Diagnostics & medtech
Wearable biodiagnostics
How enzymatic electrochemistry reads interstitial glucose and why its few-minute lag matters, what sweat genuinely reflects versus approximates, which arrhythmias two-week patches unmask, why cuffless blood pressure remains a calibrated trend rather than a measurement, and how continuous data reorganises clinical thinking.
Wearable diagnostics live or die on one engineering question: which body fluid or signal can be sampled without breaking skin meaningfully, yet still correlate strongly enough with blood to inform decisions. Four access routes dominate, each with distinct physics — and distinct ceilings.
The glucose masterpiece
Continuous glucose monitors set every standard here. A flexible filament sits centimetres inside skin, tip wrapped around immobilised glucose oxidase; the enzyme burns arriving glucose and passes freed electrons to an electrode, generating currents proportional to influx — electrochemical transduction refined into weeks-long service. The subtleties teach why the category took decades. The sensor samples interstitial fluid, one diffusion step removed from blood, so readings trail blood values by roughly five to fifteen minutes — negligible at steady state, dangerous during rapid falls when hypoglycaemia decisions hang on staleness. Living tissue also treats any implant as an invader: protein adsorption then cellular encapsulation progressively throttles analyte diffusion, which — more than battery or electronics — bounds sensor lifetime near two weeks and forces per-sensor calibration curves that track their own drift.
What sweat surrenders honestly
Sweat offers effortless non-invasiveness at the price of physiological distance. Its composition tracks serum for ions reliably — chloride testing through induced sweating has screened cystic fibrosis for half a century — and microfluidic patches now measure sodium and potassium loss continuously in athletes. Metabolites and hormones travel less faithfully: glucose concentrations sit orders below blood, lactate partly reflects local gland metabolism, and cortisol correlations wobble with sweat rate itself, since collection speed changes what dissolve into it. Reading sweat therefore means correcting for the plumbing before believing the chemistry.
Rhythms in voltage
Electrical eavesdropping avoids fluids altogether. Adhesive single-lead ECG patches worn one to two weeks record every beat while life proceeds, exposing paroxysmal atrial fibrillation that clinic snapshots miss entirely — stroke-prevention logic makes even modest detection gains meaningful at scale. The harder prize, cuffless blood pressure, exploits photoplethysmography timing: pulse arrival delays between heart and wrist stretch as vessels stiffen under pressure. But that relationship bends individually and drifts as compliance evolves, so current devices deliver trends anchored by occasional cuff references rather than stand-alone diagnoses — a distinction marketing routinely blurs and prescribing physicians cannot.
Across all routes, the deeper revolution is epistemic rather than electrical. Medicine historically sampled bodies at instants; wearables deliver trajectories — sleep architecture night over night, glucose excursions meal over meal, rhythm burdens across real months. Continuous datasets generated new clinical currencies (percent-time-in-range displacing solitary lab values in diabetes practice) and, equally, a shadow market of wellness scores whose validation lives on consumer packaging instead of peer review. The diagnostic substance concentrates wherever the measurand’s chemistry cooperates; everything else is interface design arguing with physiology.