- Open Access
Wearable sensors based on colloidal nanocrystals
© The Author(s) 2019
- Received: 8 January 2019
- Accepted: 12 March 2019
- Published: 2 April 2019
In recent times, wearable sensors have attracted significant attention in various research fields and industries. The rapid growth of the wearable sensor related research and industry has led to the development of new devices and advanced applications such as bio-integrated devices, wearable health care systems, soft robotics, and electronic skins, among others. Nanocrystals (NCs) are promising building blocks for the design of novel wearable sensors, due to their solution processability and tunable properties. In this paper, an overview of NC synthesis, NC thin film fabrication, and the functionalization of NCs for wearable applications (strain sensors, pressure sensors, and temperature sensors) are provided. The recent development of NC-based strain, pressure, and temperature sensors is reviewed, and a discussion on their strategies and operating principles is presented. Finally, the current limitations of NC-based wearable sensors are discussed, in addition to methods to overcome these limitations.
- Strain sensors
- Temperature sensors
- Pressure sensors
- Wearable devices
With the rapid development of the internet of things (IoT), wearable electronic devices have attracted significant attention in research fields and industry, as they can be used for remote health care monitoring and human–machine interfaces [1–5]. They are commonly integrated into clothes, glasses, and watches, and directly attached to human skin to collect physical, chemical, and biological signals generated by humans or their surroundings [6, 7]. Among the various components of wearable devices, strain, pressure, and temperature sensors are critical for the monitoring of human motion, health or physiological information, and external stimuli [8–13]. Significant research effort was directed toward the enhancement of the performance of the abovementioned wearable sensors using various materials such as graphene, carbon nanotubes, organic materials, and silicon nanomembranes; and/or by designing unique device structures [14–17]. However, costly and complex high temperature and/or high vacuum processes such as sputtering, reactive-ion etching, and thermal deposition are generally required to synthesize functional materials and/or manufacture devices [18–22]. This results in a high production cost, which limits their commercialization.
Colloidal nanocrystals (NCs) are considered promising building blocks for the next generation of wearable sensors, as they provide the following advantages. First, NCs can be synthesized at a large scale using wet chemical methods, and the resulting NC inks can be deposited onto various substrates in a large area under room-temperature and in an atmospheric environment using a solution based process such as roll-to-roll printing, drop casting, spin-coating, and inkjet printing [23–30]. Second, the electronic, optical, and magnetic properties of NCs can be easily controlled by adjusting their size, shape, composition, and surface state; thus enabling them to demonstrate application-specific functionality [31–37]. Based on these advantages, significant research effort has been directed toward the realization of high performance NC-based strain, pressure, and temperature sensors by the control of the interparticle distance between the NCs, or by the design of new NC structures [38–47].
In this brief review, the ligand exchange strategy of NCs for the development of conductive and functional NC thin films with application-specific properties for strain, pressure, and temperature sensors is discussed. Thereafter, a summary on the recently reported NC-based strain, pressure, and temperature sensors is presented, in addition to a brief explanation of their strategies, operating principles, and practical applications. Moreover, the review includes an overview of the current challenges, and a perspective on the future methods for the realization of advanced NC-based wearable sensors.
2.1 Surface ligand exchange of NCs for specific applications
2.2 NC-based strain sensors
There is a theoretical limit to the gauge factor of NC thin films, which is predicted by Eq. (2) . Although the gauge factor can be improved by increasing the initial interparticle distance or the tunneling decay term, the initial conductivity of the NC thin films decreases exponentially, which limits the practical applications of NC based strain sensors (Fig. 2c). To overcome this intrinsic limitation of NC thin films, novel strategies such as artificial crack formation and a NC heterostructure design were developed [50, 59]. Lee et al. introduced artificial nanocracks into MPA-treated Ag NC thin films by the application of a high pre-strain to the NC thin films (Fig. 2d, e) . The external strain opens closed cracks, which results in a significant increase in resistance. Using this approach, a high gauge factor of over 300 was achieved after the crack formation (Fig. 2f). Lee et al. also demonstrated Ag NC thin films with micro-crack based strain sensors, which exhibit a high stretchability, durability, stability, and sensitivity (Fig. 2g) .
Zhang et al. designed homogeneous and heterogeneous arrays of NCs with different surface capping ligands, and then evaluated their electrical and electromechanical properties (Fig. 3d, e) . As demonstrated, the gauge factor of hybrid structures can be tuned from 1 to 96 by adjusting the volume ratio of each NC according to the percolation theory. Lee et al. implemented the partial ligand exchange strategy to induce cracks and to simultaneously design NC thin films in a metal–insulator transition state for strain sensor applications (Fig. 3f) . The conventional ligand exchange process was conducted with sufficient treatment time for the formation of fully ligand exchanged functional NC thin films. In the case of Ag NC thin films treated with TBAB for over 60 s, fully ligand exchanged, highly conductive, and strain-insensitive NC thin films were formed. In contrast, partially ligand exchanged Ag NC thin films with naturally formed cracks were observed when the as-synthesized Ag NC thin films were treated with TBAB for 15 s, which exhibited an intermediate conductivity and high gauge factor of up to 300 (Fig. 3g).
