Facile Fabrication of Carbon Nanocolloid‐Silver Composite Ink for the Application of All Inkjet‐Printed Wearable Electronics

Currently, owing to the increasing demand for continuous monitoring of health status, it is highly expected to develop functional sensors with satisfied performance. However, traditional sensors generally suffer from the disadvantages of inherent rigidity, limited range, and complicated fabricating methods, failing to provide the required detection. Well‐developed printing technologies have opened a new era for fabricating electronics. The advantages of flexibility, easy scalability, and high resolution endow inkjet printing with the ability to manufacture devices with desired properties. In this work, a green carbon nanocolloid‐silver composite ink is facilely prepared and successfully applied in inkjet printing of strain sensors. Good performance such as high sensitivity (a gauge factor of 267.5), fast response (114 ms), long service life, and outstanding stability (>3000) is demonstrated for the as‐fabricated sensors. Moreover, the printed sensors are revealed to be capable of monitoring multiscale human motions for practical application as a wearable device.


Introduction
Flexible electronics have exhibited promising potential in the applications of wearable devices, energy storage materials, Eskin, and smart human-activity monitoring. [1][8][9] Flexible electronics are stretchable, lightweight, and able to fit well with rough skin.These flexible electronics achieve the purpose of transmission of information by converting DOI: 10.1002/adsr.20230007912][13] For these soft intelligent devices, the conductive components are crucial in achieving desired functions.Diverse conductive materials have been explored for building flexible electronics, including metal nanowires, [14] carbon nanotubes, [15] MXene, [16] transition metal dichalcogenides (TMDCs), [17] graphene, [18] etc.For example, graphene is an ideal alternative for fabricating wearable strain sensors.[21][22][23] As a mature conductive material, the addition of silver (Ag) nanoparticles will greatly improve the conductivity and promote the antioxidant.Even though these conductive elements have sound performance, fatal drawbacks such as high cost, complex synthesis procedures, and the addition of toxic dispersant would largely impede the wide-range applications.
On the other hand, functional flexible electronics are usually with complex structures, for which suitable manufacturing technologies are highly expected.Traditional manufacturing methods such as casting, molding, and forging for fabricating electronic devices are limited by the disadvantages of low precision, high price, strict reaction conditions, roughness, and timeconsuming. [24,25]Printing methods, as smart and flexible tools to fabricate nanomaterials-based devices, could provide more freedom and accuracy in controlling multiscale geometries. [26]mong different printing techniques, inkjet printing, which has been demonstrated with efficiency on multiple occasions, is one of the most extensively implemented techniques. [27,28][31] Due to the large freedom in selecting substrates and inks, high resolution, low production waste, easy scalability, and good controllability, intense research interest has been attracted for applying inkjet printing in the field of flexible electronics. [32,33]Up to now, there have already been some successful demonstrations of using inkjet printing to fabricate flexible electronics.However, it is still a big challenge for large-scale industrial applications due to the limited available inks.How to develop conductive inks with good printability, low price, facile fabrication process, and environmental friendliness to replace graphene or Ag is essential for the further expansion of the applications of inkjet printing on flexible electronics.
In this work, we experimentally demonstrated an all-inkjetprinted wearable strain sensor, which can be fabricated using the green carbon nanocolloid-silver (CN-Ag) composite as the conductive material and polyethylene terephthalate (PET) film as the flexible and wearable substrate via inkjet printing technology.The composite inks were synthesized by a low-cost, straightforward, and additive-free method.The sensing performance of the inkjet-printed strain sensors under different strains was investigated by measuring the relative resistance changes.The strain sensors exhibited excellent sensing performance, with high sensitivity (gauge factor up to 267.5), considerable durability, and fast response (114 ms).The sensing mechanism was revealed by observing the in-site microstructural images of the strain sensors at various strains.Finally, we verified that the as-fabricated strain sensors can be applied to efficiently monitor various human activities, including but not limited to the bending of fingers, ankles, knee joints, and wrists.

