Stretchable and multifunctional sensor devices that can measure and quantify various stimuli such as strain, pressure, and temperature have attracted heightened interest due to their revolutionary applications in artificial skin [1,2], robotics [3], human motion detection [4–6], and personalized health care [7–9]. Up to date, a variety of resistive or capacitive strategies by using conductive materials and structures have been developed to achieve multifunctional electronic sensors. For example, stretchable conductors made of silver nanowires were demonstrated for capacitance-based strain and pressure sensing [10]. Polyurethane fibers coupled with ZnO nanowires were employed for detections of strain, temperature, and UV by resistance changes [11]. Despite the high performance and multiple sensing capabilities of these electronic devices, they failed to distinguish different stimuli signals simultaneously. Integration of multiple sensing elements into independent electrical signals could achieve multiparametric sensors with simultaneous readout and low cross sensitivity, which, however, aggravated the complexity and the cost of fabrication processes [12]. In addition, the poor biocompatibility of metallic components and the high sensitivity to electromagnetic interferences are also some of the basic challenges that hinder their practical applications. Recent advances in stretchable, organic polymer-based optical waveguides have fueled the development of optical sensors with large deformability for wearable/implantable applications [13–18]. For example, we have demonstrated nanoparticle-embedded stretchable polymer optical fibers for wearable monitoring of single parameters, including strain [13,14] and temperature [17]. As an alternative strategy to electronic sensors, the realization of stretchable sensors in optics enables multidimensional readout (e.g., intensity, phase, polarization, and wavelength) with miniaturized size, excellent electromagnetic immunity, and low cost.

 

Fig. 1. (a) TEM image of the ${{\rm NaYF}_4}\!:\!{\rm Yb},{\rm Er}$ at ${{\rm NaYF}_4}$ core-shell UCNPs. (b) EDXA of the synthesized core-shell UCNPs. (c) Incorporation of the core-shell UCNPs in PDMS fiber. The UCNP-loaded region showed visible UCL emissions when the 980 nm laser was launched. Top, laser off; bottom, laser on; (d) Stress–strain curves of the PDMS and UCNP-PDMS fiber, respectively.

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Here, we demonstrated a stretchable and multifunctional optical sensor (SMOS) that can simultaneously detect and distinguish temperature and strain. The SMOS was fabricated by assembling lanthanide-based upconversion nanoparticles (Ln-UCNPs) in a polydimethylsiloxane (PDMS)-based optical fiber, which has a refractive index of $\sim{1.41}$. The Ln-UCNPs, consisting of an active ${{\rm NaYF}_4}\!:\!{\rm Yb},{\rm Er}$ core and a protective ${{\rm NaYF}_4}$ shell, were synthesized via the solvothermal method [19,20]. Upon NIR excitation, the ${{\rm NaYF}_4}\!:\!{\rm Yb},{\rm Er}$ core generated thermal-sensitive upconversion luminescent (UCL) emissions through the ${\rm Yb} \to {\rm Er}$ energy transfer process, while the inert shell protected the active dopant ions from nonradiative decay caused by surface defects [17]. Figure 1(a) shows the transmission electron microscopy (TEM) image of the synthesized UCNPs, which were monodisperse and uniform in shape with an average diameter of ${42.7} \pm 5.6\;{\rm nm}$. Energy-dispersive x-ray analysis (EDXA) was performed to determine the elemental composition of the UCNPs, where elements including Na, Y, F, Er, and Yb were detected [Fig. 1(b)]. The core-shell UCNPs, serving as thermal-sensitive molecules, were doped into PDMS elastomers (monomer/curing agent = 10:1) for fabrication of the UCNP-incorporated PDMS (UCNPs-PDMS) fiber. The UCNP-PDMS fiber was fabricated by a simple molding process, as described in Ref. [17]. Briefly, the fiber was produced by carefully injecting degassed UCNP-PDMS precursors (UCNP concentration, 1 mg/ml) into a polyethylene tube mold (inner diameter, 500 µm) through a syringe and leaving it to be thermally cured at 80°C for about 40 min. Afterwards, demolding of the solidified fiber was achieved by water pressure. Figure 1(c) shows photographs of the fabricated UCNP-PDMS fiber, where luminescent UCNPs were loaded in its middle section through segmental precursor injection. When illuminated by laser excitation at 980 nm, strong visible emissions were observed at doping regions as compared with those without doping, indicating successful incorporation of the UCNPs within the fiber.