2.3 NC-based pressure sensors
A pressure sensor is a device that detects a force applied to a specific area. Moreover, it is one of the most important mechanical sensors, in addition to strain gauges. Pressure sensors have attracted considerable attention in various research fields, as they can be used for medical diagnoses, touch screen, health care monitoring, and industrial applications [65–68]. Among the various types of pressure sensors, capacitive or resistive type pressure sensors, which convert applied pressure to electrical signals, are the most efficient and cost-effective [69, 70]. Significant effort was directed toward the improvement of the performance of pressure sensors, such as their stretchability, sensitivity, durability, reliability, linearity, and detection range using various materials, and by the development of unique device architectures [11, 71, 72]. In particular, nanoscale/microscale bumpy structures such as pyramids or hemispheres are generally used to improve the sensitivity and enlarge the detection range of pressure sensors [11, 21, 73]. However, complex and toxic process such as e-beam lithography or chemical etching are generally required for the fabrication of these structures.
Lee et al. demonstrated flexible pressure sensors based on NC thin films with micro-cracks (Fig. 5e) . Pressure applied to the bottom of the devices induced positive strain on the NC films with cracks, which increased the resistance. It was revealed that the sensitivity and pressure detection range can be adjusted by controlling the thickness of the substrates (Fig. 5f).
High performance NC-based pressure sensors can be utilized in various applications. The practicality and functionality of NC-based pressure sensors are demonstrated in applications that involve pulse monitoring and tactile sensors (Fig. 5g, h).
2.4 NC-based temperature sensors
There is a continuous increase in the demand for high performance temperature sensors, given that accurate temperature measurement is very important in several industries, medical fields, and research fields. The recent advancement of wearable technology promotes the rapid development of wearable temperature sensors, as they are essential components of wearable devices for health care monitoring or disease diagnoses based on body temperature measurements [13, 14]. Several studies were conducted to improve the sensitivity, stability, and durability of wearable temperature sensors, and to enlarge their temperature detection range using carbon materials, polymers, and thin metal films [74, 75]. However, complex multi-step procedures, which include high temperature and high vacuum processes, are mostly used for the fabrication of the wearable temperature sensors.
NCs can be synthesized at a large scale and deposited onto various substrates with at low-cost using solution based processes [27, 32]. Thus, to develop cost-effective and highly sensitive NC-based temperature sensors, the temperature-dependent electrical characterization of NC thin films was investigated by several researchers [38, 49, 76].
Furthermore, the MPA- or EDT-treated Ag NC thin films exhibited a negative resistance change (negative TCR) as the strain increased, whereas a positive resistance change was observed for the NH4Cl- or TBAB-treated Ag NC thin films with positive TCR values of 1.03 × 10−3 K−1 and 1.34 × 10−3 K−1, respectively (Fig. 6c–f).
Segev-Bar et al. investigated the temperature-dependent electrical properties of Au NC thin films with respect to their size and organic surface ligands. As observed, the temperature sensitivity increased as the NC size and length of the surface ligands increased (Fig. 6g, h) .
Wearable electronics have attracted significant attention, as they can be utilized in remote health care systems, human–machine interfaces, and soft-robotics, among other applications. NCs can overcome the limitations of conventional wearable devices due to their solution processability and tunable properties. Based on these advantages, significant research effort has been directed toward the improvement of the performance of NC-based wearable sensors (strain, pressure, and temperature sensors), as discussed above. However, the NC-based wearable sensors can be further improved. First, conformal contact with human skin is a critical requirement for wearable electronics, for the efficient and accurate detection of human signals [78, 79]. It is therefore necessary to design NC-based wearable sensors using soft elastomers that have stiffnesses similar to that of human skin. Moreover, the effects of the strain, pressure, and temperature on the wearable sensors should be considered, given that all these external perturbations could induce changes in the resistance of wearable sensors [49, 67, 76]. For example, changes in the applied pressure and temperature can modify the resistance of strain sensors, thus limiting the accuracy of the real strain measurement. Therefore, novel methods to decouple unwanted stimuli should be developed for the realization of NC-based wearable sensors with high accuracies. Furthermore, a power supply should be considered, to fully realize NC-based skin-mountable wearable sensors, given that conventional heavy and bulky batteries cannot be used in the system [80, 81]. Hence, self-powered NC-based wearable sensors should be developed to realize the next generation of wearable technology. Finally, while considerable achievements in printing and patterning methods have been demonstrated such as transfer printing, there is still room for improvement in the manufacturing of NC-based wearable sensors . For example, multiple steps of mask alignment, light exposure, and development using photoresists are generally required in conventional patterning methods. To reduce the fabrication steps and costs, advanced patterning techniques such as direct optical lithography using light-responsive ligands without photoresists should be developed for the realization of practical and cost-efficient NC-based wearable devices .
WSL and SJO wrote the manuscript. WSL, SJ, and SJO designed the figures. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.
This research was supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science, ICT and Future Planning (2016R1C1B2006534), and Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF-2018M3D1A1059001). This research was also supported by Korea Electric Power Corporation (R18XA06-02).
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