Industrial Electronics

Recent advances in flexible antennas for wearable devices

06 October 2021
A flexible near-field communication antenna. Source: Graphene Flagship

The advent of 5G networks with low latency, high speeds and large capacity has facilitated the emergence of the 'Fourth Industrial Revolution.' 5G networks will aid every industry, including 3D imaging, streaming services, healthcare and smart cities. Additionally, a robust 5G network is required for effective operation of internet of things (IoT) devices. One of the major impediments to the tech innovation of next-generation IoT devices is lack of component flexibility due to form factor and weight issues. Although there have been lots of advancements in miniaturization, flexibility remains a difficult characteristic to conquer. Technological breakthroughs in engineered materials have aided in the advancement of flexible electronics.

Need for flexible antennas

Flexible wireless device markets are growing fast, in part due to increased demand for implantable and wearable devices for health monitoring systems and everyday wireless gadgets, including laptop computers and cell phones. As a result, the demand for flexible printed antennas has expanded significantly during the last few years, particularly for biomedical applications. Flexible antennas are a critical component in the development of in vivo vital sign monitoring, organ function regulation, brain interfaces, intracranial sensors, continuous gait analysis and drug administration systems. To incorporate gadgets onto curvilinear surfaces and adjust to constantly changing motions of the human body, devices must be conforming and physically flexible, if not stretchy. Since the bending stiffness of a thin film structure, which quantifies its resistance to bending deformation, roughly scales with its thickness, reducing the structure's thickness is an effective way to allow bendable/flexible antennas. Besides medical benefits, government agencies, commerce, and academia are also interested in building a flexible antenna for harsh circumstances.

Substrates and materials for flexible antennas

Flexible antennas are constructed from a variety of substrates and conductive materials. The substrate is selected for its dielectric qualities, mechanical deformation tolerance (bending, wrapping and twisting), miniaturization potential and durability in the surrounding environment. In comparison, the conductive material used (depending on its electrical conductivity) determines the antenna's performance, like radiation efficiency.

The substrate material for the flexible antenna must have a low dielectric loss, a low thermal expansion coefficient, a low relative permittivity and a high thermal conductivity. This constraint is motivated by the requirement for higher efficiency (in a range of environments) at the expense of increasing antenna size. The requirement for a high dielectric constant in tiny antennas is an exception to the above statement.

Three substrate types have frequently been used in the construction of flexible antennas: metal foils, thin glass and polymers or plastics. Metal foils can withstand high temperatures and allow for the deposition of inorganic compounds; however, their surface roughness and exorbitant prices limit their applications. While thin glass is flexible, its inherent brittleness limits its applicability. Polymer or plastic materials are ideal for flexible antenna applications, which include thermoplastic noncrystalline polymers, thermoplastic semicrystalline polymers and high-glass transition temperature material.

The development of conductive patterns with exceptional electrical conductivity is critical in wireless applications to ensure efficiency, high gain and bandwidth. Furthermore, the conductive material's resistance to degradation owing to mechanical deformation is one more desirable characteristic. Owing to their high electrical conductivity, nanoparticle inks (i.e., silver and copper) are frequently utilized for constructing flexible antennas.

Manufacturing methods for flexible antennas

The effectiveness of a flexible antenna is controlled by the process of construction, which varies depending on the substrate. Inkjet printing, screen printing and 3D printing are frequently used to fabricate flexible wearable antennas.

Inkjet printing has developed as a viable option to more traditional production methods such as etching and milling. It is an additive manufacturing technique that transfers the design straight onto the substrate with no use of masks, resulting in minimal material waste. Based on accuracy and speed of prototyping, it is the favored production approach for polymeric substrates such as paper, polyimide and PET.

Screen printing is a straightforward, cost-effective and feasible method for creating flexible electronics. It has been widely used to integrate RFID antennas by spraying conductive inks or pastes onto low-cost, flexible substrates such as textile materials and paper and is a woven screen technology characterized by a range of thicknesses and thread densities. A squeegee blade is pressed against the screen, bringing it into contact with the substrate and creating a printed pattern. The required pattern is created by ink ejecting via the screen's exposed portions on the adhered substrate. Additionally, it is an additive technique, similar to inkjet printing, rather than a subtractive method, such as chemical etching, making it more cost effective and ecologically benign.

With the commercial availability of a variety of printing materials and procedures, innovative 3D printing techniques for flexible antennas have gained appeal. It has a number of advantages, including the capacity to fabricate complex 3D structures quickly and with a variety of materials, as well as the flexibility to adjust the density of the printed object. The ability to fabricate complicated 3D structures from bulk materials and the ability to 3D print flexible materials such as metals, polymers, ceramics and also biological tissues make it an interesting medium for designing antennas.

Conclusion

Flexible antennas are a vital component in achieving the flexibility of electronic devices. It is suited for present and future sensing applications and wireless communication due to its small size, minimal rates of manufacture, lower form factor and flexibility to suit non-planar surfaces. The materials used to fabricate antennas are determined by application requirements such as environmental considerations and expenses.

To contact the author of this article, email engineering360editors@globalspec.com


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