Synthesis of Printable Inks and Microstructural Characterization
In this work, the PET-based all inkjet-printed CN/Ag nanocomposite strain sensor was fabricated via a simple, low-cost, and effective strategy, which is schematically illustrated in Figure 1.The preparation of printable inks is the core of the inkjet printing process.Inks with tailored rheological properties are a prerequisite for printing functional devices with satisfied performance.For the CN/Ag/PET strain sensor, the first step is to synthesize CN-Ag composite ink with proper viscosity and particle size.In this study, the viscosity value was tested to be 7 cp.The obtained inks are then injected into the cartridge, followed by being printed onto PET thin films.After drying for 30 min under 150 °C, the substrates with sensing elements were cut into suitable shapes for various activity modes, ready for sensing performance tests.
The crystal structure of the as-prepared CN-Ag composite ink was investigated through XRD measurements.Figure 2a-c illustrates the XRD patterns of exfoliated CN, Ag nanoparticles, and the composite.As Figure 2a shows, the XRD pattern of CN shows two reflections at 2 = 26.5°and54.6°, corresponding to the (002) and (004) crystal planes of layered carbon, respectively.For the XRD pattern of Ag nanoparticles (Figure 2b), there are four diffraction peaks appearing at 2 = 38.2°,44.3°, 64.4°, and 77.5°, which corresponds to the Ag crystal planes of (111), ( 200), (220), and (311), respectively. [34]The XRD pattern of CN-Ag composite ink exhibits a superposed diffraction peak of Ag and CN, and no impurity is observed (Figure 2c).During the printing processes, the key factor for continuous patterns is to achieve small particle size with good dispersion preventing the nozzle from clogging.To demonstrate this, the flake size distribution was investigated by dynamic light scattering (DLS).According to the DLS data (Figure 2d), the CN is composed of nanoflakes with sizes between 78 and 615 nm.As Figure 2e demonstrates, the particle size of Ag paste mainly concentrates at ≈100 nm, with a small portion at ≈18.2 nm.In Figure 2f, CN-Ag composite ink is primarily composed of flakes with sizes between 8.72 and 400 nm, with an average size of ≈100 nm.The hypothesis is further verified by SEM images, as shown in Figure 2gi, which indicate that the size of the ink flakes is fairly small, mostly under 1 μm, with high concentration.Compared with carbon nanocolloid, Ag nanoparticle is relatively small, mostly under 100 nm in size.In addition, Ag-CN composite ink exhibits superior dispersibility, which is able to form an effective conductive network with good sensing ability.Therefore, the combination of Ag nanoparticles with CN is not likely to lead to nozzle blockage.To achieve high-resolution patterns for inkjet printing, there is a trade-off between viscosity, particle size, and ink concentration.The main factors found in the research that affect inkjet printing are the concentration, viscosity, and size of the flakes in the printable ink.Flakes with a large diameter can lead to nozzle  clogging, while too small particle size makes it difficult to form continuous conductive networks.Inks with unsuitable viscosity result in low-resolution printed patterns.Patterns printed with low-concentration inks exhibit inferior electrical response, and thus repeated printing is required to obtain uniform conductive films, while high-concentration conductive inks are difficult to prepare and easily clog the printer nozzles.

