PDMS Film Research Articles

Flexible strain sensor with self-healing function for human motion monitoring

Flexible strain sensors with highly similar effects to human skin have been given great attention due to their potential application in personal health monitoring, human–computer interaction systems and artificial electronic skin fields. In particular, the self-healing properties of the sensors are important for their long-term and repeated use during the actual operation. Herein, a flexible strain sensor with complete self-healing function is proposed by combining self-healable PDMS film with rich hydrogen bonds and conductive ink based on recoverable liquid metal. By adjusting the contents of different components of self-healing PDMS film and the relative mass fraction of the liquid metal ink in the strain sensor, the tensile stress and resistance of flexible sensor can be changed to match different usage scenarios. The sensor can achieve a maximum tensile stress of 0.83 MPa and an elongation at break of 843%. After self-healing for 24 h at room temperature, its tensile stress can revert to 82% of the original value, while the electrical connection can instantaneously recover to initial situation after fracture surface contacts. This hints its potential advantage as wearable sensors for motion monitoring of the human body and developable applications in medical monitoring, recyclable electronics and artificial skin.

Fast Response PDMS/GaN Optical Vacuum Microsensors

In this letter, we demonstrate a PDMS/GaN-based optical microsensor for measuring vacuum pressure. The submillimeter-sized GaN device responsible for optical emission and detection is fabricated through the wafer-scale process. The integration of low-cost porous PDMS film can provide significant reflection changes under atmospheric pressure and vacuum conditions, enabling the GaN device to sense pressure variations in 22–760 Torr. The attractive features of the sensor such as compactness, ease of operation, fast response of 1.0 s, small hysteresis of 1.3 %, and high stability make it suitable for wide applications of vacuum pressure sensing.

Enhancement of the intrinsic fluorescence of ZIF-8 via post-synthetic cation exchange with Cd2+ and its incorporation into PDMS films for selective sulfide optical sensing

In this study, ZIF-8 MOF nanocrystals were synthesized and post-synthetically modified by applying different cation exchange strategies. Addition of cadmium nitrate in either methanol or DMF followed by either magnetic stirring or gentle heating led to the incorporation of a small amount of Cd (II) ions into the crystal structure in most cases, as clearly demonstrated by several characterization techniques including PXRD, SEM-EDS and FT-IR. This novel doped material exhibits a high fluorescence with the maximum emission wavelength at 444 nm upon excitation at 380 nm, which allows its use as an effective optical sensor. The sensing capability of the Cd-doped ZIF-8 material was demonstrated by its exposure to sulfide ions in aqueous solution. The fluorescence of the doped material was gradually quenched as the concentration of S2− was increased. Sensing devices based on mixed-matrix membranes (MMMs) were fabricated by using poly (dimethyl siloxane) (PDMS) as a hosting matrix for the Cd-doped ZIF-8 crystals, giving rise to fluorescent sensing films with fast and selective responses against a broad number of potential interferents.

Synthesis and characterization of UV curable biocompatible hydrophilic copolymers containing siloxane units

Tissues are highly three-dimensional structure complexes composed of different cell types and their interactions. One of the main challenges in tissue engineering is the inability to produce large, highly perfused scaffolds in which cells can grow at a high cell density and viability. Poly(dimethyl siloxane) (PDMS) is used as a flexible, biocompatible cell culture substrate with tunable mechanical properties. However, its fragility and hydrophobicity still pose a challenge. Here, we present a new strategy for the three-step one-pot synthesis of novel biocompatible hydrophilic copolymers containing siloxane units. In the first step, free radical copolymerization of acrylic acid (AA), butyl methacrylate (BMA), and 2-hydroxyethyl methacrylate (HEMA) was carried out in dioxane (DO) solution in the presence of 2,2′-azodiisobutyronitrile (AIBN). In the second step, the copolymers were modified with diepoxypropoxypropyl-terminated polydimethylsiloxane (DE-PDMS), and in the third step, the copolymers were additionally modified with glycidyl methacrylate (GMA). The modified copolymers were characterized by FTIR, NMR spectroscopy and elemental analysis. Films of modified copolymers were prepared by UV curing. SEM studies revealed microphase separated morphology with distribution of PDMS domains. The mechanical properties of the films depended on the amount of incorporated silicone modifier. The films were more hydrophilic than PDMS films. All novel copolymers showed high biocompatibility.

