Soft, Sweat‐Powered Health Status Sensing and Visualization System Enabled by Laser‐Fabrication

Thin and flexible skin electronics have attracted great attention for their applications in monitoring human health status continuously and intelligently. However, the versatility of these electronics is impeded by the performance of power supply and sensor modules, and their portability is also restricted by the need for external devices to handle data processing and analysis. Here, this work presents a wearable electronics system with health status sensing and visualization system (HSSVS), where the power is supplied by sweat‐activated batteries (SABs). The reported system enables the detection of crucial human physiological information, such as the Na+ concentration and the pH level in sweat, as well as skin temperature. The electrodes of the sensors and the batteries are fabricated by the laser ablation method. The laser‐induced polyimide (PI)/gelatin‐based graphene (LIGA) based sensors show a wide linear range for the sensing markers with high sensitivity, and high selectivity. The SABs based on laser‐induced PI/gelatin based graphene anchored with manganese dioxide (LIGA@MnO2) can support the entire sensing and visualization system with ultrathin, excellent biocompatibility, and mechanical properties. Additionally, incorporating a visualization design such as color changes in LEDs enables users to easily identify variations in health status. This integrated system demonstrates promising potential in smart sensing devices for health management.


Introduction
The rapid advancement of flexible electronics has led to increased interest in wearable technologies for healthcare, sports, and entertainment applications, due to advantages such as portability, flexibility, and excellent mechanical properties. [1]Human skin, the largest organ of the body, offers a substantial surface area for detecting diverse physical and chemical signals, that can provide valuable insights into an individual's health status. [2]For example, the skin temperature typically ranges from 31 °C to 38 °C during exercise, which can reveal individual's health status. [3]If the skin temperature rises above 38 °C, it may lead to hyperthermia or heat exhaustion, while a temperature below 31 °C can cause hypothermia. [4]The second example, sweat, the most common body fluid, has been widely reported as a valuable source of detection samples due to its abundance of multiple biomarkers, including Na + , NH 4 + , pH, and glucose. [5]he concentration of Na + in sweat typically ranges from 10 to 70 mmol L −1 , with an average concentration of about 40 mmol L −1 . [6]6a,7] Another example associates with the pH level, as which serves as a significant indicator of health status.Usually, the body maintains the pH of sweat between 4.0 and 7.0, with a typical range from 4.5 to 6.5. [8]n abnormal pH value of sweat, either below 4.0 or above 7.0, can lead to skin irritation and discomfort. [8,9]Therefore, it is crucial to develop effective and practical methods for real-time monitoring of the physical and chemical characteristics of human skin during exercise.
Recently, efforts have been directed toward the advancement of wearable sensors, with a focus on enhancing their sensitivity, stability, selectivity, and biocompatibility.However, several obstacles still remain and hinder the development of wearable sweatsensing systems.One challenge is the power source module, which is a crucial component of the sensing system.Commercial lithium batteries are commonly utilized due to their high energy density, stability, and compact size.However, their rigid format is the biggest hurdle. [10]Flexible/stretchable lithium batteries are emerging and promising solutions, while the toxicity issue poses challenge in terms of safety concerns for wearable devices. [11]1b,5c,13a,b,14] The development of SABs has opened up new possibilities for the integration of power sources and sweat sensors into an integrated system for real-time, portable, and intelligent wearable sensing systems. [15]ere we report a soft, sweat-powered health status sensing and visualization system (HSSVS) via a simple laser processing method.The HSSVS includes four SABs as the power source modules, three types of sensors as sensing modulus, a microcontroller unit (MCU) as the data processing modules, and a specif-ically designed LED array as a health status visualization component.The cathode is made using laser-induced PI/gelatin-based graphene anchored with manganese dioxide (LIGA@MnO 2 ), enabling the Mg-air-based SABs to exhibit excellent characteristics with a maximum specific capacity of 148.10 mAh g −1 , a maximum power density of 6.31 mW cm −2 , an open circuit potential (OCP) of 1.5 V, and a short circuit current (SCC) of 1.89 mA, which can continuously power 120 LEDs for over 8 h.Moreover, the LIGA-based sensors all show good linearity between signal responses and targeting concentrations with excellent specificity.The health status visualization is achieved through the color difference on the flexible print circuit board (FPCB).Demonstrations of HSSVS directly mounted onto human skin have shown the potential for continuously visualizing an individual's health status during exercise.

