Types and Applications of Smart Materials

Curious about Smart Materials? You might want to start with slime.

Have you seen “Slime?” If you haven’t, check YouTube, I’ll wait. As you’ll see, Slime is as it sounds, a slimy material that magnetically attracts children with an affection for experimentation. Why? Two reasons – 1. It’s weird. It feels funny in your hands, giving it an addictive tangibility. 2. It’s easy to change the properties. Want sparkly Slime? Add glitter. Want glow in the dark Slime? Add “glow powder” (also known as photo-luminescent pigment). Slime is simple; the slimy part is just Borax (Sodium Tetraborate) and PVA glue (Polyvinyl acetate). Changing the properties of this material is straightforward and satisfying.

The attraction that kids have to the dynamic properties of Slime is the same as the attraction that designers and engineers have to smart materials. Slime may not have many practical applications, but it is exciting. Materials that have unexpected and powerful properties are engaging and potentially useful. But just like slime recipes on YouTube, the variety of smart materials can be overwhelming. As a materials geek running a materials company, I thought that I might be able to help provide some context and guidance.

The variable properties of slime (pictured below) make it both engaging and a tangible example of smart materials.

What are smart materials?

What makes materials “smart?” In short, a smart material is a compound which has a visible and tangible reaction to external stimuli by undergoing a material property change. These stimuli can include chemical, electrical, mechanical, thermal, and magnetic changes in the environment. The response to these changes is dependent on the material.

Changes can include paint that self-heals when it is scratched, coatings which change colour in response to the presence of chemicals, materials that have a “shape memory” when presented with a magnetic field, and metals that change shape at specific temperatures. In a simple sense, smart materials are a robust, solid-state sensor for the environment around them. It is up to the designer or engineer to use these sensors to the best effect in specific applications.

Smart materials:

  • act simultaneously as actuators and sensors
  • perform controlled mechanical actions without any external mechanism
  • are adaptive with the environmental condition
  • create the potential for new function development within applications.

The US army research office has maybe the best and most expansive definition for smart materials:

“a system or material which has built-in or intrinsic sensor(s), actuator(s), and control mechanism(s) whereby it is capable of sensing a stimulus, responding to it in a predetermined manner and extent, in a short/appropriate time, and reverting to its original state as soon as the stimulus is removed”.

This definition is useful as it covers both the traditional smart materials discussed below as well as the more contemporary approach of directly combining materials and contemporary electronics to create material systems that respond to and/or actuate in the presence of environmental stimuli.

Benefits of smart materials

Smart materials are compelling because they are often an efficient way to fulfil a specific performance objective. Effective implementation of smart materials can simplify design, reduce the part count, and increase the lifespan of an object. The primary benefit of smart materials is that their performance is inherent to the material itself. It’s up to the engineer or designer is to use that property effectively.

To help make the classes and use-cases of smart materials more tangible, I have described use cases within the types of smart materials:

  • Light-sensitive (photochromic) materials
  • Temperature-sensitive (thermochromic) materials
  • Chemical-sensitive (chemochromic) materials
  • Self-healing materials
  • Magnetic-sensitive materials and magnetorheological fluids
  • Shape-Memory Alloys

In each case, the use of a Smart Material has made a design less complicated and more robust.

The uses of Smart Material range from medical and construction to automotive industries. Devices using smart materials might eventually replace more traditional technologies in the construction of buildings, vehicles, and consumer products. Lower component weight, component size, and complexity combined with improved design flexibility, functionality, and reliability, make smart materials an attractive option. In addition, smart materials offer a level of environmental robustness not easily achieved through other technologies as they are not typically impervious to water, moisture, or dust.

Light-sensitive (photochromic) materials

Elena Cochero’s fantastic work with photochromic pigments demonstrates how a designer can take great advantage of elegant material properties within some unexpected applications. Photochromic pigments change colour in the presence of light. Depending on their chemistry, pigments can vary across various colour ranges at different rates. Elena’s work has focused on UV-sensitive pigments, a subset of photochromic pigments. She has harnessed the pigment’s ability to “see” invisible but potentially harmful UV light.

She has used these pigments to make badges for children that change colour on exposure to UV to help give a tangible indication of sun exposure. Recently she’s used these pigments to make a pendant that transforms from translucent to bright pink to indicate your sun exposure. The UV exposure is practically intangible until you get a sunburn. Using the inherent properties of these pigments is an elegant way to help anyone understand their exposure without resorting to a more complex electromechanical meter.