Sensing Performance
To investigate the sensing characteristics of the printed strain sensor, we measured the relative resistance change upon tensile strain from 0% to 7.3% and calculated the gauge factor.Figure 3 illustrates the variations of relative resistance and gauge factor as a function of the applied strain.The relative resistance increases monotonically with the increase of applied strain, which indicates outstanding sensing performance.The relative resistance climbs up to nearly 900% when the applied strain increases from 0% to 7.3%.The sensitivity of the sensors could be evaluated by gauge factor (GF), [35] which is calculated by GF = (R-R 0 )/(•R 0 ), where R 0 represents the initial resistance value, R stands for the instant resistance, and  refers to the strain calculated as the ratio of length deformation (ΔL) to the original length (L 0 ).When the applied stretching strain ranges from 0% to 3.3%, the GF of the sensor presents a relatively low value of ≈44.4.When the applied strain increases to 5.5%, the calculated GF climbs up to 148.The GF value would reach 267.5 with the further increase of the applied strain (in the strain range of 5.2−7.3%).As can be clearly observed, the printed CN-Ag strain sensor demonstrated a high gauge factor as high as 267.5, and more importantly, the sensors exhibited high sensitivity over most of the detection range.Zhang et al. [36] designed a wrinkled carbon nanotube-based strain sensor on a PDMS substrate with high sensitivity (gauge factor up to 3.01 at 44% strain) and outstanding conductivity.Wang et al. [37] fabricated flexible strain sensors based on rGO-decorated TPU, which exhibited good sensitivity (GF reached 11 under a strain of 10%).In some scenarios, large GF values can be achieved.Fu et al. [38] developed a flexible strain sensor with satisfied mechanical properties and excellent sensitivity (GF climbed up to 113).Luo et al. [39] synthesized one-step-printed strain sensors for human motion detection, with a GF value of 216 and superior durability.Compared with this work, our strain sensor demonstrated a dramatically superior sensitivity.
Figure 4a demonstrates the response of the strain sensor to repeated dynamic stretching-releasing cycles at a rate of 8 mm s −1 and a strain of 5%.The strain sensor exhibited outstanding mechanical stability and durability (small variation in the changes of relative resistance is normal fluctuation).As shown in the inserted image, when it becomes stable, the sensor exhibits good periodic cycling characteristics during a long-term test, which further reveals the superior stability and reliability of the data. [40]o demonstrate the flexibility of the obtained sensor, Figure 4b shows the response of various strains.It is obvious the strain sensor could work efficiently under various strain ranges, and with the increase of the strain values, changes of relative resistance would increase apparently.The response time, which is an important character for the sensors, refers to the time required for responding from the initial to the stretched state or being recovered from the stretched to the initial state.As depicted in Figure 4c, the response time and recovery time are 114 and 130 ms, respectively, which is quick enough to be adapted to multiple application scenarios.A supersensitive flexible strain sensor based on Ag/PDMS was reported by Li et al., [41] which maintained a fast response (125 ms).Zou et al. [42] promoted a durable sea-urchin-shaped strain sensor with a fast response (48 ms).According to the work that has been reported so far, the response time of the strain sensor is generally between 7 ms and 1 s. [43,44]s a comparison, the strain sensor developed in this study can be confirmed with a relatively fast response.
To demonstrate the good performance of the strain sensors in our work, a summary of some reported strain sensors manufactured via different materials and technologies was conducted, as shown in Table 1.Compared with other studies, the printed CN-Ag strain sensors in this work exhibit a high gauge factor, fast response, and suitable durability.Combined with the manufacturing flexibility, this inkjet printed strain sensor exhibits great advantages in the application of wearable electronics.More than 3000 (≈3500) cycles of the repeated stretching/releasing at a tensile strain of = 5%.Inserted picture is a detailed illustration of the value from 1500 to 1525 cycles.b) Sensing response under 2% strain, 4% strain, 5% strain, and 6% strain, respectively.c) Response time and recovery time of the strain sensor when strain is applied and released.

Application in Wearable Monitoring Devices
The growing demand for monitoring human activities and physiological signals requires excellent performance of sensors.The satisfied sensing performance of sensors offers quick and accurate detection of multiscale deformation, such as finger, ankle, wrist, and knee motion, and more precise movement, including swallowing, smiling, and frowning.We first use the developed inkjet-printed sensors to monitor the bending of a finger with different angles, the relative resistance changes are shown in Figure 5.It can be observed that the change in relative resistance increases significantly with rising bending angles.The relative resistance is ≈7%, 14%, 17.4%, and 67% when the finger is bent at angels of 30°, 60°, 90°, and 120°, respectively.The results indicate that the printed CN-Ag flexible sensors are promising in developing wearable health monitors.Because of the wide detection limit, high durability, and outstanding sensitivity, the CN-Ag strain sensor can be employed for various human motion monitoring.To further investigate the recognition ability of the movement of different body parts, we  attached the sensor to the surface of volunteers' ankle, wrist, knee joint, and face to measure the resistance changes.As shown in Figure 6, diverse human activities can be identified immediately.Ankle bending was first detected, eight bending-recovering cycles are shown in Figure 6a.The change of relative resistance fluctuates with the motion of the ankle, i.e., increases while the ankle is bent and recovers while the elbow moves back to the initial position.Figure 6b demonstrates the relative resistance change of the sensor to wrist bending.It can be clearly observed that the results obtained high repeatability.Figure 6c illustrates the relative change in resistance regarding knee bending.Compared with Figure 6a,b, the different human activities correspond to various amplitude characteristics.Furthermore, as Figure 6d illustrates, the subtle deformation of the throat can also be clearly reflected by the relative resistance changes of the strain sensor.Figure 6e,f displays the response of smiling and frowning, which demonstrates the differences in relative resistance changes due to the different amplitude of facial muscle movements.The results revealed that subtle facial muscle movements can also be accurately recognized.Therefore, this inkjet-printed CN-Ag strain sensor is qualified to be applied as a wearable human motion detector. [54]

