The metamaterial device contains multiple concentric layers that rotate independently. This device shown here is a nine-layer concentric ring. Credit: Chinese University of Hong Kong

Making objects disappear

12 February 2023

A research team at the Chinese University of Hong Kong has engineered a multifunctional hydrodynamic device with a system that integrates invisibility, field rotation, and concentration – all three functions that together may have biological uses in cultivating bacterial growth in cell and tissue culture, as well as influencing the rate of absorption and timing of drug delivery.

Within the past two decades, optical cloaking and invisibility has taken a leap from the pages of fantasy fiction into the realm of applied science. In 2006, British physicist Sir John Pendry developed a theoretical framework that paved the way for making optical cloaks a reality. The possibility is achievable through the design of metamaterials, which possess radical properties derived from internal microstructures that can manipulate electromagnetic waves such as visible light. The idea of the stealth cloak is that by grading the index of refraction in the cloak, light can be bent by the right amount to hide an object behind a mirage. The field of transformation optics pioneered by Pendry has inspired scientists to reassess the foundations of light and optics. They have since taken the science in this highly active field from beyond earlier theoretical developments.

Professor Xu Lei holding a multifunctional hydrodynamic device engineered by his team. Credit: Chinese University of Hong Kong

At the Chinese University of Hong Kong, collaborative research team led by Professor Xu Lei has designed and manufactured a multifunctional device that can integrate cloak, concentrator, and rotator into one system with the ability to switch across the three functions. In the mega-device created by Xu’s team, the design patterns of cloak, concentrator, and rotator make minimal disturbances to the background. By doing so, this may render an object invisible, hiding it completely as to make it almost disappear. In the past, scientists have created a multitude of novel optical devices, however, until now none have been designed to switch interchangeably from invisibility, field rotation, and concentration.

Most physical fields that physicists are interested in, such as flow field, optical field, and electric field, are determined by both magnitude and direction. With all three properties in one mega-device, they can adjust both the magnitude and direction at will. For example, from cloak to concentrator, they can adjust the field magnitude continuously; and can freely tune the direction with rotator. With all three: cloak, concentrator, and rotator in one device the user can have full control of the field inside, direction and magnitude.

The first to third rows are the experimental images of cloak, concentrator, and rotator configurations respectively. The left column shows the static images of different configurations, and the right column shows the flow fields, represented by different colours. Credit: Chinese University of Hong Kong

We might imagine the concentrator function in the optical field as a magnifying glass of sorts that can focus light on a point in the electromagnetic field to make it stronger. This concept in the liquid flow field permits a much greater velocity inside the concentrator versus the background field. Rotators manipulate the direction of field propagation, selectively tuning the angle and direct the flow.

Xu suggests a way to understand the invisibility function in a flow field: “If you have a flow field once the flow hits the object in the middle, it gets deflected and disturbed like an optical field, so the flow field gets distorted. An invisible object can be detected by this flow field distortion in the same ideal as the optical field. The invisibility function is achieved by minimizing this flow field distortion.”

Invisibility, field rotation, and concentration are valuable properties sought out in metamaterials. The invisibility property is much desired in situations such as optical invisibility to avoid optical detection and acoustic invisibility to avoid sonar detection. For human body implantation, it is also desirable to make minimal disturbance to the background environment and flow. For field rotation and concentration, it can tune the field in magnitude and direction and thus enable a full control of the field.

Xu and his team specialise in the flow field of porous media to manufacture their multi-functional metamaterial device. Their device in the flow field of porous media can either be fabricated by optical lithography or 3D printing depending on the desired resolution and device size. Optical lithography is a patterning process in which a photosensitive photoresist is selectively exposed to light through a mask, leaving a latent image in the photoresist that may be selectively dissolved to provide patterned mould. This in turn can be used to make a device with Polydimethylsiloxane, a common versatile silicon-based polymer with applications including lubricants, antifoaming agents, and implants. The advantage of optical lithography over 3D printing is its improved spatial resolution between pores and sizes on the micron scale. 3D printing can be more convenient for drawing patterns on a much larger millimetre and centimetre scale, but the spatial resolution is not as good.