Cochero’s UV-sensitive pendant (pictured below) changes color after UV exposure, helping make intangible and potentially harmful light, more tangible.

Temperature-sensitive (thermochromic) materials

Similar to photochromic pigments, thermochromic materials change colour based on temperature. They also have a range of responses to heat or cold, with the designer being able to specify different colours and responses to suit design requirements. LCR Hallcrest’s line of thermochromic thermometers is an excellent example of where the appropriate use of a smart material can dramatically simplify design and in the process, make it more scalable.

These passive thermometers are applied widely across machinery, factories, and even medical patients. These devices provide a simple, visual indication of the temperature of its mounting location. The combination of robust operation and low-cost opens up a diverse range of applications: whether a part may be too hot to touch, whether food is stored at the correct temperature, or to monitor a patient’s temperature during a surgical procedure easily. The low-cost and simplicity of these materials make it possible to create a sticker, which can be applied almost anywhere.

Hallcrest’s thermochromic thermometers (pictured below) are a cost-effective and robust way to measure and indicate temperature on almost any surface.

Chemical-sensitive (chemochromic) materials

Smart materials can play a role in even more mission-critical applications where there is no reasonable alternative. Chemochromic materials change colour in the presence of certain chemical compounds. Like photochromic and thermochromic materials, chemochromic materials can be specified to react in specific ways to different chemical compounds. NASA’s design for a Hypergol Leak Detection Sensor is an elegant response to the challenge of detecting potentially dangerous leaks of hypergolic propellants. In simpler terms, leaking rocket fuel is an extraordinarily dangerous situation, and operators must spot leaks fast.

NASA’s design uses chermochromic pigments, which change colour in the presence of hypergols to alert workers to the presence of a leak. The pigment is deployed as a tape directly to a pipe, so the sensor can take the shape of the surface. Once attached, the tape changes colour from yellow to black to visually indicate the presence of the fuel. This system has an elegantly simple design with minimal components, no electronics, and no moving parts making it fundamentally robust and scalable.

NASA’s chemochromic detector tapes are used on pipelines like those pictured below to detect the presence of dangerous fuels, just through a color change.

Self-healing materials

Self-healing materials are a class of smart materials that have recently captured the public’s imagination. From self-healing glass, textiles, and paint, it’s exciting to imagine that the materials around might have some of the self-repair capability of a biological system. But of course, the mechanism behind self-healing materials isn’t biology, but polymers. Polymers, which, when fractured, are chemically promoted to rebond or to rebound.

Out of many possible applications, the first comes from an unexpected place: self-healing paint for high-end vehicles. Finding a small scratch on the otherwise perfect surface of a new car is frustrating, but self-healing polymers make it possible for the surface to repair itself. The first commercial roll-out of such a material was by Kawasaki Motorcycles. The 2019 H2 motorcycle comes with what Kawasaki describes as a “Highly-Durable Paint,” which will self-repair certain types of scratches over time.

Kawasaki motorcycles’ self-healing paint uses a specific physical structure to make its bikes look better longer.

Magnetic-sensitive materials and magnetorheological fluids

Magnetorheological fluids might be more magical in their effect than any other smart material, in that they reliably produce “ohs, and ahs” when demonstrated. Magnetorheological fluids are reactive to magnetic fields as they contain electrically conductive particles that specifically align with the poles of a magnet. This change in alignment can change the viscosity of the fluid. These fluids are commonly known as “ferrofluids,” which perfectly illustrate the shape and presence of magnetic fields. But the most impactful application of these materials has been within the field of automotive suspension.

Magnetorheological dampers use the variable viscosity of these materials to create suspension systems that can be tuned on the fly. When a magnetic field is applied or removed, the viscosity of the fluid changes, changing the ride quality of the vehicle from soft to hard (or vice-versa) with a practically instantaneous response. These systems dramatically reduced complexity over other active suspension systems, by eliminating the complex network of valves and plumbing which would normally be required to provide such high-resolution control.

By re-orienting particles within a magnetic field, Magnetorheological dampers change the performance of automotive suspension components almost instantaneously.

Shape-Memory Alloys

One of the most widely deployed applications of Smart Materials are Arterial stents made from Shape-Memory Alloys (SMAs). A clogged or collapsed artery is a significant health risk and can directly contribute to the death of a patient. The challenge is how to remove the restriction to blood flow in a fast and minimally invasive way. Thankfully, a smart material with a shape memory function, known as NiTiNOL, and some clever engineering provide a solution that has been used in thousands of patients.

