From finger prick sampling to wearable and implantable chem/bio-sensors

Despite the wide range of applications and tremendous potential of implantable sensors targeting chemo/bio-markers, bringing actual practical devices fully to market continues to be inhibited by significant technological barriers associated with long-term reliability, which is a key requirement for implantable devices. Wearable chem/bio-sensors offer an interesting approach, intermediate between the long-term vision of implantable devices, and the single use-disposable devices that are the current dominant use model.  For example, wearable patch-type devices employing minimally invasive sampling of interstitial fluid for continuous glucose monitoring target a use period of about two weeks [1].  However, despite this apparently rather modest target, despite apparently promising breakthroughs, large-scale adoption is still frustratingly elusive, and products are still finding it difficult to establish markets.  Moves by Google into the biosensing space are an interesting development, with the focus again being on how to gain access to sample fluids through which key biomarkers like glucose can be tracked in a non-invasive manner via a limited duration use model.  Google, in partnership with Novartis, is focusing on glucose monitoring through a contact lens that can be powered inductively (no batteries), can communicate wirelessly, function for 24 hours (lenses are changed daily), has an integrated electrochemical sensor, and is in contact with a sample fluid (ocular humour) with glucose composition related (albeit somewhat fuzzily) to that of blood [2].  Similarly, the period up to the launch of the Apple iWatch witnessed a frenzy of speculation about whether it would have an integrated glucose monitoring capability [3].  In the end, the iWatch was launched, with no mention of any integrated chem/bio-sensing capability.\ud
However, once these initial applications are delivered, and the wearable platform possibilities more clearly resolved, the drive for more value will place the spotlight on other sensing technologies that can implemented on-body to provide new types of information.  In this respect, chemical sensors and biosensors are obvious candidates, particularly for conditions like diabetes that demand long-term continuous monitoring.  These devices are inherently more complex and less dependable than the well-established physical sensors, as reflected in the difficulties in bringing these sensors to market [4].  However, recent advances in real-time sweat electrolyte monitoring using wearable chemical sensing platforms are pointing the way forward [5,6].  In this paper, I will examine the issues that currently limit the applicability of chemo/bio-sensors in wearable and implantable scenarios, and present ways through which the effective autonomous lifetime of these more complex sensors might be extended from the current norm of (at most) several days, towards much longer periods (ideally years).\ud
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References\ud
1.  See for example http://diatribe.org/abbott-freestyle-libre-transforming-glucose-monitoring-through-utter-simplicity-fingersticks, last accessed 11th July 2016.\ud
2. See http://www.independent.co.uk/life-style/gadgets-and-tech/google-licenses-smart-contact-lens-technology-to-help-diabetics-and-glasses-wearers-9607368.html, last accessed 11th July 2016..\ud
 3. See for example http://www.phonearena.com/news/Apple-iWatch-to-arrive-in-October-with-curved-OLED-screen-blood-glucose-sensor-and-more_id56939, last accessed 11th July 2016.\ud
 4. Concept and development of an autonomous wearable micro-fluidic platform for real time pH sweat analysis, V. F. Curto, S. Coyle, R. Byrne, N. Angelov, D. Diamond and F. Benito-Lopez, Sensors and Actuators B-Chemical, 175 (2012) 263-270.\ud
5. T. Glennon, C. O’Quigley, M. McCaul, G. Matzeu, S. Beirne, G.G. Wallace, F. Stroiescu, N. O’Mahoney, P. White, D. Diamond, “SWEATCH”: A Wearable Platform for Harvesting and Analysing Sweat Sodium Content, Electroanalysis. 28 (2016) 1283–1289. doi:10.1002/elan.201600106. \ud
 6. W. Gao, S. Emaminejad, H.Y.Y. Nyein, S. Challa, K. Chen, A. Peck, H.M. Fahad, H. Ota, H. Shiraki, D. Kiriya, D.-H. Lien, G.A. Brooks, R.W. Davis, A. Javey, Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis, Nature. 529 (2016) 509–514. doi:10.1038/nature16521