 

Fig. 2. (a) Optical setup for multifunctional sensing based on the SMOS. (b) UCL spectra of the SMOS at various temperatures. (c) Dependence of ${\rm ln}({I_{525}}/{I_{545}})$ on the inverse temperature (${1/}T$), measured under various tensile strains up to 80%. (d) A cycling test when the temperature was periodically changed between 30°C and various settings (i.e., 40°C, 60°C, and 80°C).

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The fabricated fiber exhibited high deformability and could be easily stretched to over 2 times its initial length. Figure 1(d) shows the stress–strain curves of the fiber with/without UCNPs doping. The UCNP-PDMS fiber showed a Young’s modulus (E) of $\sim{224}\;{\rm kPa}$, slightly higher than that of the pure PDMS fiber (${\rm E} \approx {154}\;{\rm kPa}$). The rupture strain for the PDMS and UCNP-PDMS fiber reached up to 285% and 253%, with the mechanical strength being 0.9 MPa and 0.69 MPa, respectively. The soft and stretchable attributes of the UCNP-PDMS fiber make it especially promising for skin-compliance measurements. To that end, we demonstrated the UCNP-PDMS fiber as SMOS for wearable multifunctional sensing with an optical interrogation setup [Fig. 2(a)]. The SMOS (length, $\sim{1}\;{\rm cm}$) was integrated with conventional silica multimode fibers (MMFs) to facilitate light coupling and ease of wearable integration, where the coupling joints were reinforced and encapsulated with loose tubes [inset of Fig. 2(a)]. Laser excitation (980 nm, $\sim{50}\;{\rm mW}$) was launched into the SMOS through the pigtailed MMF, and the transmitted UCL emissions were measured with a portable spectrometer. A short-pass optical filter with cutoff wavelength of 850 nm was employed to suppress the incidence of the residual excitation laser into the spectrometer. To characterize its temperature response, the SMOS was stored in a digital incubator equipped with a thermocouple (resolution, $ \pm {0.1}^\circ {\rm C}$). Figure 2(b) shows the UCL spectra of the SMOS under various temperatures. The green emission bands at central wavelengths of 525 and 545 nm were attributed to the transitions of $^2{{\rm H}_{11/2}} \to ^4{\!{\rm I}_{15/2}}$ and $^4{{\rm S}_{3/2}} \to ^4{\!{\rm I}_{15/2}}$, respectively [19]. It can be seen that the intensity ratio of ${I_{525}}/{I_{545}}$ increased monotonously with the increasing temperature, where ${I_{525}}$ and ${I_{545}}$ are the UCL intensities at 525 nm and 545 nm, respectively. The changes in ${I_{525}}/{I_{545}}$ versus temperature could be described by Boltzmann distribution [21],

Taking advantage of the unique temperature-sensing attributes, we applied the SMOS for wearable sensing of the body temperature. A finger-touching test was performed to demonstrate the feasibility of the SMOS for skin temperature monitoring. An IR thermogram of the fingers was taken by a calibrated IR camera to provide a temperature reference [Fig. 3(a)]. Figure 3(b) shows a repeatable readout of the sensor during the cycling test of finger touching and removing. The sensor showed stable output at room temperature, but an immediate increase was observed once a finger touched the sensing fiber. Figure 3(c) shows a closeup of the finger-touching response with calibrated temperature readout on the right axis. The finger temperature was measured to be $\sim{34.3}^\circ {\rm C}$, consistent with results measured by the IR camera [Figs. 3(b) and 3(c)]. The response and recovery times (defined as the time required to transfer from the initial state to 90% of the response/recovery value) were about 7.5 s and 10 s, respectively.

 

Fig. 3. (a) IR thermogram of the fingers taken by a calibrated IR camera. (b) Response of the SMOS to repeated touching-removing cycles of a human finger. (c) Zoom-in readout of the SMOS to finger touching. The right axis shows the calibrated inverse value of the absolute temperature (${1/}T$).

