man with additional arms

Dear Evil Engineer: Is there any way I could acquire excess arms?

Image credit: Dreamstime

The Evil Engineer helps out an overworked villain and single parent in need of an extra pair of limbs.

Dear Evil Engineer,

I’m a single parent working full-time as a fintech consultant specialising in cryptocurrency exchange raids. Between keeping my clients’ imaginary money flowing into their wallets, volunteering with the Institution of Evil & Treachery, and homeschooling my two daughters, I wonder sometimes how I manage to keep going! I need more hours in the day, an extra adult around the house, or at least an extra pair of arms.

I can’t afford a vehicle that can be propelled to relativistic speeds, and under these coronavirus restrictions I cannot find a nanny or partner to come and help with the children, so I’m going to settle for the extra pair of arms. Would you be able to give me some advice?


An overloaded villain


Dear villain,

I am sorry that you are so burdened by your responsibilities. I have an enormous amount of respect for the villains who raise children and hell on Earth simultaneously. Let me help you as best as I can.

It is possible to surgically attach arms to a person and recover some function. In 2013, quadruple amputee Brendan Marrocco underwent an unprecedented double arm transplant. This complex process requires a long wait for the perfect donor and two teams of surgeons (one for each arm) at work for 13 hours. The arms are removed from the donor while the recipient is anaesthetised ready to... take arms. The surgeons open the skin at the site to isolate blood vessels, nerves, muscle, and bone; the same is done for the donor arms.

The bones are placed together, and the tendons and muscles are anchored to the recipient’s bone. The nerves are disconnected and carefully reattached to the nerves controlling the muscles in the donor arms; as they regrow, the recipient gains some control of the muscles. The blood flow is reinstated by reconnecting the veins and artery, and finally the site is closed.

The process is a brilliant gift for amputees. It is not, however, so useful for those already adorned with the conventional number of upper body limbs for Homo sapiens. You could conceivably create artificial shoulder joints to attach them to, but there remains the challenge of connecting them to the nervous system, finding a suitable artery, and supporting it with extra muscle and bone (arms are supported by the pectoral muscles, clavicles and scapulae). Even if you found a suitable artery, you would certainly end up with gratuitous arms flopping off your body amusingly. I’m not suggesting that’s a bad thing, but it’s not what you’re looking for.

So, let’s turn to non-fleshy extraneous arms: robotic arms. As with surgery, this field is mostly concerned with replacing missing limbs. The niche field of supernumerary robotic limbs, however, seeks to augment people with extra limbs that move independently of the wearer’s arms.

A team at MIT has been developing a pair of robotic arms that are attached to a backpacklike harness. An earlier iteration placed them directly over the spine, allowing the wearer to bear the 5kg machinery without excessive strain, but they were later relocated to around the waist. The robotic arms have three degrees of freedom, and grippers packed with sensors to monitor variables such as torque. This form factor – a pair of mirrored arms supported on the body with a harness – is standard.

The more interesting challenge is granting the wearer control; simply following or mirroring the movement of biological arms is of limited use. The MIT team used an approach well suited to a narrow range of tasks such as repetitive maintenance work. In their ‘teach by demonstration’ approach, a second human guides the robotic arms through a task manually, and the arms repeat these motions. Progress on the work is monitored via sensors on the wrists and robotic mount.

An alternative approach developed at the same university used a sensor vest to monitor four pectoral and abdominal muscles via EMG; learning to voluntarily contract these muscles allows the wearer to control the limbs. Researchers at the University of Tokyo also used muscular activity as input for robotic limbs; their system maps the motion of the wearer’s feet onto the arms. The limbs, which are worn beneath the arms, use a positional tracking system with optical markers on the toes and knees to track motion. A sock-like device allows the wearer to make a fist with the robot’s prehensile hands with a squeeze of the toes.

While these are impressive designs, the really interesting systems are those that are controlled using the mind. Mexican research scientist Christian Peñaloza is one of a handful of scientists specialising in using brain-computer interfaces (BCIs) to control robotic systems. Peñaloza’s approach monitors the brain with EEG: a simple and non-invasive procedure which uses an electrode cap to record the brain’s electrical activity. Turning these signals into movement commands is no cakewalk, particularly if the user is an able-bodied person wearing supplementary limbs. Under these circumstances, the software must untangle multiple sets of ‘intent’ within the electrical signals. Researchers tend to use machine-learning approaches to interpret these signals; Peñaloza uses deep learning. His experiments have demonstrated that a wearer can control at least one robotic arm with a BCI while simultaneously using their own arms to carry out a different task.

So, there you have it; supernumerary robotic limbs do exist and they can be controlled through various inputs, including with the mind. You just won’t be able to order them on Amazon quite yet.


The Evil Engineer

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