SAB-Based Wearable Electronics System for Health Status Visualization
Figure 1a shows the schematic illustration of the SABs-powered HSSVS, including SABs for powering the entire system, a flexible biosensing platform for detecting Na + and pH in sweat, and a thermistor integrated on a FPCB (33 × 31 mm) for monitoring skin temperature and an LED array for health status visualization.This system facilitates the quick and intuitive interpretation of an individual's health status based on the LEDs illumi-nation.Once the SABs are activated by sweat during exercise, the sensor platform can detect the Na + concentration and the pH level in sweat, as well as the skin temperature of the human body.Then the LEDs are controlled by MCU in accordance with the voltage measured by the sweat sensors and skin temperature.When the values are lower than the normal range, the LEDs generate orange light, while when the values are within or higher than the normal range, green and red light are generated, respectively.This color-coded LED array provides real-time visual feedback on an individual's health status. Figure 1b shows the schematic diagram of flexible SABs to illustrate their essential components.The SABs consist of several layers, starting with commercial medical waterproof breathable tape (150 μm thick), which not only functions as an adhesive layer to attach SABs to the skin but also allows air exchange for optimal performance.The LIGA@MnO 2 electrode is used as the cathode of the SABs (10 × 5 mm), while a thin Mg foil (10 × 5 mm, 50 μm thick) is used as the anode of SABs.To facilitate rapid sweat absorption, a thin cotton layer (10 × 5 mm, 1 mm thick) containing potassium chloride (KCl) powders is incorporated as the electrolyte.The chemical reactions are shown as follows: [16] Anode : Mg → Mg 2+ + 2e − (1) When sweat is absorbed into the cotton layer, the Mg anode undergoes a reaction with oxygen and water, producing magnesium hydrate (Mg(OH) 2 ).During this process, Mg is oxidized and releases electrons, while oxygen is reduced by obtaining these electrons.This electrochemical reaction generates a flow of electrons, which can be harnessed to produce electrical energy used for powering the sensing system.Here, we use MnO 2 , a widely used material for cathodes in batteries, due to its low-cost and environmentally friendly properties. [17]In the present study, a onestep strategy is employed to fabricate a LIGA@MnO 2 electrode through the laser ablation method.The detailed characterization of LIGA@MnO 2 -based SABs can be found in part 2.3.Figure 1c displays the schematic diagram of the biosensors in the HSSVS.
The biosensors consist of two working electrodes and one reference electrode for the detecting Na + and pH in sweat.The LIGAbased electrodes offer both good electrical properties and great mechanical behaviors.The detail discussion of the LIGA-based sensors is shown in part 2.4.

Characterization of the LIGA, LIGA@MnO 2 Composite
The procedure to fabricate LIGA-based electrodes is depicted in Figure S1 (Supporting Information).The morphologies of the asprepared electrode were examined by scanning electron microscope (SEM).As observed in Figure 1d and Figure S2 (Supporting Information), the laser-induced PI-based graphene (LIG), the laser-induced PI/gelatin based graphene (LIGA), and the laserinduced PI/gelatin based graphene with manganese dioxide anchored (LIGA@MnO 2 ) all show a grooved structure microscopically with high specific surface area, and the PI/gelatin based graphene exhibits a more pronounced porosity compared with LIG, which can provide more specific surface area for MnO 2 to anchor on.According to Figure 1e and Figure S3 (Supporting Information), the energy dispersive spectroscopy (EDS) image analysis has confirmed the formation of LIGA@MnO 2 by the presence of carbon, manganese, and oxygen.The Raman spectra of LIG, LIGA, and LIGA@MnO 2 show three characteristic bands: D-band (≈1350 cm −1 ), G-band (≈1580 cm −1 ), and 2Dband (≈2700 cm −1 ) (Figure 1f). [18]The D-band in laser-induced graphene suggests the presence of structural defects attributed to sp 3 hybridized carbons and an increased number of edge planes.
The G-band implies the E 2g phonon vibration of graphitic carbon, while the 2D-band indicates the restoration of sp 2 hybridized carbons. [19]15d,20] Here, the I D /I G ratio of LIG is 0.76, while that of LIGA is 0.91.The higher I D /I G ratio of LIGA indicates a higher density of photoinduced defects compared to LIG.This is consistent with the SEM morphologies, revealing a more porous structure of LIGA.17a,21] These electrodes also exhibit excellent mechanical flexibility, as shown in Figure S4 (Supporting Information).

