High Sensitive Flexible Sensors: Opening New Avenues with Silver Nanowires!

Flexible sensors are finding avenues in various fields such as intelligent textiles, human health monitoring devices, robotics, structural monitoring of bridges, aircraft, and more. To meet the requirements of manufacturing these flexible sensors, they need to possess the key qualities of being ultra-thin, transparent, and have good electrical performance. This is where the application of Silver Nanowires (AgNWs) comes in.

Before we proceed, let us understand the issues with traditional sensors.

The Issue with Traditional Sensors

Traditional sensors such as silicon sensors have good linear properties and piezoresistive effects [1]. Nevertheless, they are brittle and hard, bringing out the issue of creating flexible devices [1]. This is when materials like polydimethylsiloxane were used with silicon to create flexible devices. However, it was later discovered that polydimethylsiloxane and silicon were not compatible for creating micromachining technologies [2].

So, in order to synthesize flexible sensors with materials that were compatible and possessed high conductivity, active metal nanowires like Silver Nanowires, graphene, and carbon nanotubes (CNTs) were used [2].

Why are Silver Nanowires Preferred?

Graphene has a high sensitivity of (GF > 1500) [3] but it has a low strain range: (ε < 7%) [3]. CNTs on the other hand, have a low sensitivity of (GF < 1.5) [3] and a broad strain range: (ε < 200%) [3]. Silver Nanowires fixes both the challenges. It can significantly improve the low sensitivity of CNTs and the low strain range of graphene [3].

As a one-dimensional metal nanowire, AgNWs have been a widely preferred material for fabricating flexible sensors owing to their excellent flexing, endurance, and conductivity [2]. To achieve good adhesion, high stretchability, and even better sensitivity, the flexible sensors were created using Silver Nanowires that were embedded in polyimide (PI). These sensors also have the advantage of being low-cost.

Yao et al. [4] fabricated sensors that displayed good linearity and reversibility with a 50% [4] increase in strain range. Also, the pressure under detection showed a good range of 1.2 MPa [4]. Hwang et al. recently created a self-powered patchable sensor that can monitor human activities such as heart rate, breathing patterns, body position, and sleep patterns. The stretchability of this transparent sensor was reported to be almost 50–90% [4].

Jeong et al. [4] implemented ultra-long AgNWs into an elastic-composite generator to achieve a hyper-stretchability of almost 200% [4]. In addition to having excellent stretchability, the sensitivity in the sensors could be tuned by controlling the roughness and areal density of the AgNW networks.

Design & Operational Principles of the Flexible Sensors

The flexible sensors have two AgNW-PI layers which function as pressure-sensing layers. The sensor has an area of 3 × 3 cm2 [2] and can be easily bent by hand, demonstrating good flexibility. The surface of the film has AgNWs, with one end exposed to the surface and the other embedded in the polyimide (PI).

The pressure-sensing layers are further layered between the following materials:

• 1st Layer: Nickel-based electrode layers (Ni). They are created by electroplating.
• 2nd Layer: PI layers that are used as substrates.
• 3rd Layer: Polyvinylchloride (PVC) layers that are used as an encapsulation layer and substrate.

The Silver Nanowires on the top layer are randomly and partly in contact with Silver Nanowires on the bottom layer. However, when the sensor is touched, the two layers come in contact with each other. This results in multiple conduction pathways, leading to a decrease in resistance [2].

On the other hand, the sensitivity of the sensor will be quite high. Moreover, if the pressure in the sensors is increased, the conduction pathways could hit a saturation point. If the pressure continues to increase, AgNW networks in the membrane start exhibiting a change in their resistance [2].

Material & Fabrication Process

Silver Nanowires with a diameter of 120 nm and length of 20 μm are suspended in an alcohol concentration of 20 mg/mL [2]. This is followed by suspending Polyimide (PI) in the concentration. It must have a viscosity of 1100–1200 mPa·s [2].

After mixing both the materials, a composite material with a 2% [2] mass concentration is created. Next, the composite is degassed in a vacuum oven for 30 mins. The composites are then removed from the vacuum oven and placed aside. By spin-coating specific photolithography, wet etching, electroplating, and sputtering technology, the PI layer serves the function of a sacrificial layer, and the flexible AgNW-PI films are fitted on a stretchable substrate.

Conclusion:

The flexible sensors with AgNW-PI layers display the following properties:

• Easy fabrication
• Excellent durability
• High sensitivity — 3294 kPa−1 under 600 Pa [2]
• Good repeatability
• Compatibility with Micro-electromechanical systems (MEMS) technology

These flexible sensors are continuing to find applications in biomedicine, robotics, and the portable wearable device industry. What’s more, they have also opened avenues for reducing the size of the devices, achieve high sensitivity, and cover multiple large areas at a lower cost.

Nanorbital: Breaking Technological Boundaries with Silver Nanowires!

At Nanorbital, we have had a wide exposure to the global chemical industry for the last 20 years. Our experts develop and facilitate the adoption of Silver Nanowires to improve the quality of life and push technological boundaries.

We feature Silver Nanowires with diameters that range from 30±10 to 120±10. However, we are not restricted to these ranges. We are deeply rooted in Silver Nanowires synthesis and can customize them based on your requirements. This can help you fabricate highly sensitive, flexible sensors that exhibit excellent conductivity.

If you are looking to take technological applications to the next level, then look no further. Connect with us at email@nanorbital.com or call us on +91–93279 08826.

Sources:

[1] Palczynska A. et al., Prisacaru A. et al., “Towards prognostics and health monitoring: The potential of fault detection by piezoresistive silicon stress sensor” — https://ieeexplore.ieee.org/abstract/document/7463344/

[2] Hongfang Li et al., Guifu Ding et al., Zhuoqing Yang et al., “A High Sensitive Flexible Pressure Sensor Designed by Silver Nanowires Embedded in Polyimide (AgNW-PI)” — https://europepmc.org/article/pmc/6471986

[3] Jae Hyuk Choi. et al., “Fabrication and Performance Evaluation of Highly Sensitive Flexible Strain Sensors with Aligned Silver Nanowires”- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7074651/

[4] Yao et al., Hwang et al., Jeong et al., “Fabrication and Applications of Flexible Transparent Electrodes Based on Silver Nanowires” — https://www.intechopen.com/books/flexible-electronics/fabrication-and-applications-of-flexible-transparent-electrodes-based-on-silver-nanowires