Julian Ramírez

Electrochemical tattoo biosensors for real-time noninvasive lactate monitoring in human perspiration.

The present work describes the first example of real-time noninvasive lactate sensing in human perspiration during exercise events using a flexible printed temporary-transfer tattoo electrochemical biosensor that conforms to the wearers skin. The new skin-worn enzymatic biosensor exhibits chemical selectivity toward lactate with linearity up to 20 mM and demonstrates resiliency against continuous mechanical deformation expected from epidermal wear. The device was applied successfully to human subjects for real-time continuous monitoring of sweat lactate dynamics during prolonged cycling exercise. The resulting temporal lactate profiles reflect changes in the production of sweat lactate upon varying the exercise intensity. Such skin-worn metabolite biosensors could lead to useful insights into physical performance and overall physiological status, hence offering considerable promise for diverse sport, military, and biomedical applications.

Tattoo-Based Noninvasive Glucose Monitoring: A Proof-of-Concept Study

We present a proof-of-concept demonstration of an all-printed temporary tattoo-based glucose sensor for noninvasive glycemic monitoring. The sensor represents the first example of an easy-to-wear flexible tattoo-based epidermal diagnostic device combining reverse iontophoretic extraction of interstitial glucose and an enzyme-based amperometric biosensor. In-vitro studies reveal the tattoo sensors linear response toward physiologically relevant glucose levels with negligible interferences from common coexisting electroactive species. The iontophoretic-biosensing tattoo platform is reduced to practice by applying the device on human subjects and monitoring variations in glycemic levels due to food consumption. Correlation of the sensor response with that of a commercial glucose meter underscores the promise of the tattoo sensor to detect glucose levels in a noninvasive fashion. Control on-body experiments demonstrate the importance of the reverse iontophoresis operation and validate the sensor specificity. This preliminary investigation indicates that the tattoo-based iontophoresis-sensor platform holds considerable promise for efficient diabetes management and can be extended toward noninvasive monitoring of other physiologically relevant analytes present in the interstitial fluid.

Wearable textile biofuel cells for powering electronics

The fabrication and performance of a wearable biofuel cell printed directly onto textile substrates are reported. The textile biofuel cell utilizes physiologically produced sweat lactate as the fuel to generate electrical energy, producing up to 100 μW cm−2 at 0.34 V during in vitro experimentation, even after repeated bending stress. Furthermore, the wearable and flexible biofuel cell can be easily integrated with a portable energy storage device for on-demand powering of wearable electronics. To validate energy harvesting, the biofuel cell is integrated into a headband and a wristband, and with the help of an on-board DC/DC converter, extracts energy from perspiring human subjects for direct powering of an LED or a digital watch. Convenient incorporation and removal from a variety of garments are achieved by printing the biofuel cell on a detachable care label. Such textile-based non-invasive biofuel cells can be expected to serve in the future as the power unit for wearable electronics and biomedical devices.

An epidermal alkaline rechargeable Ag–Zn printable tattoo battery for wearable electronics

Herein we report for the first time the fabrication of a rechargeable, benign, skin-worn Ag–Zn tattoo battery using unconventional materials, such as screen printed electrodes, temporary tattoo paper, alkaline gel electrolytes and a PDMS cover for sealing the battery. The tattoo battery can be easily worn by a person for powering wearable devices. Detailed characterization of a typical Ag–Zn tattoo cell reveals a capacity density in the range 1.3–2.1 mA h cm−2 and stability up to 13 cycles. The tattoo cell exhibits a stable open circuit voltage of 1.5 V over a 5 days period and endures repeated stretching and bending strain cycles with minimal decrement in its performance. The lateral arrangement of the negative and positive electrodes allows the integration of several cells into a battery in series or parallel combination for tuning the discharge capacity and voltage to the desired values. The practical nature of the tattoo battery was illustrated by applying it to a human subjects skin followed by lighting a red LED. The epidermal tattoo battery thus meets the demands of wearable power sources, including mechanical compliance and tunable discharge capacity, to power body-worn electronic devices.

Human ability to discriminate surface chemistry by touch

The sense of touch is mediated by the interaction of a soft material (i.e., skin) with the texture and chemistry of an objects surface. Previous work designed to probe the limits of tactile perception has been limited to materials with surface asperities larger than the molecular scale; such materials may also have different bulk properties. We demonstrate in a series of psychophysical experiments that humans can discriminate surfaces that differ by only a single layer of molecules, and can “read” patterns of hydrophobicity in the form of characters in the ASCII alphabet. We design an apparatus that mimics free exploration of surfaces by humans and corroborate the experimental results with a theoretical model of friction that predicts the velocities and pressures that permit discrimination. These results demonstrate that forces produced, while sliding a finger along surfaces, interact with the mechanoreceptors of the skin to allow the brain to discriminate surfaces that differ only by surface chemistry. While we used intentionally simple surface modifications in this study (silanized vs. oxidized silicon), these experiments establish a precedent for using the techniques of materials chemistry in psychology. They also open the door for the use of more sophisticated, molecularly engineered, materials in the future.

