From implantables to wearables, shrinking form factors are creating the need for smaller batteries. However, miniaturizing batteries is no easy feat. Making them smaller is one thing, but how do you maintain power density while shrinking the real estate? And if you’ve made forays into the domain of minuscule printable batteries, what types of inks can you use to create anodes and cathodes that will effectively allow current to flow? The answer lies in the utilization of the right chemicals, together with the right designs.

Shrinkage vs. Power

“Making smaller batteries is certainly possible,” remarks Gary Freitag, director, product development engineering, power sources, batteries, and capacitors at Clarence, NY–based Greatbatch Medical. “However, providing the required longevity and power capability while shrinking battery size has limits.” A primary limitation is that power density is dependent on the amount of contact surface area between the two sides of the cell—the anode and the cathode. As the cell becomes smaller, the surface area decreases, reducing the power density.

One method for maintaining power capability while minimizing surface area is to incorporate multiple plates into the battery design, according to Freitag. However, this method comes at the price of increased complexity and cost. It also reduces the energy density. Another method for achieving a balance between power and size is to fabricate the cathode using a more powerful material. For example, designers can transition from the use of carbon monofluoride (CFx) to Q technology, in which a layer of high-rate lithium/silver vanadium oxide (SVO) increases the ability of the cell to supply current.

“Ultimately, however, the main obstacle to balancing size and energy needs is simple chemistry,” Freitag says. “And while a great deal of research has been conducted to develop new and modified chemistries for battery applications, finding the right combination of high energy density, current capability, safety, reliability, and longevity continues to be challenging. Moreover, it takes many years of work to establish the long-term reliability of a new chemistry and to ensure that it is suitable for use in medical implantable devices.”

Reducing battery volume has always been a huge consideration, Freitag comments, but it hasn’t been achieved solely by improving the active chemical components. The company has also succeeded in miniaturizing its battery designs by decreasing the volume of the nonactive parts of the battery—the enclosure, current collectors, insulators, and other parts that don’t provide energy. And while size is important, the ability to create complex curvatures and shapes is also critical because it enables medical device manufacturers to better utilize space in physiologically compatible ways.

Battery miniaturization has also been accomplished through more-efficient electronics or by otherwise reducing current demand. Also, manufacturers have accomplished this task by maintaining tighter tolerances, using new materials, or implementing novel assembly methods. “Worldwide, an enormous amount of effort is underway to improve battery performance,” Freitag says. “While it is difficult to know exactly what is around the corner, there will almost certainly be advances that will impact the implantable device market in the future.”

Flatter than a Pancake

As with implantables, wearable medical devices that lie inconspicuously on the skin can make the transition from the design studio to the marketplace only if miniaturized battery technologies are developed to power them. In order to work in wearable devices, however, such batteries have to be both thin and flexible.

Manufactured by Alameda, CA–based Imprint Energy Inc., a class of miniaturized batteries fits this bill. Measuring 400 µm or less in thickness, the company’s polymer cells are flexible, allowing them to be embedded into all sorts of medical devices, according to CEO Devin MacKenzie. Similar in thickness to or thinner than any of the components in a flex circuit, they can be integrated directly with flex circuit assemblies. This functionality, he adds, enables medical device designers to fit batteries and energy-storage components into all sorts of tight spaces or to develop designs that can be embedded into clothing.

“Conventionally, there are two kinds of batteries used in the medical device space that designers have attempted to make thin and flexible,” MacKenzie says. “The first type includes lithium batteries, while the second includes thin versions of alkaline cell or zinc-carbon batteries.” While lithium-based batteries can exhibit good energy density and rechargeability, they tend to fail severely when they are flexed. In contrast, zinc-carbon batteries tend to have limited capacity per unit area and limited power-delivery capability. This failing becomes important in one of the key areas of functionality required for emerging wearable medical devices—wireless connectivity. To drive wireless connectivity, the battery must supply a good amount of instantaneous power, but traditional zinc-carbon batteries lack this performance capability.

Imprint Energy builds on both of these platforms, MacKenzie remarks. “Our batteries have an electrolyte that works similarly to lithium batteries, but instead of using lithium, they use zinc. Zinc enables us to simplify battery-manufacturing processes and create very flexible substrates and simple packaging without the toxicity and flammability concerns associated with lithium-based batteries.”

A battery is like a sandwich of electrical and chemical layers that make up its positive and negative sides, MacKenzie explains. The bridge that communicates between the two is the electrolyte. Constructed by printing successive layers on top of one another, Imprint Energy’s batteries consist of a stack of positive and negative components. The end result is a dried stack that forms a continuous thin film.

As straightforward as this manufacturing process may sound, printing miniaturized batteries for medical device applications is challenging. The most daunting challenge, MacKenzie notes, centers around finding high-performance materials that can be made into printable inks. “For example, if we need to add a terminal or a little electrical interconnection to a battery, the terminal can be printed. But it must still be able to drive wireless connectivity signals, requiring quite a bit of battery current.” Thus, the printed layers must be high-performance layers.”

Furthermore, the batteries must contain electrochemically compatible materials, according to Mackenzie. The chemistry employed in the company’s batteries differs from the conventional materials used in lithium or alkaline batteries. Because it reacts differently from standard materials, all the printed layers must be compatible with the battery. “Using compatible inks and ensuring that they perform as well as materials used in nonprinted batteries is quite challenging,” MacKenzie adds.

People have made sensors, wireless devices, smart bandages, and even drug-delivery devices that rest on top of the skin, MacKenzie says. They incorporate a 3-D package on top that contains coin cells or other batteries and electronics. But such 3-D devices get caught on clothing, rub against the edge of a bed, and are uncomfortable. Eventually, they will pop off because the body is flexible and the device is rigid. “In contrast, we’re talking about truly conformal smart bandages and especially such devices as ultrathin sensors that can wirelessly transmit signals,” he adds. “Such devices would be very advantageous to the patient. But to make them, you need thin, flexible batteries.”

Bob Michaels is senior technical editor at UBM Canon.

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