from the reaction, and the heat sometimes needs to pass through multiple layers of plastic and air (which unfortunately are great insulators!). It’s also imperative to include temperature monitoring to enable feedback circuitry to adjust the drive current of the heater to ensure the actual temperature is achieved.
Onboard heating in disposable electrodes
This is where onboard microheaters come in. Rather than heating the reaction from the instrument, a microheater built directly into the consumable applies localised, low-power heating within the test strip itself. Because these are on-strip, each strip lot can have a TCR (temperature coefficient of resistance) calibration performed, meaning the temperature experienced at the reaction site is characterised, thus removing the need for onboard temperature measurement and feedback to adjust the drive current. The drive current to achieve the right temperature is set during lot calibration. This works, but it increases the development time and device cost for the consumable. However, it simplifies the instrument and reduces its cost (if this is a client requirement).
We’ve been producing onboard-heated electrodes as part of our biosensor fabrication work, and it’s changed how we think about where thermal control should sit in a system, not as a fixed rule, but as a genuine design choice. For this option, it is critical to fully characterise the temperature coefficient of resistance (TCR) for our range of microheater electrode designs we can deploy to client biosensors. This allows us to choose the right option based on clients’ needs (though they still typically require fine-tuning for each given application).
Think about the system early
Which brings this back to where it started. This isn’t a decision to make late in a project, once the instrument and the chemistry have already been designed separately. Temperature, the disposable, and the instrument are part of the same system, and treating them that way from the outset gives the development team more opportunity to make informed decisions before the system architecture becomes difficult to change.
Conclusion
Start at the start: establish the chemistry and determine what the consumable needs to manage the chemistry and clinical sample. Design your instrument based on the consumable needs. Have a clear strategy to control your core variables, such as temperature, time, concentration, pH and volume.
FMS’s role isn’t to tell every client to put the heater on the strip. It’s to understand what a client’s product actually needs and help them weigh the approaches and trade-offs against that.
That’s consistent with how we work more broadly, supporting clients from concept through to commercialisation and reducing the risk in that journey rather than adding to it.