Discussion of the Sensing Mechanism
To further understand the sensing characteristic, in situ SEM characterization was applied to investigate the sensing mechanism of the printed sensors.According to the results, the high sensitivity could be attributed to the formation of micro-cracks.Figure 7a illustrates the original state of the inkjet-printed strain sensor, which was smooth and continuous, indicating the presence of uniform conductive layers.Figure 7b,c displays the evo-lutions of the surface under various tensile strains.It confirms that as tensile strains increase, the cracks on the sensor surface would be first generated and then enlarged, which leads to an increment in the resistance.When the loaded strain is released (Figure 7d), the surface cracks would shrink slowly and restore to approach the approximately initial state.Although there are some micro-cracks still existing on the surface, they are so narrow and they do not penetrate the sensor surface.As a result, the resistance does not fluctuate significantly from the initial state, and the strain sensor still demonstrates a stable sensing response.It is worth noting that little plastic deformation occurred under the applied strains, which indicates the flexibility and elasticity of the sensor.When correlating the surface morphology under various strains with the relative resistance-strain curves, it can be derived that the evolution of cracks plays an important role in the sensing mechanism.The evolution of cracks would repeat during each tensile cycle, which results in data with high repeatability for the cyclic test.
The remarkable strain sensing performance of the CN-Ag composite strain sensor mainly attributes to the formation of abundant conduction pathways.The possible sensing mechanism is illustrated in Figure 8.To support this hypothesis, the surface of the strain sensor under different applied strain was observed by SEM (Figure 7).As shown in Figure 8a, before being stretched, the multiscale carbon nanoflakes and silver nanoparticles were interconnected with each other, forming a continuous conductive path and abundant pathways.After a 5% tensile strain was applied, the substrate would be elongated, leading to a reduction of thickness.The mismatching stiffness between CN-Ag composite film and PET matrix results in micro-cracks.Meanwhile, the relatively small carbon flakes and Ag nanoparticles in micro-cracks are still able to lap with each other and maintain  conductive networks (Figure 8b).The increment of strain leads to a small portion of networks being disconnected, resulting in an increase in resistance and gauge factor (Figure 8c).When the applied strain was released, micro-cracks would recover to nearly the original state and reconnect to generate the conduct networks, allowing resistance to gradually recover, as displayed in Figure 8d and observed in Figure 7d.

Conclusion
By adopting a green, high-throughput, effective, and facile strategy, we have fabricated an inkjet-printed CN-Ag strain sensor.The combination of Ag and CN enhances the electrical conductivity and promotes the sensing performances of sensors, while the PET substrate improves the stretchability.The sensors are demonstrated with good flexibility, high sensitivity (GF max = 267.5),and fast response (114 ms).Through observation of surface morphology, the sensing mechanism is confirmed.In addition, the printed CN-Ag strain sensor exhibits promising reproducibility and long service life, demonstrating the capability of monitoring various human motions.The results prove the functionality of the as-printed devices.Looking into the future, the research interest will focus on integrated flexible sensors on different substrates, to meet the growing demand for continuous human health monitoring.