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Except as a stretchable temperature sensor, the SMOS can also be used for strain measurement since its light transmission characteristics are strongly affected by stretching [Fig. 4(a)]. Upon stretching, the SMOS showed decreased UCL intensities at both 525 and 545 nm with the increasing strains, attributed to the longer path of light passing through the attenuating fiber [Fig. 4(b)] [16]. Moreover, as revealed in Fig. 2(b), the UCL emission at 525 nm is temperature-insensitive. Harnessing this feature, we may simultaneously measure and distinguish temperature and strain with the proposed SMOS, where the temperature and strain can be detected from $ \ln ({I_{525}}/{I_{545}}) $ and $ {I_{525}} $, respectively. Figure 4(c) shows the normalized intensities at 525 nm versus tensile strain, where the SMOS exhibited reversible intensity changes during the stretching-relaxing cycle. In particular, the strain response is almost independent of temperature with a sensitivity cross talk of 6% in the temperature range of 30–80°C and as low as 2% in the normal body temperature range (30–40°C) [Fig. 4(d)]. Stability and durability are crucial for long-term wearing applications. We evaluated the long-term stability of the UCL emissions by keeping the SMOS at a constant temperature of 35°C, where the intensity drift of the UCL emission at 525 nm was less than 1% in 5 days [Fig. 4(e)]. Furthermore, the SMOS exhibited stable performance in its strain response even during 5000 cycles of a loading and unloading test with a maximum strain of 60%, demonstrating excellent durability [Fig. 4(f)].

 

Fig. 4. (a) Photographs showing ${2\times}$ stretching of a luminescent SMOS. (b) UCL spectra of the SMOS under stretching. Inset shows the temperature readout, ${\rm ln}({I_{525}}/{I_{545}})$, which is strain-independent. (c) Normalized UCL intensities at 525 nm versus tensile strain during both the stretching-relaxing cycle. (d) Normalized UCL intensities at 525 nm as a function of temperature under strains of 0% and 80%, respectively. (e) Long-term stability of the UCL emissions, where the SMOS was kept at a constant temperature of 35°C. (f) 5000 cycles of loading/unloading cycles with a maximum strain of 60%, showing long-term durability.

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The flexible and stretchable SMOS could be mounted on an elastic kneepad to monitor strains induced by sport activities [Fig. 5(a)]. Figure 5(b) shows the capability of the SMOS in real-time detection of the vigorous extension and flexion motions of the knee joint. Moreover, various knee-related sport activities such as marching, jogging, jumping, and squatting-jumping could be detected and discriminated by virtue of the distinguishable response patterns [Fig. 5(b)]. These results demonstrate the great potential of the SMOS as a wearable and real-time sensing device for applications such as monitoring athletic performance, assessing rehabilitation progress following knee injuries, and assisting robot walking [22,23]. Furthermore, the SMOS could be directly attached on human skin to simultaneously monitor both the body temperature and movements during human activities. As a demonstration, we attached the SMOS on a volunteer’s finger joint to simultaneously monitor skin temperature and finger motion [Fig. 5(c)]. As evidenced by Fig. 5(d), during the monitoring of the temperature change in response to skin attaching, the motions of finger wagging could be simultaneously recorded with negligible cross talk.

 

Fig. 5. (a) Integration of the SMOS in an elastic kneepad to monitor knee-related motions. (b) Strain response to knee motions of flexion/extension, marching, jogging, jumping, and squatting-jumping. (c) Photograph of the SMOS attached on the skin of a finger joint for simultaneous detection of skin temperature and finger motion. Inset shows a photograph of finger bending. (d) Simultaneous temperature and strain response of the SMOS during finger wagging.

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In summary, we have demonstrated a SMOS for simultaneous sensing of temperature and strain based on UCNP-assembled polymeric optical fiber. As an optical temperature sensor, the SMOS detected temperature from the intensity ratio of the green UCL emissions centered at 525 and 545 nm, which showed a linear response in the range of 30–80°C. Especially, by virtue of the ratiometric characteristic, the readout of the SMOS was free from strain-induced interferences, enabling wearable and continuous monitoring of skin temperature during vigorous body motions. Furthermore, the SMOS exhibited reversible changes in its light transmission under mechanical deformations, allowing simultaneous detection of strains from the temperature-insensitive emission at 525 nm. We demonstrated the applications of the SMOS in real-time monitoring of both the thermal and motion activities of the human body. Our investigations will inspire a novel direction for developing next-generation intelligent devices towards humanoid robotics, human–machine interfaces, and health monitoring.