Multilayer Heterogeneous Membrane Biosensor Based on Multiphysical Field Coupling for Human Serum Albumin Detection.

A factor closely associated with renal disease status in clinical diagnosis is abnormal human serum albumin (HSA) concentration levels in human body fluids urine, serum, etc. The surface stress biosensor was developed as a new type of biosensor to detect protein molecule concentration and has a wide range of clinical applications. However, further sensitivity improvement is required to achieve higher detection performance. Herein, MXene/PDMS/Fe3O4/PDMS of the multilayer heterogeneous membrane biosensor (MHBios) based on the coupling of the magnetic field, electric field, and surface stress field was successfully developed to achieve high sensitivity HSA detection through magnetic sensitization. The modified antibody specifically binds to HSA at the AuNP layer, allowing the biosensor to convert the surface stress caused by PDMS film deformation into an electrical signal. When the biosensor was exposed to a uniform magnetic field, the conductive path of the conductive layer was reshaped further as the magnetic force amplified the deformation of the PDMS film, enhancing the conversion of biological signals to electrical signals. The results exhibited that the detection limit (LOD) of the MHBios was 78 ng/mL when HSA concentration was 0-50 μg/mL, which was markedly lower than the minimum diagnostic limit of microalbuminuria. Furthermore, the MHBios detected HSA in actual samples, confirming the potential for early disease screening.

Robust PFMA/CNTs composite PDMS superhydrophobic film via SI-CuCRP method for efficient anti-icing

A PFMA/CNTs composite PDMS superhydrophobic film with excellent anti-icing effect is prepared by adhering bromide carbon nanotubes to a PDMS film surface forming organic-inorganic composite reinforced structure, and then grafting PFMA brush by SI-CuCRP method. The composite PDMS films with contact angle of 170° present excellent superhydrophobicity, and still maintain the dewetting ability after 120 times friction cycle of sandpaper. After 10 finger mechanical wear and heating self-healing cycles, the PFMA/CNTs composite PDMS films can restore its superhydrophobicity, showing good wear resistance and self-healing properties. In addition, delayed icing time and ice adhesion strength of PFMA/CNTs composite PDMS superhydrophobic film are 14 min 32 s and 61.9 kPa, respectively, exhibiting efficient anti-icing. Besides, due to photothermal conversion ability of carbon nanotubes, the PFMA/CNTs composite PDMS superhydrophobic films present outstanding de-icing effect under NIR irradiation.

Stretchable flexible sensors for smart tires based on laser-induced graphene technology

Continuous feedback on a tire is an essential means to ensure tire safety. Smart tires are an important part of the future vehicle control system, which affects the safety and comfort of vehicles by combining sensors with traditional tires to achieve continuous monitoring of real-time dynamic parameters. A stretchable and flexible sensor made of laser-induced graphene (LIG) and PDMS, designed for use in smart tires, is presented in this work. The sensor is known as a LIG-PDMS sensor. Using transfer printing, LIG is formed on a commercial polyimide film under the scribing of a laser beam following the predesigned route before being transferred to a PDMS film. This technology is used to successfully prepare flexible sensors for measuring the tire road interaction at different driving speeds due to its flexibility and shape-following characteristics. The real-time monitoring of the wheel speed and the shape of the tire grounding mark during the driving process is realized by embedding multiple LIG sensors in the tire to monitor the strain information of the tire grounding. Results show that the tire deformation can be accurately feedbacked with the LIG sensors, demonstrating our method's capability for designing and manufacturing intelligent tires.

