The ability to control the propagation of waves at small scales forms the foundation of many technology applications today. Metamaterials have become a material of interest due to their small size, unique properties, and ability to propagate and control different electromagnetic waves. 

Optical and thermal metamaterials have gathered some of the most attention. For wave propagation, optical metamaterials have certainly gained a lot of attention because of their ability to control light waves, which has led to their inclusion in very small and precise optical and photonic components, such as photonic integrated circuits (PICs) and advanced optical gratings. 

Outside of optical metamaterials, acoustic metamaterials are another promising metamaterial for controlling and propagating electromagnetic waves. Acoustic metamaterials are nanoscale engineered materials that contain a lattice of subwavelength resonators. The collective action of all these nanoscale resonators gives the metamaterial interesting properties, such as a negative mass density that can influence waves in ways not found in nature. This includes the ability to stop wave propagation in specific frequency bands. Their small size and weight can be used to create a compact device, but many metamaterials only have fixed properties once they have been manufactured into a device, limiting their use. Researchers have turned to actively tuned metamaterials to overcome this, with a specific focus on using them for haptic technologies. 

Interest in Acoustic Metamaterials in Haptics 

Following their success in optics, metamaterials have gained interest in haptic devices because they can operate on stiff and non-dissipative panels, such as glass screens, unlike many other haptic motor arrays. Many haptic systems use control methods and geometry on a surface to provide localised vibrotactile feedback (vibrations to the user when touched, like we have in many touchscreens today). On the other hand, metamaterials can contain the elastic waves and provide localised vibrotactile feedback. However, once manufactured, they are often not as versatile as initially planned because they end up being manufactured with fixed properties and spatial arrangements, which limits how much functionality they have in haptics and other technologies. 

One of the main ways to overcome these issues has been to develop metamaterials with active unit cells, which compensate for potential functional losses and can help tune the metamaterial to specific frequencies. Piezoelectric transducers have been the preferred way to create active unit cells because they have an adjustable stiffness that can alter the band structure of a metamaterial through variable junctions, reconfigure waveguides, and steer wavefronts. This has meant that these active unit cells are well-suited for real-time, high-frequency and low-amplitude applications, and they can operate in the kHz to MHz frequency ranges. 

Using electromagnetic actuation is also a promising active approach, especially because it offers complementary capabilities to piezoelectric transducers. This approach uses a permanent magnet to toggle the modulus of the metamaterial between positive and negative. Electromagnetic actuation can be included within each unit cell of a haptic device by embedding an energised coil and can be used to set the resonators to specific states, and control the frequency bands, phase changes and polarisation across the metamaterial. 

Through the real-time configuration of waveguides, active metamaterials could be used to develop a multi-touch, programmable vibration-based display on rigid surfaces. However, human touch is extremely sensitive to low-frequency vibration up to 1 kHz, which requires deep subwavelength unit cells the size of a finger pad. As it stands, current active unit cells are not suitable for haptic technology because piezoelectric unit cells work at very high frequencies, and electromagnetic unit cells don’t provide real-time responses. Researchers have now created an active metamaterial that is made from a dual-state unit cell, combining both aspects, that is suitable for haptic technology. 

New Metamaterial Class Used for Haptic Technology 

This new active acoustic metamaterial class is made from dual-state unit cells made from off-the-shelf electromagnetic linear resonant actuators (LRAs), which are found in many smartphones and gaming consoles. It has been found that a 1D array of LRAs can be used to create a self-tuned metamaterial with deep subwavelength bandgaps within touch-relevant frequencies. The dual unit cells act as resonators when they are left disconnected and act as vibration sources when powered. One of the key advantages is that it can be used with easy-to-access components, making it simpler to create for haptics designers. 

The researchers are using the dual unit cells to create a 2D tactile display. The unit cells were constructed using the LRAs and made into a square tessellation with a square footprint of 10 × 10 × 4 mm3. Unit cells can behave in either a resonator or actuator state on the display, giving the unit cells dual-state capabilities, and the actuators contain a mass attached to a spiral flexure spring to restrict oscillations in the z-axis. A polyimide flexure was overlaid over the LRAs, and an analogue accelerometer and H-bridge MOSFETs were laid on top of the polyimide.  

For the LRAs themselves, all the required active elements were already embedded in the off-the-shelf LRAs, including a FR-4 substrate, a voice coil, a flexure spring, a moving mass with a neodymium magnet, and a steel casing. When an alternating current (generated from an external stimulus) is applied to the moving mass attached to both the magnet and coil, it induces a Lorentz force. This causes the mass to oscillate and generates transverse waves that produce strong vibrotactile sensations. This is when the LRAs are in an actuator state. When the coil is disconnected, it creates an open circuit, so no current can be established across the unit cell. This creates a passive resonator state, so each of the LRA components across the display has dual-state capabilities for responses to stimuli, leading to them being more suitable for haptic displays where there is periodic stimulation. 

The haptic devices could quickly switch between the two states in under 25 ms, leading to a sharp reconfiguration of waveguides and shaping of vibration patterns. This led to the dynamic shaping of vibration patterns with a bandwidth of around 120 bits/s. The metamaterials were also found to have a deep subwavelength behaviour and could stop wave propagation at wavelengths 17 times their size. 

To test the devices, the researchers performed a series of experiments with human participants and showed how information relevant to the human touch (binary words and time-varying patterns) could be encoded for tactile perception applications, such as multi-touch haptic displays and mechanical computing. These experiments showed that the human participants could retrieve 3-bit spatially ingrained messages. It was also found that the vibration could be steered along a path to create the illusion of motion, which could potentially have direct applications as a communication device for the visually impaired because it follows the Perkins Brailler layout. 

While the spatial resolution of 50 mm is already good enough for haptic uses, the researchers have stated that the resolution could be further improved by making LRAs with a higher quality factor or LRAs that operate at higher frequencies. The researchers also stated that the layout of the LRAs on the display could accommodate a wider range of hand and finger morphologies if they were moved to 10 mm increments. It’s also believed that adding a closed-loop system with a lead-lag or proportional-derivative compensator could reduce response times, but it would make it more complex to design and potentially less reliable, as the off-the-shelf system is built on simplicity and reliability.  

Outside of being used in multi-touch refreshable displays, the researchers believe that these dual-state metamaterial systems could be used in more analogue processing applications and could help to develop haptic computing systems, where reactive metasurfaces could dynamically process and deliver feedback in response to user input. 

Reference: 

Daunizeau T. et al, Dynamic shaping of multi-touch stimuli by programmable acoustic metamaterial, Nature Communications, 16, (2025), 8562