Wrinkled Silver Nanowire (AgNW) Sensors: Next-Generation Flexible Devices

1. Introduction

Flexible and stretchable electronic devices are widely desirable which has led to discoveries of materials that can work under high strains without compromising performance. These electronic devices can be electronic wearables or soft electronic devices that are required to be attached to deformed surfaces like skin and should withstand large deformations. Generally, materials with high conductivity, such as metals incorporated with elastomers are used for such purposes. Metals having distinct morphologies such as nanowires, films, nanoparticles, etc., can be used to form various structures including wrinkles, meshes, waves, etc. Silver nanowires (AgNWs) have emerged as promising materials due to their high conductivity and mechanical properties. The wrinkled structures are formed when an elastomeric substrate is coated with a rigid or stiff thin layer and the system experiences compressive stress or the removal of tensile stress. The contraction of these elastomeric substrates can be caused by thermal, solvent, or mechanical treatment. Stretchable electrodes made with wrinkled AgNWs have shown high stretchability and are widely used in chemical and biological sensors with increased electrochemical signal and analytical sensitivity.

The wrinkle patterns have become a simple and cost friendly solution that is emerging as an effective method to enhance flexible sensor performance. The wrinkle structure not only provides improved sensitivity by increasing the contact area but has also been a widely researched topic in recent years due to its controllability and low cost.

2. Mechanical Limitations in Flexible Sensors

Flexible sensors are often used in wearables, robotics, and health monitoring, where these sensors are required to have accuracy, stretchability as motion is required and durability. Many high-performance flexible strain sensors are made by the deposition of a rigid functional film, like metal or conductive nanomaterials, coated on a stretchable substrate like polydimethylsiloxane (PDMS), silicone rubber (Ecoflex), and polyurethane (PU). But these sensors face a big challenge in reliability and durability. The rigid film and flexible substrate have distinct mechanical properties, where the rigid films can tolerate very small deformations while the flexible substrate has wide stretchability. This creates high interfacial stress during bending, stretching, or motion, leading to interfacial failure and buckle delamination. Interfacial failure means that the rigid film starts to peel off or crack from the stretchable substrate while under strain, and buckle delamination means the rigid layer starts to buckle or wrinkle uncontrollably, forming random deformations. Furthermore, materials like PDMS, Ecoflex, and PU are viscoelastic, which under high strain and repeated usage can lead to instability of film microstructures.

3. AgNW Wrinkled Structures: A Solution for Durability and Sensitivity

Silver nanowires (AgNWs) are used as a conductive layer due to their high conductivity and mechanical flexibility. When spray-coated on PDMS, AgNWs offer low cost and high efficiency. The random network structure provides mechanical flexibility, allowing the nanowires to twist, bend, and slide, which makes the structure less prone to breakage under high strains. The entire fabrication process is inexpensive and does not require complex techniques. The wrinkle morphology and sensor sensitivity can be enhanced through UV irradiation treatment, motor speed control, applied pre-strain, and optimizing the PDMS thickness. Furthermore, recent studies have explored mask technology for the generation of patterned wrinkle structures on the PDMS surface to enhance sensor performance. This method enables precise control over the shape and distribution of the wrinkles at the micrometer scale for improving sensor performance. The wrinkled structures increase the contact surface area for the AgNWs, leading to improved sensitivity. When stretched, the wrinkles unfold, accommodating strain without stressing the interface or film. This eventually helps to prevent delamination and crack formation on the AgNW layer.

Further, some studies have proposed the incorporation of gradient-wrinkle structures to further expand the measurement range of flexible sensors. A study has proposed a new method for preparing AgNW-coated PDMS flexible piezoresistive sensors with gradient wrinkles. Gradient wrinkles have different sizes across the substrate surface. This method includes precisely controlling the mask position and UV irradiation time while using a stepper motor to pull the mask at a uniform speed. This gradient wrinkle design provides the sensor with a wide sensing range, with a continuous increase in the contact area through gradual activation of the wrinkles.

4. Fabrication Techniques for Wrinkled AgNW Sensors

Method 1: Wrinkles are formed because the layers of the flexible substrate and AgNWs used have different mechanical properties. When they are stressed or released, the mismatch causes one layer to buckle and create wrinkles. The wrinkle structures are created by first stretching the flexible substrate, i.e., the PDMS substrate. While stretched, PDMS is treated with ultraviolet (UV) light and ozone (UV/O₃ treatment). While still in the stretched configuration, a layer of silver nanowires (AgNWs) is coated onto the surface of the substrate using the bar coating technique. Then the stretched PDMS is released, and it returns to its original length. Since the AgNWs layer is stiffer than the inner PDMS, it forms wrinkles due to mismatched mechanical properties.

