Flexible Force Sensor: A Close Look at Applied Pressure Mechanical Sensors

In the recent past, monitoring human health status has become a lucrative business because it helps with preventive medicine. Think of wearables like smart watches, glasses, activity trackers, etc. The primary component in these devices is a flexible force sensor. Here's an in-depth look at these sensors, including their types and the materials used to make them. Let's get right into it!

What Are Flexible Force Sensors?

Flexible force sensors are super-thin flexible PCBs used for measuring the force between two surfaces. They are core components in wearable devices because they help convert strain and stress into electrical signals. With this signal, you can map the forces into readings on a screen.

A smartwatch with a heart rate monitor

Flexible Force Sensor Sensing Mechanism

Flexible force-sensitive sensors come in four varieties with different operating mechanisms. They include the following.

Resistive Sensors

Resistive sensors convert the change in pressure into a resistance change in the sensor. The unit features a conductive dielectric material whose contact area changes with the stress force applied to it. Once you apply pressure on the material, it deforms and increases the length of the conductive channel. This increase in size alters the resistance.

Piezoresistive pressure sensors

Resistive sensors have two sub-categories: strain and piezoresistive. Piezoresistive sensors have biocompatibility and stability issues, so the strain type is better.

Piezoelectric Sensors

Piezoelectric sensors translate pressure signals into electrical signals (voltage) using the piezoelectric behavior present in piezoelectric materials. An electric dipole moment controls this behavior.

Piezoelectric sensors

How does it work? When external pressure causes deformation in the material, it creates electric polarization, which produces opposite polarity charges on the two surfaces. And a potential difference forms afterward when you remove the applied pressure.

Capacitive Sensor

These flexible pressure sensors translate the pressure changes into varying capacitance levels. An applied pressure reduces the space between the plates in the parallel plate capacitor, altering the capacitance.

Triboelectric Sensor

Triboelectric flexible sensors create a charge after applying friction force or when under pressure contact. When relieved, the flexible sensor generates a potential difference as they separate, converting the mechanical stimuli into electrical parameters.

Like piezoelectric sensors, this type only generates an electrical signal during separation (when the applied load gets removed).

This flexible force-sensitive sensor integrates the electrostatic and triboelectric functions using a triboelectric nanogenerator.

Materials Used To Fabricate Flexible Force Sensors

The materials used to make these flexible sensors fall into three primary categories: carbon, metal, and polymers.

Carbon-Based Materials

Carbon Black

This material is an amorphous form of carbon with a structure resembling disordered graphite. You can fill carbon black in the elastic matrix to enhance its electrical conductivity and mechanical strength.

Carbon black powder

Constructing a piezoresistive sensor with a layered porous sensing architecture with carbon black gives it high sensitivity and a wide sensing range.

Carbon black is cheaper and offers better electrical conductivity than other carbon-based materials. So it is ideal for making wearable sensors.

Carbon Nanotubes

Carbon nanotubes are carbon allotropes that appear as single or multi-layer graphene sheets rolled into a cylinder. They have a superb strain-sensing capacity in piezoresistive composites. But they are not durable because they peel and crack after hundreds of deformation cycles. This condition decreases their conductivity, which affects their sensing performance.

The structure of carbon nanotubes

Graphene & Graphene Oxide

Although graphene is a carbon nanomaterial in 2D form, it has an atomic thickness. It has superb optical, electrical, thermal, and mechanical conductivity properties, plus a high specific surface area. But the material is challenging to handle during fabrication because it lacks hydrophilic functional groups on its 2D plane structure.

On the other hand, graphene oxide has several functional groups containing oxygen. Therefore, it has unique viscoelasticity in aqueous solutions and good printability.

A graphene oxide solution

With a graphene PDMS composite material strain sensor, you can adjust its sensitivity by altering its microstructure at 20% strain. This adjustment is ideal for devices in the field of health monitoring because it can adapt to the stress distribution of different body parts.

Metal Materials

Metal Nanomaterials & Films

The Metal films include:

• Silver film
• Gold film
• Aluminum film
• Zinc film
• Copper film

Besides their excellent conductivity, some films have anti-interference and anti-corrosion functions.

On the other hand, metal nanomaterials include:

• Nanowires
• Nanoparticles
• Nanosheets

Metal nanowires have tiny aspect ratios & diameters and have superb flexibility & light transmittance.

Gold, copper, and silver nanowires have the desired high conductivity and stretchability properties. The gold type has superb sensing properties but is expensive. Copper nanowires are affordable but can oxidize.

Silver nanowires hit the sweet spot because they are less costly than gold and do not oxidize like copper. Additionally, they have excellent antibacterial and electrical conductivity properties.

Liquid Metals

Liquid metal is an amorphous material that exists as a liquid at room temperature. The typically used ones include mercury and eutectic gallium indium.

Mercury is harmful to human health and toxic to the environment. But eutectic gallium indium is biocompatible, making it the ideal candidate for making flexible electronics.

Metal Oxides

Metal oxides with nanostructure and microstructure morphology have a high specific surface area. This structure gives them 2-3 times the capacity of carbon-based materials.

MXenes

MXenes exhibit superb oxidation resistance, conductivity, and mechanical flexibility, making them ideal for making flexible electronic devices. However, they have a limited linear range.

Polymer Materials

Polymers are the ideal supporting materials for making sensors, and the typical ones include:

• Polydimethylsiloxane PDMS
• Polyurethane PU
• Polyimide PI
• Polyethylene terephthalate (PET)
• Polyethylene
• Parylene
• Polyvinylidene fluoride (PVDF)
• Polyethylene naphthalate (PEN)

These materials have high tensile strength, chemical stability, good thermal conductivity, and are easy to compound with conductive materials.

Flexible Force Sensor Manufacturing Processes

The primary 3D printing methods used to make these sensors include:

• Digital Light Processing (DLP)
• Direct Ink Writing (DIW)
• Stereolithography (SLA)
• Fused Deposition Modeling (FDM)
• Selective Laser Sintering (SLS)

An SLA-DLP 3D printer

Benefits of Flexible Force Sensors

• Thin and flexible (fits in tiny spaces)
• Durable (high-temperature options can withstand temperatures up to 204°C)
• Energy efficient
• Custom sensors are available in various sizes and solutions

Flexible Force Sensor Applications

• Medical applications (wearable sensors)
• Consumer products
• Industrial motion control
• Automobiles
• Robotics
• Aerospace

But the health monitoring field (in-vitro and in-vivo) is the most typical application area.

A health monitoring device

Wrap Up

In conclusion, flexible force sensors have a wide range of applications and are particularly critical for health monitoring. Also, building them requires a wide range of materials to create a layered structure using any of the 3D printing technologies listed above. That's it for this article! Thanks for your time.