High-performance flexible self-powered triboelectric pressure sensor based on chemically modified micropatterned PDMS film

Flexible pressure sensors have played an increasingly important role in the Internet of Things (IoT) and human–machine interaction (HMI) systems. However, it is in a desire to fabricate the sensor with high sensitivity and low power consumption. In this paper, we have developed a self-powered triboelectric pressure sensor based on chemically modified micropatterned PDMS. Different wire meshes serve as template to fabricate microstructures on the triboelectric layer. To enhance the triboelectric output performance, introducing functional groups to the surface of the triboelectric layer to expand triboelectric polarity difference. By virtue of the cooperative effect of enhanced contact area variations from the PDMS microstructure’s gradual deformation, the reduced dielectric thickness as well as the increased charge density from the extended triboelectric polarity difference originated from chemical modification, the self-powered triboelectric sensor displays a sensitivity of 44.31 V kPa -1 in the range of 0.006-2 kPa and 7.16 V kPa -1 in the range of 2-8.5 kPa. Additionally, the self-powered triboelectric sensor is efficient in human motions measurement. This work provides a route to fabricate high-performance pressure sensor with morphological and chemical modification on the triboelectric polymers. • A self-powered triboelectric pressure sensor based on chemically modified micropatterned PDMS was developed. • Amino-containing and fluorine-containing functional groups were introduced to the mPDMS surface to expand triboelectric polarity difference to enhance the triboelectric performance. • The pressure sensor shows a high sensitivity, fast response time and high reliability.

Lotus leaf mastoid inspired Ag micro/nanoarrays on PDMS film as flexible SERS sensor for in-situ analysis of pesticide residues on nonplanar surfaces

Conventional surface-enhanced Raman scattering (SERS) molecular detection are based on hard and brittle substrate, which are not suitable for in-situ detection of analytes adsorbed on nonplanar surfaces. Here, we report a simple biomimetic synthesis method to fabricate lotus leaf mastoid structured AgNPs micro/nanoarrays as reliable SERS substrate. By ideal replicating mastoid structure of lotus leaf into a flexible and transparent PDMS film, followed by depositing plasmonic AgNPs, a powerful chemical sensor with high sensitivity and multiplex detecting capability is demonstrated. The employ of periodic mastoid structure array endows the sensor with high signal repeatability (RSD ∼ 8.6 %), solving the general repeatability problem of SERS substrates. In addition, the detailed designed flexible and transparent PDMS substrate is capable of identifying trace analytes on curved surfaces with excellent durability. In the proof-of-concept experiment, a limit of detection (LOD) of (10−5 M to 10−7 M) was achieved on a portable Raman device for three common pesticides residues (thiram, fonofos and triadophos) on dendrobium leaves and stem according to the molecular fingerprint, indicating its excellent in-situ detection capability. Further, the multiplex detection ability of the Ag/PDMS film is also demonstrated by analyzing the mixture of four typical analytes. Benefiting from its high signal uniformity, this flexible Ag/PDMS substrate also showed good quantitative detection capabilities.

A flexible surface-enhanced Raman Spectroscopy chip integrated with microlens

A novel flexible Surface-enhanced Raman Spectroscopy (SERS) chip integrated with microlens was proposed and designed, which consisted of PDMS film, planoconvex microlens, and silver nanoparticles (AgNPs) monolayer, and was of high signal collection efficiency. The flexible PDMS film integrated with microlens was designed by optical simulation, and fabricated by optimized micromachining process. AgNPs monolayer were uniformly assembled on the other side of the PDMS film through a liquid–liquid interface self-assembly method to form SERS chip. The prepared chip revealed excellent SERS performance with a Raman enhancement factor of about 107 and a signal variation of <11.5 %. The SERS chip was successfully utilized for in-situ detection of thiram residues on tomato skins, and its characteristic peaks could still be clearly distinguished when the concentration was down to 2.5 μM. It was shown that the proposed SERS chip was suitable for in-situ detection of a real sample on complex surface morphology and shown potential prospect in the fields of chemical and biomedical detections.

Contactless and robust dielectric microspheres-assisted surface-enhanced Raman scattering sensitivity improvement for anthrax biomarker detection.

