Air-permeable redox mediated transcutaneous CO2 sensor

Abstract

Standard clinical care of neonates and the ventilation status of human patients affected with coronavirus disease involves continuous CO₂ monitoring. However, existing noninvasive methods are inadequate owing to the rigidity of hard-wired devices, insubstantial gas permeability and high operating temperature. Here, we report a cost-effective transcutaneous CO₂ sensing device comprising elastomeric sponges impregnated with oxidized single-walled carbon nanotubes (oxSWCNTs)-based composites. The proposed device features a highly selective CO₂ sensing response (detection limit 155 ± 15 ppb), excellent permeability and reliability under a large deformation. A follow-up prospective study not only offers measurement equivalency to existing clinical standards of CO₂ monitoring but also provides important additional features. This new modality allowed for skin-to-skin care in neonates and room-temperature CO₂ monitoring as compared with clinical standard monitoring system operating at high temperature to substantially enhance the quality for futuristic applications.

Keywords: CO2 sensor; Permeable; Polyaniline; Single wall carbon nanotube; Stretchable sensor; Transcutaneous.

Fig. 1

Fabrication and structure of the stretchable CO₂ gas sensor. (a) Photograph of the porous PDMS sponge (PP), oxSWCNT-PANI ink and fabricated sensor (oxSWCNT-PANI/PP). (b) oxSWCNT-PANI/PP SEM micrographs under 0% and 60% strain. (c) ADF-STEM micrograph of oxSWCNT-PANI with EELS mapping of the region defined by the blue box. (d) ADF-STEM micrograph of the oxSWCNT-PANI ink with EELS mapping of particular region in blue box. (e) Raman spectra of SWCNT, oxSWCNT, oxSWCNT-PANI and oxSWCNT-PANI ink. The insets showed enlarged views of the RBM peaks (left) and quantitative shifts in the G-peaks in all the samples (right). (f) Raman spectra of the sensor at 0%, 30% and 60% strain. The inset shows the enlarged G-peak region.

Fig. 2

CO₂ gas sensing performance of the fabricated sensor (oxSWCNT-PANI/PP). (a) CO₂ gas sensing response of oxSWCNT-PANI/PP and oxSWCNT/PP to 5 ppm CO₂ gas. (b) Schematic representation of the CO₂ sensing mechanism of the sensor. (c) Response and recovery time of the oxSWCNT-PANI/PP sensor in CO₂ gas sensing (5 ppm). (d) Concentration-dependent CO₂ gas sensing response of the sensor (oxSWCNT-PANI/PP). (e) Comparative CO₂ gas sensing responses of oxSWCNT-PANI/PP and oxSWCNT/PP. The inset shows the changes in the responses of the sensor (oxSWCNT-PANI/PP) at lower and higher CO₂ concentrations. (f) CO₂ gas sensing performance of the oxSWCNT-PANI/PP sensor at 0, 30 and 60 % strain. (g) Response and recovery time of the sensor (oxSWCNT-PANI/PP) for consecutive CO₂ sensing cycles at different strains. (h) Strain-dependent CO₂ sensing response of the sensor at different CO₂ concentrations. (i) Langmuir and Freundlich equation fits of the concentration-dependent CO₂ sensing response at different strains.

Fig. 3

In situ Raman and DRIFT spectroscopic studies and adsorption interaction energetics for the sensor (oxSWCNT-PANI/PP). In-situ Raman spectra of the sensor under unstretched (a) and stretched (60%) (b) conditions in the presence (ON) and absence (OFF) of CO₂ gas. The insets show an enlarged region of the PANI peaks (left) and G peak (right). (c) In-situ DRIFT spectra of the sensor before and after exposure to a continuous CO₂ flow. (d) Enlarged region of in-situ DRIFT spectra corresponding to C-NH-C of PANI. (e) CO₂ adsorption–desorption isotherms for oxSWCNT-PANI and oxSWCNT-PANI with Zn-Al dispersant. (f) Isosteric heats (Q_iso) of CO₂ adsorption for PANI, oxSWCNTs, oxSWCNT-PANI and the sensor (oxSWCNT-PANI/PP).

Fig. 4

Electromechanical properties, deformation stability, and permeability of the sensor (oxSWCNT-PANI/PP). (a) Current (I)-voltage (V) characteristics of the sensor under different conditions. The insets show photographs of the sensor during bending, twisting, and rolling of the sensor. (b) Cycling stability of the sensor during 500 cycles at 60 % strain. The insets show time vs strain curves and the corresponding resistance changes of the sensor before and after cycling. (c) Time-dependent quantitative water-vapour permeation for different substrates attached to a vial cap (10 mm dia.) along with closed and open conditions for 50 days: polyurethane (PU), polyethylene terephthalate (PET), polydimethylsiloxane (PDMS) and the sensor (oxSWCNT-PANI/PP, PPS). (d) Air permeabilities of different substrates and our sensor along with closed and open conditions.

Fig. 5

Real-time application of the sensor. (a) Photograph of the sensor in the Teflon cell only (1) with a finger (2) and with a metal shield (3) during CO₂ gas monitoring of the finger. Transcutaneous sensing of CO₂ released from the first finger of the right hand corresponding to settings 1, 2 and 3 (shown in the coloured regions). (b) Response of the sensor to CO₂ gas released from the wrist when the sensor is placed directly on the skin. (c) Response when the sensor is kept in a Teflon cell with a circular opening in the middle (wrist band). The inset shows the arrangement of the sensor on the wrist.

Fig. 6

Comparative performance of our sensor with a commercially available TCM TOSCA monitor. (a) Schematic showing a normal volunteer subjected to a steady exercise load on a treadmill. (b) Corresponding measurements of transcutaneous CO₂ recorded by the TCM TOSCA monitor (black colour) and our sensor (red colour). (c) Schematic showing a normal volunteer with a steady exercise load on a treadmill in the presence of supplemental oxygen. (d) Corresponding measurements of CO₂ concentration with a TCM TOSCA monitor (black colour) and our sensor (red colour). (e) Schematic showing observation of a volunteer with obstructive sleep apnea. (f) Measurements of CO₂ concentration by a TCM TOSCA monitor (black colour) and our sensor (red colour). (g) Table depicting the advantages of our sensor compared with the TCM TOSCA monitor. *End-tidal measurements are used for monitoring CO₂ from the breath unlike TOSCA monitor and our sensor (transcutaneous CO₂ monitoring). It is compared here since commonly used in medical field.