The flow field in the microfluidic device of the cloak, concentrator, and rotator. Credit: Proceedings of the National Academy of Sciences.

Primarily, the metamaterial of the device has patterns that can lead to different functions such as cloak, concentrator, and rotator. “The technology is about designing the pattern to minimize diffraction, so the flow field does not feel a disturbance when it comes across our device,” Xu said. The device contains multiple concentric layers that rotate independently with designed handles at the back of the instrument. This allows for manipulation of flow velocity and direction to correspond with the three specific functions offered by an invisibility cloak, concentrator, and rotator.

There are several conventional invisible metamaterial designs: a cloak can isolate the influence between the internal and external fields, a concentrator can concentrate the external field to form an intensified internal field, and a rotator can rotate the internal field by a specific angle with respect to the external field. Our study brings advancement to the theoretical design and the practical application of multifunctional metamaterial.
Chen Mengyao, Shen Xiangying, Chen Zhen, Jack Lo Hau Yung, Liu Yuan, Wu Yilin and Xu Lei, Proceedings of the National Academy of Sciences, 28 November 2022

In a recently published article in the scientific journal Proceedings of the National Academy of Sciences, the team asserted the significance of such multifunctional metamaterial with potential applications in biomedical areas such as tissue engineering and drug release. For example, the team demonstrated one potential metamaterial device application in the effective control on bacterial biofilm growth. If the size and direction of bacteria growth can be controlled using a cloak, concentrator, or rotator, then this can become a model system for many complex living matters. They manufactured the cloak, the concentrator, and an intermediate structure inside a background porous medium. A bacteria-carrying fluid was flowed from left to right under a constant rate and temperature. The team compared the resultant biofilm formation at the centre of these three devices.

The image shows the cloak (left), concentrator (right), and an intermediate structure (centre) inside a background porous medium. Below, we see the resultant biofilm formation at the centre of these three devices indicated by green GFP fluorescence. Credit: Chinese University of Hong Kong

The cloak exhibited the largest amount of biofilm whereas the concentrator showed the least. For the cloak the inside field was isolated from the outside so the velocity inside it is very small, almost zero as it does not get affected by the outside flow. For the concentrator function, there is a very strong flow that makes the bacteria difficult to attach and grow because the flow field is high. The ability to tune the device to switch aligning angles between layers can manipulate the biofilm growth quantity.

Xu’s team also manufactured two different rotators side by side to tune the direction of biofilm growth. The rotators successfully changed the direction of the flow field that influenced the direction of bacteria growth in different velocity directions.

“The main purpose of our device here was to change the magnitude and direction of bacteria growth with negligible disturbance to the background field,” said Xu. “From the bacteria experiments, the device can tune both quantity and direction by adjusting the configurations. The cloak configuration can be chosen for a fast-growing speed whereas the concentrator configuration can perform the opposite function. This exhibit of growth control can expand to study the controlling of bacterial growth and cell culture.”

The tuning of velocity magnitude and direction in the 3D-printed device. Credit: Proceedings of the National Academy of Sciences

For a potential cell and tissue culture application, the device can be used either in vitro or in vivo, depending on circumstances. For example, the device can be used to create different cell and tissue culture environments outside the body, or it can be directly implanted inside the body to culture tissues in vivo.

The organs and tissues within a human body consists of porous media with various internal flows. Medical implantations of devices can have a profound effect on the surrounding flow field and can subsequently have a cascade disturbance on the body. One possible future application for the invisibility mega-device would be controlling the release of drug delivery. Pharmaceutical drugs have many inherent challenges such as absorption, dosage, metabolism (fast half-life), within the body. With an adjustable flow field within the mega-device, a sustained and smart drug release from slow to fast near the target drug receptor could have a significant improvement for patients and the healthcare industry.

Xu said, “Our main contribution is to have this multifunctional device realised experimentally so that all three functions can be integrated into one mega device, and we can tune this device to the requirement of user’s need.”