This report presents a contactless and robust dielectric microspheres (DMs)-assisted surface enhanced Raman scattering (SERS) enhancement method to improve SERS detection sensitivity detection sensitivity. DMs that could focus and collect light were embedded within the polydimethylsiloxane (PDMS) film to avoid direct contact with the analytical solution and improve detection reliability. The as prepared DMs embedded PDMS DMs PD MS film was integrated with a microfluidic technique to enhance the SERS signal of a liquid substrate. Detection in microfluidic systems can reduce reagent consumption, shorten assay time, and avoid evaporation of the colloid substrate solution. The robustness and potential influencing factors of DMs PDMS film assisted SERS enhancement (DERS) were evaluated using 4-aminothiophenol (4-ATP) as the Raman probe. The sensing performance of the proposed method toward dipicolinic acid (DPA) was evaluated, and an evident signal intensification was obtained. Remarkably, the DMs PDMS film can also be implemented on solid substrates. A proof-of-concept experiment was performed by covering the DMs PDMS film directly over an AgNPs@Si solid substrate wherein a 5.7-fold sensitivity improvement was achieved.

Opportunities and challenges in CsPbX3@Glasses@Film with High Quality for LCD by Process Optimization towards commercialization

Although backlight display screens based on CdSe quantum dots (QDs) have basically solved the remaining problem of small display gamut, its high preparation cost limits its commercial development. In contrast, perovskite nanocrystals (NCs) with narrow band emission, simple preparation and low cost have made leaps and bounds in next-generation backlit displays luminescent material. However, the instabilities are the main issues to solve before we can apply these materials in modern displays. Researchers have precipitated perovskite NCs in inorganic glass substrates and compounded them with PDMS films for backlighting, but the important factors for commercialization have been ignored: Long-term stability under strong blue light irradiation and preparation cost. In this work, a unique melt-water quenching method combined with composition control was applied to prepare a series of glass powder. The optimal photoluminescence (PL) emission and color coordinates in the color coordinates of glasses powder were elaborately regulated by heat treatment temperature control and the introduction of rare earth Tb. Moreover, the passivation of the glass powder surface through special water treatment greatly improves its stability under long time blue light exposure. After irradiation under 20000 cd/m2 blue light (460 nm) for 500 h, the PL intensity retention rate reaches 85% of commercial CdSe film. The red and green-emitting NCs glasses powder with high photoluminescence quantum yield (PLQY) and stability were combined with PET, PS and PDMS to form a composite film, afterwards, combined with Liquid crystal display (LCD)module shows a fuller color image. Finally, after comparison, PS film was demonstrated to be the most suitable choice for commercialization backlighting.

Energy harvesting properties of the Nafion thin films

In this report, energy harvesting properties of Nafion films were investigated using piezoionic generator (PIOG) and triboelectric generator (TEG) devices. Nafion films were prepared by simple doctor blade deposition of a Nafion ionomer solution onto the aluminum electrode. The Nafion films were characterized for their surface morphology, composition, and crystallinity. Furthermore, the Nafion film exhibited strong substrate adhesion, a smooth surface, and amorphous nature. The deposited Nafion film was used directly as an active layer to fabricate the PIOG and TEG devices with ITO electrodes. Output voltages of ∼450 mV and ∼1.9 V have been observed for the PIOG and TEG devices against biomechanical energy. The maximum instantaneous power produced by these devices is approximately ∼0.205 μW cm−2, and ∼0.128 μW cm−2 for the PIOG and TEG, respectively. The PIOG performance can be further improved using Nafion composite films. Similarly, the TEG performance can be enhanced with other friction layers such as PVDF, PDMS, PMMA, and PVC films instead of ITO. Therefore, the proposed nanogenerators can be used as touch sensors and energy sources for wearable electronic devices in the future.

Direct X-ray Detectors Based on PDMS Films With Low Detection Limit and High Flexibility

The excellent mechanical properties of Polydimethylsilane (PDMS) has been widely used in hybrid flexible photodetectors, but there is hardly any research on it as the active layer of the direct X-ray detectors. In this letter, the low detection and high flexibility X-ray detector was fabricated based on pure PDMS. The metal/polymer/metal (MPM) vertical structure detector has a detection limit of 49.69 nGy <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{air}\text{s}$ </tex-math></inline-formula> −1. Carrier migration occurs in PDMS under X-ray irradiation with the sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$57.7~\mu \text{C}$ </tex-math></inline-formula> Gy <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{air}^{-1}$ </tex-math></inline-formula> cm−2 at the electric field of 2 V/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> and the dose rate of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$23.9~\mu $ </tex-math></inline-formula> Gy <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{air}$ </tex-math></inline-formula> s−1. The device still maintains a good performance after bending tests (different bending radius and 10000 cycles). This work provides a new possibility for the next development of direct X-ray detectors.