Performance Characterization of the SABs
The battery performance characterization of SABs is shown in Figure 2. The anode of SABs is a commercial Mg foil, while the cathode is the LIGA@MnO 2 .To achieve optimal battery performance, the content of MnO 2 and the types of electrolytes are carefully optimized.The content of MnO 2 is highly associated with the concentration of precursor solution (Mn(Ac) 2 ) and the spin coating speed of the precursor solution.As illustrated in Figure 2a,b, the power density of LIGA (4.06 mW cm −2 ) is higher than that of LIG (3.66 mW cm −2 ) due to its more porous structure, as confirmed by SEM images and Raman spectra.All the LIGA@MnO 2 -based electrodes exhibit higher performance than those counterparts without MnO 2 , indicating the laser-induced PI/gelatin-based graphene with MnO 2 anchored can significantly enhance the performance of SABs.
Through optimization experiments, it is determined that the optimal concentration of precursor solution (Mn(Ac) 2 ) is 1 M, and the optimal spin coating speed is 1000 rpm, resulting in the highest power density of 6.31 W cm −2 .The sheet resistance of LIGA@MnO 2 , OCP, and battery efficiency after 2000 s at different concentrations of Mn(Ac) 2 solution and spin coating speeds are investigated to obtain the optimal parameters.According to Figure S5 (Supporting Information), the LIGA@MnO 2 electrode spin-coated with 1 M Mn(Ac) 2 solution at 1000 rpm exhibits the lowest sheet resistance of 18 Ω □ −1 , relatively high OCP of 1.6 V, and 98% of initial OCP after 2000 s once activated by the electrolyte, which is consistent with the parameters optimized for the highest power density.Then, KCl, NaCl, and NH 4 Cl are chosen for the optimization of electrolytes of SABs.As demonstrated in Figure 2c, the NaCl and KCl-based electrolytes show a high specific capacity of 153.63 and 148.10 mAh g −1 , both of which provide enough energy to power the HSSVS.
To minimize the interference from the Na + sensor, the KCl is selected as the electrolyte component in the present study.The OCP of SABs is about 1.5 V (Figure S6, Supporting Information) and the short circuit current is about 18.69 mA by linear sweep voltammetry (LSV) testing (Figure 2d). Figure 2e,f shows the electrical response of the SABs in relation to the injected sweat volume.The SABs exhibit activation upon injecting 1.3 μL cm −2 of sweat, and as the sweat volume increases to 2 μL cm −2 , approximately 98% of their maximum OCP can be achieved.This that the SABs can be effectively activated with a small sweat volume and within a relatively short period of time (<10 s).The fast response and low sweat volume required for the SABs present a great advantage in powering the sensor and controlling, and visualization modules promptly during human body perspiration.Figure S7 (Supporting Information) displays the performance of the SABs when operating in sweat under different pH levels.The specific capacity is around 120 mAh g −1 , which demonstrates that the SABs provide a sufficient energy supply for the HSSVS among different pH ranges.To further verify the superiority of the SABs, we connected two, three, and four SABs in series.These series configurations of SABs result in an increase in OCP of 2.836, 4.181, and 5.474 V, respectively, demonstrating the potential of SABs can be designed to meet various power consumption requirements of common flexible electronics (Figure S8, Supporting Information).The four SABs in series can also power 120 green LEDs array for over 8 h once activated (Figures S9 and S10, Supporting Information).In addition, the mechanical stability is also investigated through a bending test.After repetitively bending the SABs over 500 cycles at a constant angle of 90°and a frequency of 2.6 Hz, the OCP outputs stabilize at 1.5 V (Figure 2g). Figure 2h and Figure (Supporting Information) also exhibit the output performance at different angles (45°, 90°, 135°, 180°).The OCP of the SABs fluctuates between 1.458 and 1.471 V, which is about a 1% variation from the initial OCP.Furthermore, the OCP of the SABs will recover to their initial voltage once released, indicating the excellent flexibility and robustness of the SABs.Moreover, The SABs have its unique advantages in energy supply field, particularly in wearable electronics, due to their good biocompatibility.
In this study, The SAB uses a cathode made of LIGA@MnO 2 , KCl as an electrolyte, and a commercial Mg foil as the anode.A commercial medical waterproof breathable tape is used to secure the entire battery to the arm and allows air exchange for optimal performance.All materials used have been proven to have high biocompatibility, making it safer and more feasible for prolonged contact with the skin. [22]According to Figure S12 (Supporting Information), the SABs exhibit good biocompatibility, as there is no skin irritation reaction observed after wearing them for 3 h.

