How Biomechatronics Works

In the TV series "The Six Million Dollar Man," scientists restore a crippled test pilot, who lost legs, one arm and an eye. They have the technology, so they rebuild Steve Austin and give him superhuman qualities. While this is total science fiction, modern robotics is inching ever closer to this vision in a field known as biomechatronics.

Biomechatronics is the merging of man with machine -- like the cyborg of science fiction. It is an interdisciplinary field encompassing biology, neurosciences, mechanics, electronics and robotics. Biomechatronic scientists attempt to make devices that interact with human muscle, skeleton, and nervous systems with the goals of assisting or enhancing human motor control that can be lost or impaired by trauma, disease or birth defects.

Consider what happens when you lift your foot to walk:

The motor center of your brain sends impulses to the muscles in your foot and leg. The appropriate muscles contract in the appropriate sequence to move and lift your foot.
Nerve cells in your foot sense the ground and feedback information to your brain to adjust the force, or the number of muscle groups required to walk across the surface. You don't apply the same force to walk on a wooden floor as you do to walk through snow or mud, for example.
Nerve cells in your leg muscle spindles sense the position of the floor and feedback information to the brain. You do not have to look at the floor to know where it is.
Once you raise your foot to take a step, your brain sends appropriate signals to the leg and foot muscles to set it down

This system has sensors (nerve cells, muscle spindles), actuators (muscles) and a controller (brain/spinal cord). In this article, we will find out how biomechatronic devices work using these components, explore the current progress of biomechatronics research and learn about the benefits of such devices.

Biomechatronic Components

Any biomechatronic system must have the same types of components.:

Biosensors

Biosensors detect the user's "intentions." Depending upon the impairment and type of device, this information can come from the user's nervous and/or muscle system. The biosensor relates this information to a controller located either externally or inside the device itself, in the case of a prosthetic. Biosensors also feedback from the limb and actuator (such as the limb position and applied force) and relate this information to the controller or the user's nervous/muscle system.

Biosensors may be wires that detect electrical activity such as galvanic detectors (which detect an electric current produced by chemical means) on the skin, needle electrodes implanted in muscle, and/or solid-state electrode arrays with nerves growing through them.

Mechanical Sensors

Mechanical sensors measure information about the device (such as limb position, applied force and load) and relate to the biosensor and/or the controller. These are mechanical devices such as force meters and accelerometers.

Controller

The controller is interfaces the user's nerve or muscle system and the device. It relays and/or interprets intention commands from the user to the actuators of the device . It also relays and/or interprets feedback information from the mechanical and biosensors to the user. The controller also monitors and controls the movements of the biomechatronic device.

Actuator

The actuator is an artificial muscle that produces force or movement. The actuator can be a motor that aids or replaces the user's native muscle depending upon whether the device is orthotic or prosthetic.

See the animation below to view how the system works. Next, learn about the progress made in the field of biomechatronics.

Orthotics vs. Prosthetics

Orthotic devices artificially assist human movement without replacing the impaired limb. In contrast, prosthetic devices replace the lost or injured limb to restore movement.

Why use biomechatronics rather than conventional orthotic/prosthetic devices? While many new orthotic/prosthetic devices use microelectronics and robotic components, they cannot accurately emulate the complex motions of human limbs. Current orthotic/prosthetic devices do not feedback to people or adjust to variable loads or complex terrains. They do not adjust on a moment-to-moment basis to the individual wearer. Biomechatronic devices promise to overcome these limitations by interfacing directly with the wearer's muscle and nervous systems to assist/restore motor control.

Biomechatronics Research

Several laboratories around the world conduct research in biomechatronics, including MIT, University of Twente (Netherlands), and University of California at Berkeley. Current research focuses on three main areas:

Analyze human motions, which are complex, to aid in the design of biomechatronic devices
Study how electronic devices can be interfaced with the nervous system (implantable electrodes in brain and muscle, surface galvanic electrodes on skin)
Test ways of using living muscle tissue as actuators for electronic devices

Analyze Human Motions Human motions are complex, whether it be reaching for a glass or walking over rough terrain. We must understand how humans move so that we can design biomechatronic devices that effectively mimic and aid human movement.