Conformal PDMS films for strengthening flexibility and retaining absorption in pyramid ultra-thin c-Si solar cells

Ultra-thin c-Si solar cells with the advantages of flexibility, translucence and portability are weakened in the performance of flexibility due to the surface light trapping structure, which reduces its future application potential. In this work, we designed a conformal PDMS film on the Si surface with pyramid structure to greatly inhibit the stress concentration on the surface and enhance the flexibility of ultra-thin c-Si solar cells. The maximum decrease of the Si surface stress can reach 71.19%, from 232.07 N/m2 of the pyramid structure to 66.86 N/m2 of the conformal PDMS film. In this process, it is found that at the stress concentration position, the conformal PDMS film can change the included angle of stress directions from 180° to 90° for reducing the probability of Si wafer rupture. In addition, the refractive index change in the vertical direction of the conformal PDMS film is more moderate, which can achieve the reflection reduction to improve the light absorption from 91.36% to 94.17%. Using multi physical simulation to evaluate the mechanical and photoelectric properties, the ultra-thin c-Si solar cell with conformal PDMS film has the best comprehensive performance, with the maximum surface stress of 74.59 N/m2 and the power conversion efficiency (PCE) of 19.04%, which is reduced by 49.49% and increased by 4.27%.

Boosting performance of triboelectric nanogenerator via polydimethylsiloxane modified with perovskite BiFeO3 nanoparticles

ABSTRACT Triboelectric nanogenerators (TENGs) have great potential applications in the field of microelectronics. However, the low surface charge density of the triboelectric layer is one of the key problems greatly limiting its output performance. Herein, we report a high-performance TENG using BiFeO3@polydimethylsiloxane (BFO@PDMS) composite film as a triboelectric layer. The results indicate that doping a small amount of BFO particles can significantly increase the dielectric constant and surface area of the PDMS film, which maximises the surface charge density of the triboelectric layer. With an optimized doping concentration , the dielectric constant of BFO@PDMS composite film is 14.2% higher than that of the pure PDMS film. The BFO@PDMS-based TENG delivers a maximum output voltage and current of 180 V and 30 μA, corresponding to 200% and 500% increases compared with pure PDMS film, respectively, which can drive an electronic watch and light 52 LEDs, indicating potential applications in microelectronic devices.

Developing an Effective and Durable Film for Marine Fouling Prevention from PDMS/SiO2 and PDMS/PU with SiO2 Composites.

Polymer film coating with a highly hydrophobic surface property is a practical approach to prevent fouling of any structures in the marine environment without affecting marine microorganisms. The preparation of a polymer coating, from a simple and easy method of solution blending of hydrophobic polydimethylsiloxane elastomer and hydrophilic polyurethane with SiO2, was carried out in this study, with the aim of improving characteristics, and the coating demonstrated economic feasibility for antifouling application. Incorporation of SiO2 particles into PDMS and PDMS/PU polymer film improved mechanical properties of the film and the support fabrication of micropatterns by means of a soft lithography process. Observations from field emission scanning electron microscope (FESEM) of the PDMS/SiO2 composite film revealed a homogeneous morphology and even dispersion of the SiO2 disperse phase between 1–5 wt.%. Moreover, the PDMS film with 3 wt.% loading of SiO2 considerably increased WCA to 115.7° ± 2.5° and improved mechanical properties by increasing Young’s modulus by 128%, compared with neat PDMS film. Additionally, bonding strength between barnacles and the PDMS film with 3 wt.% of SiO2 loading was 0.16 MPa, which was much lower than the bonding strength between barnacles and the reference carbon steel of 1.16 MPa. When compared to the previous study using PDMS/PU blend (95:5), the count of barnacles of PDMS with 3 wt.% SiO2 loading was lower by 77% in the two-week field tests and up to 97% in the eight-week field tests. Subsequently, when PDMS with 3 wt.% SiO2 was further blended with PU, and the surface modified by the soft lithography process, it was found that PDMS/PU (95:5) with 3 wt.% SiO2 composite film with micropatterns increased WCA to 122.1° ± 2.9° and OCA 90.8 ± 3.6°, suggesting that the PDMS/PU (95:5) with 3 wt.% SiO2 composite film with surface modified by the soft lithography process could be employed for antifouling application.

