3D printing makes manufacturing regular lenses, and more recently Fresnel lenses, easy and cost-efficient, but now, additive manufacturing can be used to extend their functionality with enhanced optical properties and sensing abilities.  

Dr. Haider Butt, Associate Professor, Murad Ali, Ph.D. candidate, and Dr. Fahad Alam, Postdoctoral Fellow, all Department of Mechanical Engineering, published their results in.

In optical designs, spherical and aspherical lenses form and guide light. Aspherical lenses have a more complex surface profile, making them difficult to manufacture, but a single aspheric lens can often replace a much more complex multi-lens system.

Fresnel lenses concentrate light using a stepped surface that bends the light as much as a thick, heavy glass lens. They are made of concentric rings, and each ring bends the light slightly more than the one below it, so all the light rays emerge in perfect, parallel beams. 

“Fresnel lenses are innovative spherical lenses characterized by optimized mass and materials,” Dr. Butt said. “While they originated in lighthouses, they are now used in field lenses, magnifiers, smartphones, photovoltaic panels, ultrasonic devices, and miniature spectrometers, among many other uses.”

Fresnel lenses in lighthouses are used to create powerful beams of light that stretch long distances. Unlike the conventional lenses in a telescope, for example, the optical quality of the light beam emerging from a lighthouse lens doesn’t matter. This means the Fresnel lens can be made from plastic, such as acrylic or polycarbonate, as well as glass, making their manufacture cheaper and easier.

“Acrylic exhibits excellent optical characteristics for multiple applications in solar technology, for example, especially in concentrating photovoltaic systems,” Dr. Butt said. “Alternative silicon Fresnel lenses are also used in space applications such as solar concentrators with glass protection. These are easily produced by casting, injection molding, and compression molding.”

However, if the Fresnel lens is to be used to collect light from a distance and bring it into a sharply focused image, precision manufacturing is key. Inexpensively made Fresnel lenses make poorer quality images than traditional glass lenses due to spherical aberration: Light rays travelling through a Fresnel lens at different angles will come to a focus at slightly different points, creating a blurred image. Because the surface of the lens is discontinuous, the image is distorted, and because different colors are refracted by the lens to different degrees, chromatic aberration is also a concern.

Adjusting the angle of the steps in a Fresnel lens is crucial to minimizing aberrations, although current manufacturing techniques limit design and processing flexibility.

“High resolution printers have made it possible to print 3D micro- and nano-optical components to perform complex optical operations,” Dr. Butt said. “For instance, optical waveguides and lenses are immensely popular as light-guiding devices, using complex geometric shapes integrated with optical fibers, gas, and optofluidic sensors. Advances in additive manufacturing are pushing optical and photonic devices into new and unexplored architectures with immense commercialization potential.”

In this research, the lens was designed with 15 rings of a constant width of just 0.833mm.

 “3D printing processes are more promising to explore the design strategies and complexity of Fresnel lenses,” Dr. Butt said. “3D printing also allows for multimaterials-based lens production, for sensing and multifunctional optics.

Thermochromic materials undergo a coloration or discoloration process at specific temperatures: When the temperature reaches a particular value, a color change occurs. To add thermal sensing to the lenses and make them four-dimensional, a thermochromic pigment powder was added as a responsive material to the transparent hydrogel resin that constitutes the Fresnel lens itself. This powder causes a reversible change, turning the lens from transparent to pink when temperatures drop. Various concentrations of the pigment can be used for parameter-specific optical applications, making the manufacturing process tunable to different applications.

The fifth dimension was introduced by embedding microscale holographic patterns on one side of the lenses. A variety of textured surfaces with holographic effects can be embedded into 3D parts, with the holographic film applied to the print bed of the 3D printer. A microsize holographic pattern in a Fresnel lens enables the lens to focus light and simultaneously exhibit the holographic effect. And the holographic effect is more than just aesthetically pleasing: The rainbow pattern generated could easily be combined with an image sensor, providing a miniature spectrometer for mechanoluminescence-sensing applications.