Dr. Peter Veltink and colleagues at the University of Twente are analyzing walking movements (gait analysis) by measuring body movements with camera systems, ground reactive forces with force meters, and muscle activity with electromyograms (recordings of the electrical activity produced by muscle contractions). The analysis of normal and impaired patients will help understand free walking motions and diagnose specific gait problems in impaired patients. Veltink's group similarly evaluates balance control while walking and standing.

Dr. Hugh Herr's Biomechatronics group at MIT uses computer models and camera analyses of movement to study balance , leg retraction during running , and angular momentum conservation during walking.

Interfacing Electronic Devices with Humans An important aspect that separates biomechatronics devices from conventional orthotic and prosthetic devices is the ability to connect with the nerves and muscle systems of the user so he can send and receive information from the device.

Peter Veltink's group in the Netherlands is using implantable electrodes to stimulate the calf muscles. They are developing sensing and control methods for the dorsiflexor muscles, which lift the foot during walking. This will help to treat paralysis and stroke victims who cannot control this foot during walking (i.e. dropped foot).

Hermie Hermens and Laura Kallenberg of Veltink's group are using electrodes placed on the skin to monitor the electrical activity of the underlying muscles (electromyography) rather than using electrodes implanted directly into them. This reduces pain and discomfort and may also be a pathway for 2-way communication. (http://bss.ewi.utwente.nl/research/biomechatronics/rsi.doc/index.html).

Veltink's group is also using electromyogram surface electrodes for feedback and control of lower-leg prosthetics. In the prosthetic, the knee angle is detected and the information is relayed by electromyography to the stump muscles in the amputated leg. The wearer can sense the activity and be taught to interpret it. Eventually the electrical activity of the stump muscles might be used to control the prosthetic.

Next, learn about biomechantronic research at MIT.

I, Cyborg

Dr. Kevin Warwick of the Reading University in the United Kingdom has tested human-computer interfaces on himself. Warwick was surgically implanted with a tiny array of 100 silicon electrodes into his medial nerve of his left arm below the wrist. Wires from the implant ran under his skin and out of his elbow where he connected with amplifiers and other circuits to convert his nerve impulses to digital electronic signals. With this implant, he was able to operate a separate robotic hand remotely by making movements with his own hand. The experiment ran for 3 months and Warwick described it in his book titled "I, Cyborg."

Hugh Herr and MIT Biomechantronics

Hugh Herr's group at MIT is developing a sieve integrated circuit electrode (an integrated circuit is a tiny plastic chip with an entire electrical circuit imprinted on it). In this setup, two stumps of nerve are connected through a guidance channel (a small tube that keeps the nerve endings close to each other). In the channel, there is a sieve with each hole connected to an electrode on an integrated circuit board. As the nerve fibers grow through the holes to connect with each end, they contact the electrodes, thereby creating an interface.

Advanced Orthotics and Prosthetic Devices

Hugh Herr's lab is also making prosthetic devices that better mimic true human movements:

A knee prosthesis senses knee force, torque, and position and adjusts the swing and movement of the knee to the individual user. In the knee is a magnetorheologic fluid, which is oil containing a suspension of tiny iron particles (0.1-10 microns in diameter). An electromagnetic field applied across the oil can change the thickness or viscosity of the fluid because the iron particles form chains as they align with the magnetic field. Because the viscosity of the fluid can be adjusted by fine tuning the electromagnetic field, this controls and adjusts the resistance of the knee on a moment-to-moment basis, thereby giving the user a realist gait. (See http://biomech.media.mit.edu/research/pro3_1.htm for a video of this device). Commercially, this knee is a product called Rheo-KneeTM, which is made by Ossur.
To treat drop foot gait, an orthotic device was developed that controls and varies the stiffness of the ankle joint on a moment-to-moment basis as the user steps forward. This device gives the user a more normal gait than current orthotic devices.