Sensing Performance of Biosensors
Figure 3 exhibits the sensing performance of biosensors.Firstly, a thermistor was integrated into the FPCB to enable skin temperature monitoring.As demonstrated in Figure 3b, the thermistor has a linear range of 20 °C to 40 °C, which is sufficient for detecting skin temperature.Moreover, its short response time of approximately 1.09 s and quick recovery time of about 2.57 s make it feasible to measure human skin temperature quickly.
To further enhance the functionality of the device, a flexible integrated sensor patch based on LIGA was fabricated for in-situ and real-time monitoring of Na + concentration and pH level in sweat.The schematic is illustrated in Figure 3a.The pH sensor is prepared through electroplating polyaniline (PANI) onto the LIGA electrode, while the Na + sensor is formed by first electroplating poly (3, 4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) onto the LIGA electrode and then coating it with a Na + ion-selective layer.The sensing performance of the LIGA/PANI-based pH sensor is demonstrated in Figure 3d,e.the sensor exhibits excellent linear response within the pH range from 3 to 9. The selectivity of the pH sensor is also evaluated by adding various interference substances, including NaCl and glucose (Figure 3f).Negligible responses are observed when these interferences are injected into the blank solution, while a significant voltage response is observed when the solution pH is changed from 5 to 7. All these results verify that this pH sensor exhibits good linearity between responses and pH levels with high selectivity in complex environments.Then, the LIGA/PEDOT:PSS/Na + selection layer electrode is selected as a Na + sensor with a linear range from 10 to 150 × 10 −3 M (Figure 3g,h).Compared with the addition of 10 × 10 −3 M KCl and NH 4 Cl, an obvious voltage response is observed upon adding 10 × 10 −3 M NaCl (Figure 3i).Benefiting from the excellent sensing performance of Na + , pH, and skin temperature sensors, this integrated sensor patch is able to provide real-time and accurate health information for subjects during exercise.

On-Body Real-Time Health Status Visualization
Visualization is a critical aspect of health monitoring as it allows individuals to easily recognize their health status.This involves the interaction between sensor information and users, which is vital for building a closed-loop healthcare system.Encouraged by the excellent performance of SABs and biosensors, here we develop an HSSVS that incorporates the SABs as the power supply module, the biosensors as the sensing module, an MCU as the control module, and a designed LED array as the health status visualization module.The schematic of the circuit is shown in Figure 4a.The circuit design of HSSVS is demonstrated in Figure 4b.Firstly, the SABs provide about 3 V once activated by sweat.Then the voltage regulator stabilizes the voltage output at 3.3 V for operating the MCU and other electronics.According to the relationship between biomarker concentration and voltage response, the Na + concentration, pH level, and skin temperature can be displayed by the specific color of the LEDs.Based on previous research into the typical level of Na + and pH in human sweat, as well as skin temperature, [3,9,14a] here we classify each of the three markers into three grades that are low (color of orange), normal (color of green), and high (color of red) to reflect health status (Figure 4c, the color difference of the LED array on the FPCB is shown in Figure S13, Supporting Information).The MCU will control the color of the LED array to indicate different statuses of the human body intuitively.Figure 4d depicts the biosensor patch and FPCB of the sensing system, which demonstrate excellent flexibility even when bent under a wide range of angles, allowing it to operate continuously during exercise.The entire system, HSSVS, can be worn on the arm, once activated by sweat, the LEDs display different colors to indicate the wearer's health status (Figure 4e).Furthermore, the lighted LEDs can also serve as a warning signal when running at night.Based on the comprehensive design, all the results demonstrate the potential of this soft, sweat-powered HSSVS for continuously monitoring human physiological information and visualization feedback about health status during exercise.Additionally, compared to recent work on visualization methods for closed-loop systems (as shown in Table S1, Supporting Information), HSSVS has the advantages of portable, biocompatible, easy to read, and without requiring any external devices.This promising approach has broad prospects in many application fields such as outdoor activities and construction work.