Effects of flexibility and branching of side chains on the mechanical properties of low-bandgap conjugated polymers

This paper describes effects of the flexibility, length, and branching of side chains on the mechanical properties of low-bandgap semiconducting polymers. The backbones of the polymer chains comprise a diketopyrrolopyrrole (DPP) motif flanked by two furan rings and copolymerized by Stille polycondensation with thiophene (DPP2FT). The side chains of the DPP fall into three categories: linear alkyl (C8, C14, or C16), branched alkyl (ethylhexyl, EH, or hexyldecyl, HD), and linear oligo(ethylene oxide) (EO3, EO4, or EO5). Polymers bearing C8 and C14 side chains are obtained in low yields and thus not pursued. Thermal, mechanical, and electronic properties are plotted against the number of carbon and oxygen atoms in the side chain. We obtain consistent trends in the thermal and mechanical properties for branched alkyl and linear oligo(ethylene oxide) side chains. For example, the glass transition temperature (T g) and elastic modulus decrease with increasing number of carbon and oxygen atoms, whereas the crack-onset strain increases. Among polymers with side chains of 16 carbon and oxygen atoms (C16, HD, and EO5), C16 exhibits the highest T g and the greatest susceptibility to fracture. Hole mobility, as measured in thin-film transistors, appears to be a poor predictor of electronic performance for polymers blended with [60]PCBM in bulk heterojunction (BHJ) solar cells. For example, while EO3 and EO4 exhibit the lowest mobilities (< 10-2 cm2 V-1 s-1) in thin-film transistors, solar cells made using these materials performed the best (efficiency > 2.6%) in unoptimized devices. Conversely, C16 exhibits the highest mobility (≈ 0.2 cm2 V-1 s-1) but produces poor solar cells (efficiency < 0.01%). We attribute the lack of correlation between mobility and power conversion efficiency to unfavorable morphology in the BHJ solar cells. Given the desirable properties measured for EO3 and EO4, the use of flexible oligo(ethylene oxide) side chains is a successful strategy to impart mechanical deformability to organic solar cells, without sacrificing electronic performance.

RAFT Polymerization of an Intrinsically Stretchable Water-Soluble Block Copolymer Scaffold for PEDOT

Despite the common association of π-conjugated polymers with flexible and stretchable electronics, these materials can be rigid and brittle unless they are designed otherwise. For example, low modulus, high extensibility, and high toughness are treated as prerequisites for integration with soft and biological structures. One of the most successful and commercially available organic electronic materials is the conductive and brittle polyelectrolyte complex poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). To make this material stretchable, additives such as ionic liquids must be used. These additives may render the composite incompatible with biological tissue. In this work, we describe the synthesis of an intrinsically stretchable variant of the conductive polymer PEDOT:PSS that is free of additives. The approach involves the synthesis of a block copolymer comprising soft segments of poly(polyethylene glycol methyl ether acrylate) (PPEGMEA) and hard segments of poly(styrene sulfonate) (PSS) using a reversible addition-fragmentation chain transfer (RAFT) polymerization. Subsequently, we used the newly synthesized ionic elastomer PSS-b-PPEGMEA as a matrix for the oxidative polymerization of EDOT. The resulting polyelectrolyte elastomer, PEDOT:PSS-b-PPEGMEA, can withstand elongations up to 128% and has a toughness up to 10.1 MJ m-3. While the polyelectrolyte elastomer is not as conductive as the commercial material, the toughness and extensibility are each more than an order of magnitude higher. Moreover, the electrical conductivity of the polyelectrolyte elastomer exhibits minimal decrease with strain within the elastic regime. We then compared the block copolymer to physical blends of PEDOT:PSS and PPEGMEA. The blend material had a much lower failure strain of only 38% and a maximum toughness of 4.9 MJ m-3. This approach thus emphasizes the importance of the covalent linking of the PSS and PPEGMEA blocks. Furthermore, we demonstrate that the conductivity of scratched films can be restored upon exposure to water.

Metallic Nanoislands on Graphene for Monitoring Swallowing Activity in Head and Neck Cancer Patients

There is a need to monitor patients with cancer of the head and neck postradiation therapy, as diminished swallowing activity can result in disuse atrophy and fibrosis of the swallowing muscles. This paper describes a flexible strain sensor comprising palladium nanoislands on single-layer graphene. These piezoresistive sensors were tested on 14 disease-free head and neck cancer patients with various levels of swallowing function: from nondysphagic to severely dysphagic. The patch-like devices detected differences in (1) the consistencies of food boluses when swallowed and (2) dysphagic and nondysphagic swallows. When surface electromyography (sEMG) is obtained simultaneously with strain data, it is also possible to differentiate swallowing vs nonswallowing events. The plots of resistance vs time are correlated to specific events recorded by video X-ray fluoroscopy. Finally, we developed a machine-learning algorithm to automate the identification of bolus type being swallowed by a healthy subject (86.4%. accuracy). The algorithm was also able to discriminate between swallows of the same bolus from either the healthy subject or a dysphagic patient (94.7% accuracy). Taken together, these results may lead to noninvasive and home-based systems for monitoring of swallowing function and improved quality of life.