Current and Future Uses of Biomechatronics

Most actuators that are used in orthotic and prosthetic devices are electrical motors or electrical wires that shrink when current is passed through them. While these devices can provide contractile force, they do not come close to mimicking the dynamic flexibility of living muscle tissue. But what if you could make real muscle actuators? In Hugh Herr's laboratory at MIT, they have made a robotic fish that propelled by living muscle tissue taken from frog legs. The robotic fish had the following components:

A styrofoam float allows the fish to float
Electrical wires make the connections
A silicone tail provides the swimming force
Lithium batteries provide power
A microcontroller control the robot's movement
An infrared sensor enables the microcontroller to communicate with a handheld device
A stimulator unit electrically stimulates the muscles

The frog muscles were attached to either side of the tail and to the plastic spine of the robot and electrodes from the stimulator were attached to them. The muscles on either side were alternately stimulated to produce a swimming motion. The robotic fish was placed in a tank of salt solution designed to keep the muscles alive. The fish swam for 4 hours out of 42 with a velocity greater than 75 percent of its maximum. The robot could swim forward, backward, turn, and stop. This was a prototype of a biomechatronic device with a living actuator.

While advancements in body armor technology can help soldiers in the Iraq War survive explosions, many of these survivors suffer damaged limbs (hands, arms, feet, legs) that require amputation. This situation has spurred research into advanced orthotic/prosthetic devices such as the ones developed in Hugh Herr's lab at MIT. The Department of Veterans Affairs and the Dept. of Defense are funding several biomechatronics groups to meet the needs of veterans and soldiers in the field.

**Claudia Mitchell, the world's first "bionic woman."**
Photo courtesy ©2007 The Associated Press

Recently, Claudia Mitchell, a former Marine and amputee, has tested a prosthetic arm developed by Dr. Todd Kuiken at the Rehabilitation Institute of Chicago. A plastic surgeon, Dr. Gregory Dumainian at Northwestern Memorial Hospital in Chicago re-directed the nerves that control her missing arm to her chest. The nerves re-grew close to the skin of her chest. Tiny electrodes on her skin pick up the electrical activity of these nerves and send signals to the motors in the arm. She is able to control the arm's movements by thinking about it. As of now, the prosthetic arm is not truly biomechatronic in that signals only go one way, from Claudia to the arm. Dr. Kuiken is working on the next step of having the arm provide feedback to her, including sensations such as pain and pressure.

Until now, we have primarily addressed how biomechatronic devices can help people with impaired motor function. But what could these devices do to a normal person? Could they give him/her superhuman strength like Steve Austin, the "Six Million Dollar Man"? To this effect, investigators at the University of California at Berkeley have developed a machine or exoskeleton to enhance the walking ability of a normal human. The Berkeley Lower Extremity Exoskeleton (BLEEX) uses metal leg braces that powered by motors to make it easier for the wearer to walk. Sensors and actuators in the device provide feedback information to adjust the movements and the load while walking. The device's controller and engine are located in avest attached to a backpack frame. While the device itself weighs 100 pounds, it enables a person to haul a 70-pound backpack, while feeling as if he/she is merely carrying 5 pounds.

**The Berkeley Lower Extremity Exoskeleton (BLEEX) helps lighten the load for the human user.**
Photo courtesy ©2003 Berkeley Robotics and Human Engineering Laboratory

BLEEX could have many uses for the military as well as civilians. With BLEEX, soldiers could carry heavy loads across rugged terrain without fatigue. Similarly, military medics could carry injured victims off the battlefield. Fire and rescue workers could carry heavy gear or supplies great distances where vehicles could not travel.

When fully developed, biomechatronic devices will be useful in many ways:

They can provide improved motor function that better mimics normal biological function to impaired individuals
They can be used to train individuals with impaired motor function (physical therapy uses)
They can adjust to each person without requiring a third party
They can enhance performance of normal individuals

The major drawbacks to biomechatronics devices are the possibility of infection (as non-biological parts are implanted into living tissue) pain and discomfort. However, the promises of this new technology provide hope that those individuals with impaired motor function will be able to have restored functions that mimic normal human movements and, perhaps, make them better, stronger or faster than before.

For lots more information on biomechatronics and related topics, check out the links on the next page.