Conclusion
In this study, we developed a soft, sweat-powered HSSVS for monitoring health status during exercise.The entire system is powered by LIGA@MnO 2 -based SABs, fabricated using a laserinduced strategy, which exhibit a maximum specific capacity of 148.10 mAh g −1 and a maximum power density of 6.31 mW cm −2 .The sensing module includes LIGA-based sensors that can detect the level of Na + and pH in sweat during exercise, as well as the skin temperature by the sensitive thermistor.By integrating the SABs, biosensors, MCU, and designed LED array, the system provides real-time visual feedback on the human health status based on the LED illumination.The materials selected for the power module, sensing module, integration strategies, and LED reminder design in this study offer a practical and feasible approach to managing health status.
Fabrication of Sweat-Activated Battery: The cathode of SABs was fabricated by the laser-induced method.Firstly, a piece of PI (polyimide) film was used as the flexible substrate, which was cleaned three times with DI water and alcohol and then dried in the air.The PI film was attached to the glass plate to make it flat, and the wettability was improved by using plasma.Next, the precursor solution consisting of 1 M Mn(Ac) 2 and 1 g mL −1 gelatin was coated on the PI film using the spin coating method at 1000 rpm for 60 s to create the functional layer.After drying in air.The PI/gelatin-Mn(Ac) 2 was treated by a CO 2 laser machine (Mingchuang Laser,  = 10.64 μm) with a power of 2 W, a speed of 20 mm s −1 .The anode used was a commercial Mg foil (10 × 5 mm), and a cotton layer with KCl powder was sandwiched between the Mg foil and LIGA@MnO 2 to form the SAB.Finally, another cotton layer (15 × 10 mm) and a commercial medical waterproof breathable tape were used to fix the entire SAB.To extend the battery's storage time, the whole battery can be vacuum sealed in a thin bag.
Fabrication of Electrochemical Sensors: The electrode was obtained using a similar method by CO 2 laser machine (Mingchuang Laser,  = 10.64 μm).Firstly, a gelatin layer (gelatin solution: 1 g mL −1 ) was coated on the PI film using the spin coating method at 1000 rpm for 60 s.The dried PI/gelatin layer was then treated using the laser machine with a power of 2 W and a speed of 20 mm s −1 to fabricate the laser-induced PI/gelatin based graphene (LIGA).After the LIGA is fabricated, the Na + and pH sensors were made with specific modifications.For the Na + sensor, a PEDOT:PSS layer was electrodeposited on the LIGA electrode using the constant current electrodeposition method (2 mA cm −2 , 900 s) in a mixed solution of 3,4-ethylene dioxythiophene (EDOT, 0.01 M) and polystyrene sulfonate (NaPSS, 0.1 M).Next, 5 μL of Na + sensitive cocktail (100 mg mixture of Na ionophore X (1% w/w), sodium tetrakis [3,5bis(trifluoromethyl)phenyl] borate (Na-TFPB, 0.55% w/w), polyvinyl chloride (PVC, K-value 72-1, 33% w/w), and bis(2-ethylehexyl) sebacate (DOS, 65.45% w/w) dissolved in 610 μL of tetrahydrofuran17 was drop-casted on the PEDOT:PSS layer and dried overnight in a 4 °C refrigerator.For the pH sensor, PANI was coated on the LIGA electrode using the cycle voltammetry method (−0.2 to 1 V at a scan rate of 100 mV s −1 ) in a solution of 0.1 M aniline and 1 M H 2 SO 4 .The reference electrode was fabricated by using chloride silver paste on the LIGA electrode.
Operation of Sweat-Powered HSSVS: For operating the HSSVS, a low-power programmable MCU (STM32L031K6U6TR, STMicroelectronics Inc.) was adopted to read the analog signals from sweat sensor arrays (Na + and pH) and skin thermal sensor, and meanwhile controlled the bicolor LEDs (green and red) for health status information visualization.The electronic components for system operation and health status visualization, as well as a temperature sensor, were integrated into the FPCB.The sweat sensor is connected to the FPCB via silver paste.The entire system was powered by the SAB.As the electronic system operated under 3.3 V, an LDO (LP3990MFX-3.3/NOPB,Texas Instruments Inc.) was exploited to step down the voltage from the battery.During application, the measured data of Na + and pH from the sensing panel were converted into digital signals through the 12-bit ADC function embedded in the MCU.Furthermore, skin temperature was obtained by the NTC thermistor under the bottom layer of the controlling panel through a voltage-dividing circuit.Similar to the sweat sensors, the analog signal from the temperature sensor was also converted to a digital signal, and then, based on the data, the temperature can be calculated back.Finally, for the health status visualization function, three bicolor LEDs were controlled by digital pins of the MCU in accordance with the voltage measured by the sweat sensors and temperature.If the temperature, pH, or Na + concentration fell below the set range (31-37 °C for temperature, 5-7 for pH, and 20-60 × 10 −3 M for Na + concentration), the LEDs emitted an orange color, which was created by combining green and red lights.When the values were within the set range, the LEDs emitted green light.Conversely, when the values exceeded the set range, the LEDs emitted red light.(Figure S13, Supporting Information).As a result, the concentrations of Na + , pH, and the skin temperature of a user can be detected in real-time.Any imbalance within the body can be noticed by the user through HSSVS.
Characterization: The SEM images were obtained by FEI Quanta 250.The open circuit was measured by the data acquisition (DAQ)/multimeter system (PowerLab 16/35, AD Instruments).Polarization curve measurement, chronopotentiometry, constant current electrodeposition, and cycle voltammetry deposition were achieved by the electrochemical workstation of CHI 660E.The Raman spectrums were obtained on a microscopic laser confocal Raman spectrometer (Horiba LabRAM HR800).
Statistical Analysis: All data figures in this paper were analyzed using Origin software.The open-circuit voltage data were presented in the form of mean ± standard deviation (SD) that was calculated by the stable voltage value measured by an electrochemical workstation (CHI 660E) with a sampling interval of 0.1 s.The sheet resistance values were presented in the form of mean ± SD, which were calculated based on three times of measurements of the sheet resistance using four probe resistivity tester (Helpass Electronic Technologies.Inc, HPS2523).

