Highly Strain‐Stable Intrinsically Stretchable Olfactory Sensors for Imperceptible Health Monitoring

Abstract Intrinsically stretchable gas sensors possess outstanding advantages in seamless conformability and high‐comfort wearability for real‐time detection toward skin/respiration gases, making them promising candidates for health monitoring and non‐invasive disease diagnosis and therapy. However, the strain‐induced deformation of the sensitive semiconductor layers possibly causes the sensing signal drift, resulting in failure in achievement of the reliable gas detection. Herein, a surprising result that the stretchable organic polymers present a universal strain‐insensitive gas sensing property is shown. All the stretchable polymers with different degrees of crystallinity, including indacenodithiophene‐benzothiadiazole (PIDTBT), diketo‐pyrrolo‐pyrrole bithiophene thienothiophene (DPPT‐TT) and poly[4‐(4,4‐dihexadecyl‐4H‐cyclopenta[1,2‐b:5,4‐b′]dithiophen‐2‐yl)‐alt‐[1,2,5]thiad‐iazolo [3,4‐c] pyridine] (PCDTPT), show almost unchanged gas response signals in the different stretching states. This outstanding advantage enables the intrinsically stretchable devices to imperceptibly adhere on human skin and well conform to the versatile deformations such as bending, twisting, and stretching, with the highly strain‐stable gas sensing property. The intrinsically stretchable PIDTBT sensor also demonstrates the excellent selectivity toward the skin‐emitted trimethylamine (TMA) gas, with a theoretical limit of detection as low as 0.3 ppb. The work provides new insights into the preparation of the reliable skin‐like gas sensors and highlights the potential applications in the real‐time detection of skin gas and respiration gas for non‐invasive medical treatment and disease diagnosis.

The gas permeability of the semiconductor is confirmed by sealing the container containing the gases (including TMA, NO2, NH3) with a polymer semiconductor film, and then checking the degree of gas leakage.The photograph of the container sealed by the polymer semiconductor is shown in Fig. S2a, c.
Herein, a stable atmospheric pressure gas environment can be obtained by slowly injecting the target gas into the container and then closing the two outlets.In order to avoid the influence of the disturbance of ambient gas on the leakage gas, we put the semiconductor film-sealed container into a larger airtight tank (7000 ml).Then a low-flow vacuum pump (~500 ml/min, calibration by comparison with mass flow meter bubble velocity) is used to pump the gas from the tank into another gas detection chamber to detect changes in concentration.Due to the low flow rate, gas replenished from the outside of the tank for a short period (100s) has less effect on its internal gas concentration.
Two types of semiconductor thin films were used in the specific experiments.
First, the suspended semiconductor films were used to confirm the gas permeability of the unstretched film, which can be obtained via the peel-off process using a PDMS film with small holes.As shown in Figure S2b, when the gas was injected into the semiconductor-sealed container, the sensor quickly exhibited a strong response and a slow recovery when the container was removed.This directly demonstrates the excellent gas permeability of unstretched polymer semiconducting films.However, the further stretched suspended films require PDMS films (self-supporting elastomeric films) with regular and small pores and smoother pore/film interfaces.It is currently still challenging to obtain high-quality through-holes in elastomer with a thickness of tens of micrometers, which is not the focus of this work.
The air permeability of stretched semiconductor films was determined using semiconductor films that were attached to ultra-thin and complete PDMS substrates.
Because PDMS as an amorphous polymer has exhibited gas permeability in numerous previous works.As shown in Figure S2d, a significant response was also observed in the black line (unstretched film), but the time point of the response lagged behind the time point of gas injection.This may be because the thickness of PDMS reaches tens of microns, and gas molecules need to undergo slow diffusion or dissolution to diffuse out.When the film was stretched at 30% strain, the sensor's response increased slightly by ~10% (red line), compared to the former.This may be because the smaller thickness caused by stretching shortens the path of gas permeating the film.In summary, polymer semiconductor films are gas-permeable regardless of whether they are stretched or not, that is, gas molecules are allowed to enter the interior of the film through free diffusion and then pass through the film.
Further, the gas permeability of the polymer film to NH3 and NO2 was visually demonstrated using PH test paper.Specifically, a container containing a solution of concentrated nitric acid and ammonia was sealed with a polymer film, and PH paper was placed on top of the film.As NH3 and NO2 permeate the film, it can be observed that the pH test paper undergoes an obvious discoloration reaction corresponding to the acidity and alkalinity, as shown in Figure S2e.The color change of the pH test paper placed on the stretched and unstretched film was consistent, which may be related to the lower resolution.Nevertheless, these results still provide intuitive evidence that gases can permeate polymer semiconductor thin films.Figure S3 shows that the morphology of PIDBTT film is uniform without any cracks, even after stretching to 90%.DPPT-TT starts to crack at ~20% strain.As the strain increases, cracks with larger gaps appear.PCDTPT starts to crack at ~5% strain.
As the strain increases, dense but small gap cracks appear.

Figure S1 .
Figure S1.The detailed fabrication process of the intrinsically stretchable gas sensor.

Figure S2 .
Figure S2.The real photograph of the container sealed by the two types of polymer semiconductor film (a, c) and the corresponding response curve to leakage of gas (b, d).

Figure
Figure S3 the Optical micrographs of polymer semiconductors in different stretched states.Insets show crack-onset strains of DPPT-TT and PCDTPT, respectively.