“Although the lenses we made have suitable optical properties, further improvements are always possible,” Dr. Butt said. “Reducing the thickness of each layer could improve optical performance, and a different curing technique could make the surface of the lens smoother. Regardless, we have shown that additive manufacturing can fabricate optical components with promising applications in the fields of sensing and communication.”

Jade Sterling
Science Writer
22 June 2022

The post Color-Changing, Holographic Fresnel Lenses Made Possible with Additive Manufacturing appeared first on Khalifa University.

Pressure sensors are used in electronic devices across all industries and making them as accurate as possible means making them as thin as possible. Researchers from Khalifa University have developed a method to use a novel 2D material for highly-sensitive and tunable flexible pressure sensors. 

Compared with conventional rigid silicon-based electronics, thin, flexible electronics can withstand various deformations such as tension, compression, bending and twisting. Pressure sensors that can transform external pressure into electrical signals are an indispensable application of flexible electronics, particularly for biomedical applications.

A team of researchers from Khalifa University has investigated how to develop a pressure sensor using a novel 2-Dimensional (2D) material, which is a single sheet of material that is just one atom thick, and 3D printing. They published their results in The research team includes Jing Fu, Research Associate, Somayya Taher, PhD candidate, Prof. Rashid Abu Al-Rub, Director of the Advanced Digital and Additive Manufacturing Group and Professor of Mechanical Engineering, Prof. TJ Zhang, Professor of Mechanical Engineering, Prof. Vincent Chan, Professor of Biomedical Engineering, and Prof. Kin Liao, Professor of Aerospace Engineering.

“Pressure sensors can be divided into various categories, including piezoelectric pressure sensors and piezoresistive pressure sensors,” Dr. Kin explained. “The working principle of a piezoresistive pressure sensor capitalizes on the change in the electrical resistance of the sensor against applied pressure. Such sensors have a simple structure, high sensitivity, fast-frequency response and low-energy consumption, making them popular candidates for various applications.”

An effective pressure sensor needs to be sufficiently thin. A sensor that is too thick may give erroneous readings as the sensor would press into a soft material, decreasing the load between the objects and increasing the measured pressure. To be as accurate as possible, researchers have turned to 2D materials to achieve sensors that are thin as possible.

“The engineering performance and robustness of a piezoresistive sensor mainly hinge on the sensor’s embedded active material,” Dr. Kin explained. “So far, different kinds of conductive materials have been used, such as metal nanoparticles, conductive polymers, graphene, and transition metal compounds. More recently, 2D materials have captured researchers’ attention worldwide, particularly transition metal carbides and nitrides or MXenes.”

MXenes are a family of 2D materials comprised of a pretransition metal, such as titanium (Ti), zirconium (Zr) or hafnium (Hf), with carbon and/or nitrogen, and hydroxyl, oxygen or fluorine surface functional group. These combinations give MXenes excellent electrical conductivity and hydrophilicity, making them promising candidates for applications such as piezoresistive sensors. 

As a 2D material, MXenes can be used as sheets and stacked on top of each other via van der Waals forces or hydrogen bonding between the functional groups. This way, MXenes can be formed into flexible and stable films, although the resulting material shows a very weak piezoresistive effect because when compressed, the structure of the sheet doesn’t allow for much deformation. Using MXenes in a 3D structure with similar length scales in all three dimensions would overcome this issue and make best use of the novel MXene material.

The Khalifa University team used additive manufacturing to develop the 3D structures. Traditional methods use templates upon which MXene layers are deposited before the templates are removed. While this does work, it does not allow for precise control of the internal structure of the resulting 3D scaffold. 3D printing overcomes this, with the technology able to fabricate flexible pressure-sensitive sensors with a high dynamic range through an easy to manipulate and large-scale manufacturing method.

“There are enormous possibilities in the design of internal structures that could be produced by 3D printing, but the triply periodic minimal surface (TPMS) structure is one of the more interesting,” Dr. Kin said. “The TPMS structure is known for possessing characteristics of surface area, mechanical robustness and thermal conductivity with an edge-free structure. Fabrication of 2D MXenes into the periodic, porous TPMS structure will lead to the development of novel 3D scaffolds with excellent electrical conductivity and mechanical properties.”