Figure 1 .
Figure 1.a) Demonstration of sweat-activated batteries (SABs) powered health status sensing and visualization system (HSSVS) for health status visualization.The LEDs display three different colors, that is orange, green, and red representing low, normal, and high levels of three markers (temperature, pH, and Na + ), respectively.b) Schematic illustration of SABs.c) Schematic illustrations of Na + and pH sensors.d) Scanning electron microscope (SEM) image of LIGA@MnO 2 .e) Energy dispersive spectroscopy (EDS) image of LIGA@MnO 2 .f) Raman spectra of LIG, LIGA, and LIGA@MnO 2 .

Figure 2 .
Figure 2. a) The power performance of sweat-activated batteries (SABs) with different concentrations of Mn(Ac) 2 solutions.LIGA@MnO 2 -1 and -2 stand for 1 M, and 2 M of Mn(Ac) 2 solutions.b) The power performance of SABs at different spin coating speeds of Mn(Ac) 2 solutions.c) Specific capacity performance of SABs at different electrolytes.d) Linear sweep voltammetry (LSV) curve of SABs.e) Electrical response of the SABs with increasing injected sweat volume.f) V/V max of the battery as a function of the added sweat volume.g,h) Effect of mechanical deformation on the electrical response of SABs.g) Bending over 500 times.h) Bending at 45°, 90°, 135°, and 180°.

Figure 3 .
Figure 3. a) Schematic of LIGA-based sensors.b) Sensing performance of the temperature sensor.c) Response and recovery time of the temperature sensor.d) Sensing performance, e) corresponding fitting curve, and f) anti-interference performance of the pH sensor.g) Sensing performance, h) corresponding fitting curve, and i) anti-interference performance of Na + sensor.

Figure 4 .
Figure 4. a) Schematic of the designed circuit.b) Simplified block diagram of the circuit design of the whole health status sensing and visualization system (HSSVS).c) A range of different biomarkers of the human body during exercise, also the color range of the corresponding health status.d) Optical images of the microelectronics system integrated with sensors and flexible circuits.e) Optical images of the health status visualization system mounted on the human arm with enlarged details.