A Glass-Ultra-Thin PDMS Film-Glass Microfluidic Device for Digital PCR Application Based on Flexible Mold Peel-Off Process.

The microfluidic device (MFD) with a glass–PDMS–glass (G-P-G) structure is of interest for a wide range of applications. However, G-P-G MFD fabrication with an ultra-thin PDMS film (especially thickness less than 200 μm) is still a big challenge because the ultra-thin PDMS film is easily deformed, curled, and damaged during demolding and transferring. This study aimed to report a thickness-controllable and low-cost fabrication process of the G-P-G MFD with an ultra-thin PDMS film based on a flexible mold peel-off process. A patterned photoresist layer was deposited on a polyethylene terephthalate (PET) film to fabricate a flexible mold that could be demolded softly to achieve a rigid structure of the glass–PDMS film. The thickness of ultra-thin patterned PDMS could reach less than 50 μm without damage to the PDMS film. The MFD showcased the excellent property of water evaporation inhibition (water loss < 10%) during PCR thermal cycling because of the ultra-thin PDMS film. Its low-cost fabrication process and excellent water evaporation inhibition present extremely high prospects for digital PCR application.

D-shaped optic fiber temperature and refractive index sensor assisted by tilted fiber bragg grating and PDMS film

A sensor combining a polydimethylsiloxane-coated Mach-Zehnder interferometer with a tilted fiber Bragg grating is proposed for dual-parameter sensing of liquid temperature and refractive index. Mach Zehnder interference is used for high sensitivity temperature sensing while the core mode of tilted fiber Bragg grating is used to distinguish the temperature range and avoid spectral overlap. Meanwhile, the cut-off mode of tilted fiber Bragg grating is used for refractive index sensing, which can effectively eliminate crosstalk between temperature and refractive index. The average temperature sensitivity of the sensor can reach 4.88, 5.15, 4.53 and 4.38 nm/°C over the range of 20, 30, 40 and 50 °C, respectively. The refractive index is in the range of 1.3330–1.3614, and the sensing sensitivity can reach 521.92 nm/RIU. Compared with other optical fiber sensors, the sensor proposed in this paper has the advantages of simple fabrication, compact structure, stable mechanical structure and high sensitivity, which is expected to be applied in the sensing field where refractive index and temperature need to be detected simultaneously.

An energy self-circulation system based on the wearable thermoelectric harvester for ART driver monitoring

With the popularity and application of Autonomous-rail rapid transit in China, the status of drivers has also attracted more attention to reducing the number of accidents on complex urban roads. The previously used sensors to monitor driver status require external power, limiting the further development and application of the sensor. This paper proposes a sensor energy self-circulation system based on a copper foam heatsink using the wearable thermoelectric harvester. The proposed method consists of a thermoelectric harvester, boost circuit, and sensor module. The thermoelectric harvester module uses the wearable thermoelectric harvester to convert the waste heat of the human body into electric energy, the copper foam heatsink is used to enhance the heat dissipation capacity, and the PDMS film is used to reduce the heat loss. The DC-DC boost circuit module uses the LTC3108 to pull up the voltage to the starting voltage of the sensor and realizes power management. The sensor module uses the ultra-low-power sensor ADLX362 to detect changes in acceleration values. The experimental results show that the maximum output power of the system to the human body at a wind speed of 1.5 m/s is 145.7 μW, which can realize the self-powered applications of the proposed sensor. The result proves the feasibility of a sensor energy self-circulation system using the wearable thermoelectric harvester based on a copper foam heatsink to monitor the status of drivers.