The team developed a simple and efficient method to combine MXene with a uniquely designed TPMS gyroid structure to create a 3D MXene-based gyroidal structure for use as a piezoresistive sensor with extremely high sensitivity, good response time and improvable durability. This method can be used to fabricate 3D MXene structures with any size, shape and internal structure.

More recently, Prof. Liao’s group has been working on constructing 3D structures of heterogenous 2D materials – different types of 2D materials organized in layered manner – for applications such as sensors, electromagnetic interference shielding, as well as energy-related applications.

Jade Sterling
Science Writer
25 May 2022

The post Advances in Flexible Pressure Sensors Using 3D Printing and 2D Materials appeared first on Khalifa University.

In a landmark review paper published in , Dr. Rashid Abu Al-Rub, Acting Chair of Aerospace Engineering, Professor of Mechanical Engineering, and Director of the Advanced Digital and Additive Manufacturing (ADAM) Group at Khalifa University, and Dr. Oraib Al-Ketan, Post-doctoral Fellow in the ADAM Group, who recently moved to New York University Abu Dhabi (NYUAD) as a Research Scientist, have reviewed the state-of-the-art in the design, additive manufacturing, and multi-functional properties of novel types of architected metamaterials. These architected metamaterials have opened the door for more research and technological applications thanks to recent advances in digital design and additive manufacturing (so-called 3D printing).

The ADAM Group is the first R&D and educational additive manufacturing (AM) group in the region focusing on advancing state-of-the-art in AM. It provides R&D services to industries across the UAE and abroad, ensuring that the UAE remains at the forefront of AM and its application in Industry 4.0.

These architected metamaterials, which are based on mathematically-designed triply periodic minimal surfaces (TPMS), were recently lauded in for their use in desalination techniques developed by KU researchers under the leadership of Professor Hassan Arafat, Director of the KU Center for Membrane and Advanced Water Technology. In fact, TPMS have been mathematically discovered more than 160 years ago, but thanks to recent advances in 3D printing they have become a reality only recently.

Now, the researchers are focusing on designing scaffolds made of the novel material for tissue engineering and bone growth. Their research is described in a paper recently published in the . This work has also been done by Dr. Dong-Wook Lee, a Research Scientist in the ADAM group, and in collaboration with Dr. Reza Rowshan, Executive Director of Core Technology Platforms Operations at NYUAD, to print the scaffolds using the metallic 3D printer available at NYUAD’s Core Technology Platforms facility.

Technology is often directly or indirectly inspired by nature, with recent studies showing many biological organisms exhibit spectacular surface topography such as shape, size, and spatial organization to allow them to adapt dynamically to a wide range of environments. Topology is the study of geometrical properties and spatial relations unaffected by the continuous change of shape or size of figures. In a broader sense, these topological features seen in biological systems can change the way engineers understand surfaces and their applications in a wide range of sectors.

“The distinctive topology-driven multi-functionalities of nature’s biological systems have motivated the materials science research community to design and synthesize materials for different engineering disciplines,” explained Dr. Abu Al-Rub. “As such, cellular materials with a wide range of topological features, length scales, and structurally controlled characteristics that include high stiffness-to-weight ratio, heat dissipation control, and enhanced mechanical energy absorption have been designed for different applications, including in the biomedical sector.”

There are many biomedical applications in which an understanding of natural topography can lead to improved material designs. One such application is in tissue engineering and bone growth. According to evolutionary theory, organisms use the least materials to achieve the best performance and optimize surface topological characteristics such as geometry, density, and spatial organization. This results in sophisticated multiscale structures which offer versatile functionalities and characteristics.

Bone scaffold tissue engineering is a rapidly advancing technology, thanks in large part to the advent of additive manufacturing, or 3D printing. An ideal bone graft or scaffold should be made of biomaterials that mimic the structure and properties of natural bones. Bone comprises an open cell composite material, but its mechanical properties vary with anatomical location and the loading direction. Therefore, a biomaterial that can be changed to suit differing requirements is highly sought after.

“In biomedical applications, a large mismatch between the bone and the implant leads to stress shielding due to the uneven stress distribution at the bone-implant boundary,” explained Dr. Abu Al-Rub. “As such, cellular materials are used to tailor the properties of the implant and avoid bone resorption around the implant.”