Method 2: In a study, silver nanowire (AgNW)-based stretchable sensors with wrinkled structures were fabricated using a two-step method involving water-induced swelling followed by AgNW deposition. Sodium chloride particles, which are highly soluble additives, were incorporated into the elastomer matrix. Then, the elastomer substrate was soaked in dopamine aqueous solution, causing significant swelling in the substrate. Dopamine was deposited onto the surface, significantly increasing its hydrophilicity. As dopamine acts as a highly adhesive layer, the dopamine-modified swollen samples captured the nanowires when immersed in AgNW/ethanol suspension. After removal of residual water, sensors with a wrinkled conductive network were produced. These fabricated sensors exhibited high sensitivity when subjected to external deformation from 10% to 100% strains, with good electrical repeatability during 8,000 stretching/releasing cycles, and exhibited consistent responses at various deformation frequencies.

5. Sensors Employing Wrinkled Electrodes

5.1. Piezoresistive Sensors

Piezoresistive sensors are devices used to convert mechanical pressure or strain into a change in electrical resistance, making them highly suitable for detecting physical stimuli such as force, touch, or pressure. In a study, silver nanowires (AgNWs) were integrated onto a wrinkled polydimethylsiloxane (PDMS) substrate surface, and the sensor's sensitivity and operating pressure range were finely tuned. Under optimized conditions, the sensor achieved a high sensitivity of 2.588 kPa⁻¹ and operated across a wide pressure range, with the capability to detect ultra-low pressures below 1 Pa. Additionally, this microstructured elastic sensor demonstrated rapid response and recovery times of 10 ms and 20 ms, respectively, along with outstanding durability and stability during repeated loading cycles.

5.2. Capacitive Pressure Sensors

Capacitive pressure sensors are devices that operate by detecting variations in capacitance that result from mechanical deformation of the electrodes and the dielectric layer when subjected to pressure. By enhancing the structural and material design of both the electrodes and dielectric components, the sensor sensitivity for wearable applications can be improved. For instance, in a study, a wrinkle-structured dielectric layer was fabricated by bonding an AgNWs/PMMA composite film onto a pre-stretched PDMS substrate. Upon releasing the strain, wrinkles were naturally formed on the surface of the AgNWs/PMMA film. A second layer comprising AgNWs blended with PEDOT:PSS on PDMS was laminated on top to serve as the upper electrode. This approach created a capacitive sensor with a wrinkled dielectric interface, leading to enhanced performance. Specifically, the wrinkled structure increased the effective contact area and the dielectric constant, resulting in a high sensitivity of up to 2.76 kPa⁻¹ under low-pressure conditions (below 100 Pa).

5.3. Triboelectric Pressure Sensors

Triboelectric sensors are devices that operate by converting pressure inputs into electrical signals. In a study, a fingerprint-like hierarchical wrinkle pattern was formed by applying mechanical strain to a PDMS film embedded with silver nanowires (AgNWs), which was then subjected to Ar (argon) plasma treatment. This process produced a well-defined, periodic herringbone wrinkle pattern that enhanced the triboelectric effect. Further, a textile-based triboelectric sensor was constructed using an ethyl cellulose (EC) nanofibrous membrane as the positive layer and a PVDF/AgNWs nanofibrous membrane as the negative layer. By roughening the textile fibers, a high sensitivity of 1.67 V kPa⁻¹ was achieved within a low-pressure range of 0-3 kPa.

6. Emerging Applications of Wrinkled AgNW Flexible Sensors

6.1. Wearable Electronics

Wrinkled AgNW sensors operate primarily as pressure sensors that detect deformation from body movements in wearable electronics. In a study, a pressure sensor based on AgNW was effectively utilized for real-time monitoring of human respiration. While flexible pressure sensors have seen widespread use in wearable health-monitoring systems, the lack of air impermeability has limited their practical implementation in continuous physiological detection. The pressure sensor developed had an air-permeable design, offering prolonged monitoring of human physiological signals such as respiration. The breathable sensor was integrated into a smart mask, making it useful for tracking respiratory activity. Specifically, when the sensor is attached to the inner surface of a mask, it detected the mechanical force of breathing in real time.

6.2. Health Monitoring

The importance of real-time monitoring of human movement is becoming increasingly prominent due to rising personal health concerns. Wrinkled AgNW sensors, due to their high flexibility and high sensitivity, are capable of monitoring a wide range of body movements, which helps in assessing human health in real time. Movement and body pressure are detected by directly fixing the sensor onto the skin. In a study, the sensors were fixed at the finger joints using transparent tape, and the relative resistance of the sensors changed as the finger joints bent at four different angles. With the increase in the bending angle of the finger joint, the force exerted on the sensor gradually increased, and the change in relative resistance increased accordingly. Also, attaching the sensor to the elbow joint showed a similar change in relative resistance to that of the finger joint when the elbow was bent at different angles.

