Although batteries and other portable energy sources may have been developed independently from bionic limbs, their level of advancement also largely affected the functions and limitations of advanced prostheses. Without these milestones, the relative convenience of using typical bionic limbs today wouldn’t be possible.
As such, when looking at the prospective future of artificial body parts, the conceptualization of various bionic energy methods must also be taken into account. Because in the future, it may not just be natural movements that make bionic limbs superior; never having to take them off would become just as crucial to their design.
The simple but tricky goals of any bionic energy system
Bionic limbs aren’t just designed to replace lost body parts functionally. They are also built to be used for as long as possible before requiring a connected energy source. As such, all bionic energy systems have the same goals: to create the perfect source that is lightweight, lasts for an entire day, and can integrate well with all other energy-saving gimmicks of the bionic limb itself.
For the very first bionic limbs, this was… kind of a challenge. Much earlier than our contemporary bionic woman of 2006, the 1993 Edinburgh Modular Arm System was initially developed with the use of rechargeable nickel-cadmium (Ni-Cd) batteries. With the limited space for the mechatronic sections of the arm, it was only able to fit inside a 0.3 kg unit, arrayed in ten cells that have a total capacity of 1400mAh.
It’s operational longevity? Only four hours on a full charge. Not exactly the duration you’d want for a seamless user experience within a single typical workday. But, it is kind of inevitable given the inherent limitations of nickel-cadmium batteries.
By contrast, the current-generation Hero Arm, a “budget” entry-level bionic arm by all standards can last for twelve hours on a single charge. Even with its relatively simpler design, it already ticks all the boxes for almost natural usage that reflects the original function of the arm. As you have guessed, it uses a modern-day fully-developed lithium-ion (Li-ion) battery, which is at least six times more energy-dense than Ni-Cd. The cells are arrayed in a configuration that provides a total capacity of 2600mAh, with the entire unit easily fitting the tiny case pocket located in the forearm.
Economic challenges, component miniaturization, and design integration
It doesn’t exactly take a genius to realize, but many of the design aspects of bionic limbs coincide with the challenges of mobile device development. After all, independent remote use, portability, and overall system weight are critical factors that are closely related to how easily a bionic arm or a smartphone can be used.
1. Prevalence of lithium-ion batteries in mobile applications
In terms of portability, it took a while for Lithium-ion batteries to become the norm before serious consideration for extended use was even made. Even though Li-ion batteries were available as early as the 1980s, the economies of scale had yet to make them affordable. We had to see the technology mature for at least three whole decades before they were affordable enough for the smallest of devices.
But what makes Li-ion batteries so perfect for bionic limbs, or even mobile devices in general? Apart from having the highest energy density discovered yet among the current battery types that we have, Li-ion batteries are also easily moldable, and maintain their electric potential even when pressed into thin shapes and strips. Indeed, we saw the largest boom in bionics-related startups during the years 2012-2016 (Ekso Bionics, Mobius Bionics, Open Bionics, etc.), where the biggest mobile device revolution in the form of high-capacity smartphones and tablets also took place.
2. Examining options for optimizing total weight and its distribution
The weight of the energy source (and power supply unit), as well as where to place it, is also somewhat of a default consideration when assessing bionic energy systems. Thankfully, this isn’t really something of a problem anymore, even when we include the added energy of motion and risk of overstressing its materials/components. In fact, the repercussions are pretty minor even when not using Li-ion batteries, so long as the energy use is efficient enough.
Apart from certain bionic limb types such as osseointegrated prosthesis, you are more likely to run into hardware fitting issues than adjusting the center of gravity or systematically reducing every gram of weight.
3. Integrating bionic energy systems with smart electronics
Like laptops and smartphones, modern bionic limbs are also built to harness various intelligent management systems that allow automatic power optimization for everyday use, such as low-power idle states, calculated power distribution, and accurate load estimation, among other fundamental features. This is also often backed up with minimal use of motors and actuators, only providing enough complexity for the necessary functions to work when they are needed.
Perhaps the best example of this at the moment is the Genium X3 knee prosthesis. Developed by long-time prosthetic expert Ottobock, it can last for five whole days when fully charged thanks to its intelligent energy management system. Though, of course, the company still recommends recharging the bionic limb at least once a day when not in use.
Notable innovations for bionic energy systems
Of course, this couldn’t be a dive into technology development without mentioning some of the innovative ways to improve on the basic concepts of bionic energy systems. A few notable ones are the following:
1. Regenerative power systems
Basically, this concept aims to provide passive energy to the bionic limbs by harnessing the movement and internal activity of the entire body itself. One method is by using electrogenic cells, which is a substrate (similar to those found in electric eels) that checks for ionic currents within cellular activity and converts them into usable power. The plan is to create artificial interfaces filled with these cells, implanting them inside the body to produce and release electricity.
Research into these ideas is, of course, still in its very infancy, even after several years of work. The main challenge is output control, or how to accurately quantify the energy. Surprisingly, the level of power generated was not much of a problem, given the relatively low power requirements of bionic limbs (provided that consistency is maintained). Also, many of the concepts of regenerative power systems on high-tech prostheses are only applied for the signal and data relay between the motors and actuators, not powering them directly.
2. Implantable fuel cells
Similar to the extension of battery technology with hydrogen-based power generation, implantable fuel cells also promise to either extend the use of internal batteries for bionic limbs or at least provide energy to less demanding components of the system. Of course, this will also eventually include outright replacing batteries, though we most likely won’t see any practical use for such a concept in the next few decades.
For example, a rather old concept was proposed by researchers at MIT, where a fuel cell could be designed to process sugar (yes, the very glucose that we also use) as a source of energy. The idea is that it would be combined to highly optimize brain implants, which would provide control to the bionic limbs, or at least give enough energy so that data transfer between interfaces would no longer require an active battery connection.
If anything else, there’s still good ol’ mechanical ingenuity
Miniaturization, optimization of design, and energy management algorithms go a long way in making a bionic energy source look and feel more reliable and long-lasting. But if there is no more room for improvement (in the future), we can at least be confident that components outside of standard bionic limb design would make their way into practical use by sheer ingenuity.
This was the case when researchers at the University of Michigan unveiled their decision to adopt a gear component design that the robot arm at the International Space Station had previously used. The more stringent requirements of using hardware for mechatronic applications in space meant the robotic arm was the perfect fit for an improved bionic arm design.
Among the criteria list, of course, are energy efficiency and weight, as both are several times more important when bringing and using hardware in space.