Experimental Section
Raw Materials: High-purity graphite plates, deionized water, and KCl were purchased from Aladdin Reagents Co., Ltd.Ag nanoparticle colloid (BoradCON-INK550, 30-40 wt.%) with green solvent, and flexible PET substrates (50 μm) with special coatings were obtained from BroadTeko Co., Ltd.Deionized water was utilized throughout the experimental processes as electrolyte.
Preparation of CN-Ag Composite Inks: The CN dispersion was synthesized using an electrolytic stripping method. [55]High-purity graphite was used as both anode and counter electrode, and then the electrodes were immersed in deionized water in an electrolysis cell.The distance between the anode and counter electrode was 5 mm.A constant current of 20 mA was applied and the electrolysis process was kept for 7 days at ambient temperature, after which the CN dispersion with concentration of ≈3-5 mg mL −1 was achieved without any organic additives.Thereafter, the concentration of the obtained CN dispersion was magnified to 40 mg mL −1 by evaporating the solvent.
Currently, various high-performance Ag-based flexible electronics have been reported. [56,57]To reduce the usage of the amount of high-cost Ag dispersion while keeping good conductivity and ensuring environmental friendliness, the treated CN dispersion was mixed with the silver solution with a volume ratio of 1:5, for which no toxic solvent was added to adjust the viscosity.Then, the mixture solution was ultrasonically treated for 20 min to generate printable dispersion and eliminate agglomeration.The viscosity of composite inks was tested by a viscometer to justify the printability.Actually, different ratios of CN to Ag had been treated to generate the composite ink, while it was found that the viscosity of the ink under a ratio larger than 1:5 would be too low for the inkjet printers, which generally required the viscosity to be with a value of 7-14 cp.Although the adjustment of the ratio could be easily achieved by involving in some toxic solvent, this work does not consider this strategy.In general, the current method can be easily scaled up and greatly reduce the cost of mass production.
Fabrication of Inkjet-Printed Wearable Strain Sensor: A DP500 inkjet printer (BroadTeko Co., Ltd., Beijing, China) was used for manufacturing flexible strain sensors.The printing nozzle has a diameter of 20 μm.The obtained CN-Ag composite inks were injected into the ink cartridge, followed by printing the inks with a speed of 200 mm s −1 on the PET substrate.Five layers were applied with a droplet spacing of 10 μm to form a continuous conductive network.The printing pattern was designed as a 2D dog-bone shape in this study (any desired shape can actually be processed).As-printed object was dried at a temperature of 150 °C for 30 min to remove the solvent.The PET substrate was cut along the printing zone to obtain a strain sensor with a suitable shape.
Microstructure Characterization and Sensing Performance Test: To prepare samples for microstructure observation, the obtained CN dispersion, commercial Ag ink, and CN-Ag composite were diluted, followed by being dropped onto the silicon wafer, and drying at 80 °C for 30 min to remove the solvent.Scanning Electron Microscopy (SEM, JSM-IT200, JEOL) was used to investigate the particle size distribution and reveal the sensing mechanism of strain sensors.The resistance of the printed sensors was measured by a precise resistance/capacitance test machine (RC01, LinkZill).The structural information of CN, Ag nanoparticles, and CN-Ag composite was investigated by an X-ray diffractometer (XRD: Ultima IV, Rigaku).The viscosity of printable inks was revealed by a viscometer (NDJ-5S, SHANGHAI FANGRUI INSTRUMENT Co., Ltd.).The size of nanoparticles in printable inks was estimated by Dynamic Light Scattering (NanoZS90, Malvern Instruments Ltd.).A homemade motorized mechanics platform was used to conduct the uniaxial tensile and cyclic tensile strain stretching experiments.To assess the sensing performance of the obtained strain sensor, two ends of the printed CN/Ag strain sensor were attached to copper tapes by sliver paste and fixed onto the homemade motorized mechanics platform, then regular tensile strain was applied.To evaluate the multiscale human motion sensing performance, the sensors with suitable widths were attached to various parts of the volunteer to monitor different human motions.The volunteer gave written informed consent before the experiment.

Figure 1 .
Figure 1.Schematic illustration of the typical fabricating procedure of CN-Ag-based wearable strain sensors.

Figure 2 .
Figure 2. a-c) XRD patterns of the CN, Ag nanoparticles, and CN/Ag composite.d-f) Dynamic light scattering characterization of the printable inks, indicating the flake size distribution of the colloids.g-i) SEM images of the CN, Ag nanoparticles, and printable CN-Ag composite ink.

Figure 3 .
Figure 3.The relative resistance change (ΔR/R 0 ) and GF values as a function of the applied strain for the printed sensors.

Figure 4 .
Figure4.a) More than 3000 (≈3500) cycles of the repeated stretching/releasing at a tensile strain of = 5%.Inserted picture is a detailed illustration of the value from 1500 to 1525 cycles.b) Sensing response under 2% strain, 4% strain, 5% strain, and 6% strain, respectively.c) Response time and recovery time of the strain sensor when strain is applied and released.

Figure 6 .
Figure 6.Response signal of the sensor for various human motions.a) Ankle bending, b) wrist motion, c) knee bending, d) swallowing, e) smiling, and f) frowning.The insets exhibit the printed sensor attached to various body parts of the volunteer.

Figure 7 .
Figure 7. Investigation of the sensing mechanism of the printed CN-Ag strain sensor via in-site SEM.SEM image of the strain sensor surface under the conditions of a) without load, b) 5% tensile strain, c) 8% tensile strain, and d) release of tensile strain.

Figure 8 .
Figure 8. Illustration of the sensing mechanism of the printed CN-Ag sensor.a) The initial state of the strain sensor.b) The diagram of the strain sensor under 5% strain.c) The diagram of the strain sensor under 8% strain.d) Surface after the tensile strain was released.

Table 1 .
Summary of some recently reported strain sensors fabricated by different 2D materials or Ag.