NiTiNOL is a “Shape Memory Alloy” (SMA), a sub-category of smart materials and shape memory materials which can also include shape memory polymers. SMAs are metals which are frequently presented in wire or strip form and can be ‘programmed’ to actuate in a specific way when subjected to a change in temperature. The effect was first demonstrated in 1962 in the US Naval Ordinance Laboratory.

NiTiNOL has a shape memory that is retained over time and it is possible to switch between states by varying the temperature of the material as demonstrated below.

The arterial stent takes advantage of the temperature differential between the human body and the surrounding environment. Outside of the body, the NiTiNOL bulb is collapsed. The actuation of the device occurs when it is inserted into the body, the warmth of the patient expands the device to hold the artery open. The continual heat of the patient’s body ensures that the device will remain expanded, promoting healthy blood flow.

This extraordinarily elegant design takes advantage of NiTiNOL’s fundamental properties to make a profoundly simple and effective life-saving device. Beyond arterial stents, the structures possible with memory alloys as actuators are very exciting and hold potential for applications that haven’t even been considered yet.

Arterial stents use NiTiNOL’s unique properties to make a device that is collapsed when inserted, then expands in the presence of body heat, keeping the artery open

The next generation of smart materials

Beyond the types of smart materials discussed above, it is necessary to note that the use of the term “smart” has recently expanded to include materials with embedded electronic functionality. Though these materials don’t necessarily undergo a physical change, they are frequently responsive to the environment. They are worthy of our attention as it is likely that the continuing miniaturization of electronics and innovations in materials science will create new classes of materials that have amazing new properties.

As a short introduction to this new world, I think it worth looking at three materials, printed photovoltaic cells, piezoelectric coatings, and printed capacitive sensors.

The concept of a painted solar cell is fantastically engaging. Sunlight bathes our world in energy, which we benefit from in the form of light and heat. But printed photovoltaic cells create the potential to transform this energy into electricity. These energy-generating surfaces produce a voltage in the presence of sunlight. The vision of applying energy-generating paint on any surface in any shape is getting closer to commercial reality every day.

Bare Conductive’s Electric Paint (pictured below) was designed to integrate with contemporary electronic tools, making it a perfect example of a new class of Smart Materials.

Similar to photovoltaics, piezoelectric materials also produce a voltage, but as a response to vibration, rather than light. Piezoelectrics themselves are typically classified as smart materials but have physical restrictions in many applications. But by creating a coating composed of piezoelectric particles, this functionality can be widely extended. Each particle works together to create an entire surface which is sensitive to vibration. So far, the primary application of these new materials seems to be the measurement of vibration for either monitoring machine performance or predicting fractures in surfaces.

Finally, printed sensors and specifically printed capacitive sensors help create a compelling new class of smart materials by using the electrical conductivity, manufacturing and form-factor freedom of electrically conductive inks.

Printed capacitive sensors use a combination of electronic hardware and careful materials selection to generate an electric field that detects the presence of humans, liquids, and more. The high-precision, low-cost, and wide-coverage area capabilities of Bare Conductive’s smart sensors demonstrate the leverage that you can create by combining smart materials with contemporary electronic hardware.

Bare Conductive’s Electric Paint and Hardware is a great example of the next generation of smart materials which combine material properties with contemporary electronics as demonstrated in the video below.

How to choose smart materials

Getting started with smart materials in your applications is easier than it might seem. Depending on your skill and knowledge, a materials selection tool like Granta’s CES selector or MatLab’s Online Material Resource can help narrow your choices. But these sophisticated search engines aren’t always useful early in the design process.

As discussed above, smart materials demonstrate a tangible reaction to changes in the environment. And there is no better way to understand that behaviour than by experiencing it yourself. Thankfully, several suppliers make it easy to acquire small samples of smart materials for testing. Companies such as Mindsets Online, Rapid and Less EMF, typically purchase large quantities of hard-to-access materials and dispense them into sample sizes, perfect for experimentation.

With materials in hand, it’s easy to explore and evaluate their properties within a design. Whether designing magnetorheological fluids, piezoelectric compounds, shape memory alloys, the structures, applications, and benefits of smart materials are truly exciting. Like children experimenting with Slime on YouTube, no one is immune to the compelling structures, shape, applications, and magic of smart materials.