The sensor was also secured to the human larynx using adhesive tape, and when swallowing occurred, a significant change in the relative resistance of the sensor was observed, which indicated that the sensor was able to successfully detect the physiological changes associated with swallowing.

6.3. Human-Machine Interfaces (HMI)

Communication between humans and machines can be facilitated through the integration of hardware and software systems. Flexible sensors serve as critical components by converting human mechanical actions into electrical signals that machines can easily interpret and respond to.

Several studies have been conducted to demonstrate these human-machine interactions. In one study, a pressure sensor incorporating a nanofiber dielectric layer made of AgNW/TPU was developed, which was positioned between two identical Au/PDMS layers to facilitate human-machine interaction. They created a piano glove equipped with 10 separate sensors, each of which corresponded to a musical note, allowing the user of the glove to play the piano. These sensors were linked to a circuit board that wirelessly transmitted the capacitance changes to a smartphone application through a Bluetooth module. Similarly, in another study, a wearable touch-sensitive keyboard for human-computer interaction with these flexible sensors was constructed. The keyboard detected changes in capacitance, and the signals were sent to a microprocessor and then transmitted through a wireless module. In addition, sensors attached to human fingers could be used to control a shooting video game by transmitting input data to a data acquisition (DAQ) board.

6.4. Soft Robotics and Electronic Skin

Various flexible electronic skins, also called E-skins, are developed to replicate the diverse sensing capabilities of human skin. Pressure sensors are essential components of electronic skin and are required to be flexible, conformable, and have small sensing pixels. In addition to pressure sensing, E-skin can also incorporate additional features like temperature sensing, self-healing properties, and the ability to distinguish between different external stimuli.

For example, in a study, a leaf-shaped electrode sensor was integrated onto the fingertip of a 3D-printed robotic hand, which demonstrated the sensor's ability to detect human touch. Similarly, a study disclosed that a crack-based pressure sensor was capable of two-dimensional color mapping of external forces as well as monitoring subtle human movements, including slow muscle contractions.

7. Key Challenges to the Practical Implementation of Wrinkled AgNW Sensors

While silver nanowire (AgNW)-based flexible sensors, with wrinkled structures, offer significant advantages in terms of stretchability and sensitivity, several challenges remain that hinder their widespread adoption and long-term performance.

7.1. Weak Adhesion on Low-Energy Elastomer Surfaces

Even though wrinkled structures formed by releasing pre-strain can improve tensile stability, they do not solve the problem of weak adhesion between the silver nanowires (AgNWs) and the elastomer substrate (e.g., PDMS), which has inherently low surface energy. This weak adhesion can lead to delamination or mechanical failure during stretching over a prolonged period.

7.2. Environmental Stability and Durability Concerns

AgNWs are susceptible to oxidation and sulfurization, which can lead to their degraded electrical conductivity over time. To mitigate these effects protective coatings or surface modifications are often required but ensuring the stability and uniformity of these protective layers over extended periods is challenging. Additionally, exposure to moisture and varying temperatures can further compromise the performance and longevity of the sensors.

7.3. Achieving Multidirectional Stretchability without Damage

While making wrinkle structure, most conventional approaches rely on uniaxial or multiaxial stretching, which limits the ability to form evenly distributed, direction-independent wrinkle patterns. This constraint also hinders the development of flexible electrodes capable of stretching effectively in all directions. Therefore, the designing of stretchable conductive electrodes faces a challenge in producing wrinkle structures that exhibit consistent, controllable deformation and high mechanical durability.

8. Future Outlook

Wrinkled AgNW-based flexible sensors have demonstrated remarkable potential in a wide range of applications, whether in wearable health monitors or human-machine interfaces or soft robotics. However, for these technologies to transition from small laboratory-scale levels to commercial products, development is required in several key areas. Future research work should focus on scalable, cost-effective fabrication techniques thereby improving the adhesion between AgNWs and elastomeric substrates without compromising in performance and stability.

As the demand for flexible, high-performance sensors grows across various industries, continued interdisciplinary collaborations will be essential in optimizing the materials, device functionalities, and system-level integration for next-generation smart electronics.

9. Conclusion

Wrinkled AgNWs sensors represent a promising advancement in the field of flexible and stretchable electronics. Their unique structural morphology, when combined with the excellent electrical and mechanical properties of AgNWs, makes them highly suitable for applications in wearable devices, health monitoring, human-machine interfaces, and soft robotics. By forming controlled wrinkle patterns, the sensitivity and durability of these sensors can be enhanced, along with their ability to withstand large mechanical deformations.