Functionally graded and multi-morphology porous lattices are one such material of interest because of the ability to control their physical, mechanical and geometrical properties spatially. Dr. Abu Al-Rub and his team investigated the relative density grading, cell size grading and multi-morphology (lattice type grading) for sheet-based lattices with topologies based on triply periodic minimal surfaces (TPMS).

A minimal surface is a surface that is locally area-minimizing­—a small piece has the smallest possible area for a surface spanning the boundary of that piece. Minimal surfaces necessarily have zero mean curvature; the sum of the principal curvatures at each point is zero. Minimal surfaces that have a crystalline structure, repeating themselves in three dimensions, are triply periodic.

“In principle, TPMS are smooth and continuous surfaces that can be described mathematically,” explained Dr. Abu Al-Rub. “These surfaces have fascinating and distinctive geometrical characteristics, for instance, a minimal surface is smooth in nature, has no sharp edges or corners, and splits the space into two or more nonintersecting, intertwined, and infinite domains that can be repeated periodically in three perpendicular directions.”

Lattices are regular, 3-dimensional, repeating structures, seen in nature as honeycombs and bones. While traditional manufacturing techniques have historically limited the ability to produce complex porous lattice structures, additive manufacturing has broadened the horizon of applications for lattice-based materials.

“Lattices are an attractive subclass of cellular materials and are widely used in the design of scaffolds and body implants,” explained Prof. Abu Al-Rub. “Advancements in fabrication techniques, specifically in additive manufacturing, facilitated their fabrication with lattices increasingly employed in biomaterials.”

Certain applications may require altering the lattice material’s volume, surface area, or pore size, depending on the intended functionality. This is known as functional grading. The ability to control unit cell size, unit cell type, and unit cell porosity can help better tailor the lattice material to meet the necessary engineering requirements.

“Functional grading is particularly important when the mechanical, physical and geometrical properties need to be tailored specifically to meet both biological, mechanical or thermal requirements concurrently,” explained Dr. Abu Al-Rub. “One example is bone implants, which are required to imitate the spatial distribution of mechanical and biological properties of natural bone. The implant should be highly porous in the middle section to allow for bone cell migration and proliferation, while the outer section needs to be highly dense to provide the desired mechanical properties.”

Trabecular bone is the porous bone tissue found at the ends of long bones like the femur, where the bone is not solid but rather full of holes connected by thin rods and plates of bone tissue. Even though it is porous, its spatial complexity contributes the maximal strength with minimum mass as it is optimized to resist loads imposed by functional activities such as running and jumping. To design the optimal bone-implant system, the mechanical properties of trabecular bone must also be understood.

“It was found that tissue regeneration progresses faster with curved surfaces as opposed to flat surfaces,” said Dr. Abu Al-Rub. “TPMS topologies exhibit mean curvatures equal to zero which resembles the mean curvature of trabecular bones.”

Previous work, covered by Dr. Abu Al-Rub and Dr. Al-Ketan in their research paper, investigated the relationship between grading approaches and the resultant mechanical and physical properties of the material. TPMS-based lattices can be categorized into solid-networks and sheet-networks depending on their scaffold architectures.

“Remarkably, the deformation behavior and mechanical properties of functionally graded and multi-morphology sheet-networks lattices have not been explored much,” said Dr. Abu Al-Rub. “We found that the deformation behavior exhibited by these lattices is very different from that observed in solid-networks or strut-based multi-morphology lattices.”

Further research into these differences will lead to improved lattice structure design for superior biomedical performance, while continued developments in additive design and manufacturing can be harnessed to integrate these complex designs into modern implants and improve clinical outcomes. Furthermore, Dr. Abu Al-Rub and his team are currently exploring other engineering applications of the developed functionally-graded TPMS metamaterials such as heat sinks and heat exchangers for thermal management, ultra-lightweight sandwich panels for aerospace structures, vibration absorbers for spacecraft systems, and catalytic supports for oil & gas industry.

Jade Sterling
News and Features Writer
21 January 2020

The post Novel Multifunctional Metamaterials Developed at KU Investigated for Biomedical Applications appeared first on Khalifa University.