Proposal
Back to TopPhysical Description
What are the objectives?
There is much to be improved in this field. We believe we have developed a better concept of a bionic hand than the existing ones. The objective of our design is to create a more realistic hand while keeping it affordable. Though it is impossible to duplicate the complex human hand with the current technology, we can use concepts of mechanical engineering to reduce the cost of the hand while maintaining or even improving the efficiency. Our hand is more affordable, more durable, more efficient, and more realistic. It takes up less power because it has less motors. It's more realistic because the skin can feel. It's more durable because of less moving parts.
Our bionic hand will replace of the forearm, the wrist, and the hand. The battery and controller will be placed inside the forearm. The wrist will attach to the prosthetic forearm while the hand attaches to the wrist.
How do the digits function?
The degrees of freedom of the human hand can be credited to the five fingers as well as the wrist. The index finger, the middle finger, the ring finger, and the pinky finger can be grouped together, known as the vertical digits. These fingers have 3 joints that allow them to flex. Usually, three joints mean three motors; however, having three motors would take up massive volume as well as use up an enormous amount of energy. To solve this problem, we have developed a way to flex these fingers with only one motor. The motor moves the primary joint that attached to the palm of the hand and it in turn flexes the two other secondary joints. By doing this, we have effectively reduce the cost and the energy usage by two folds.
The opposable thumb is different from the vertical digits because it is much more flexible. The flexion of the thumb only has two joints. However, the thumb is also capable of rotating in and out of the palm. The rotation is essential in the capability of our hand because it facilitates any movement that requires a grip. So for the opposable thumb, we have one motor that does the flexion of the thumb and another motor for the important rotation.
What motors power the digits?
Our motors for digit flexion need to be cost-effective and small, require low maintenance, and balance between efficiency and torque. We have discovered that custom servo electric motors are the most suitable types of motors for our purpose because of their torque and precise positioning. Although they can be more expensive, the low-maintenance will be well worth the price. [4] Another important feature of our servo electric DC motors is that it is not the conventional type of motor. To optimize space, the electric motors inside our servos are positioned in a long cylinder. The position feedback potentiometer, reduction gear, and actuator arm are placed at the top of the cylinder. See picture.
The servo motor is useful because it features a closed-loop system. Because of our neural control system, the closed-loop system will allow proper feedback and adjustment of the motors. Compared to a stepper DC motor which also provides torque and precision but is a open-loop system, the servo offers more control.
Motor Specifications:
Custom servo electric motors
Operating Speed: 0.09sec/60° at 15.8V
Output Torque: 1kg/cm at 15.8V
Mass: 125g
Volume: 55mm x 10mm x 10mm
What about the wrist? How does it work?
The human wrist allows many degrees of freedom, as seen in the ability to rotate 360 degrees (at an angle). In reality, the most important part of the rotation is the ability to rotate about the arm axis, such as turning a key or using a screwdriver. To replicate this motion, we insert a servo motor that will allow the wrist to turn 360 degrees, without even moving the arm. The servos used for the wrist rotation are standard rotational servos. The wrist is able to rotate up and down, powered by another servo motor, like the one used for the digits.
How does the hand feel? Is there a skin?
With all these internal metal parts, the hand may rust. We cover all the aluminum alloy backbone with a layer of plastic so that they won't be oxidized. This also gives us a basic frame of the hand so that we can cover it with a new kind of artificial skin (FILMskin)[5] made from thin layers of polymers and carbon nanotubes that give patients the sensation of hot, cold, and pressure. The biocompatible nanotube can be designed to behave as both a temperature and pressure sensor, as well as a flexible electrical conductor. It combines with polymer material to provide mechanical and thermal properties similar to those of human skin. The feedback from the FILMskin can work with our peripheral nervous system. The FILMskin is a huge improvment to the LIVINGSKIN, a mere silicon covering, used by Touch Bionics for the i-Limb [6].
What are the main materials required?
- Neutral Controller x 1
- Custom DC Electric Servo Motor x 7
- Rotational DC Servo Electric Motor x 1
- Aluminum Alloy Backbone
- Plastic Covering for Backbone
- FILMskin
Back to TopFunctionality
Up until recently, most prosthetic hands were fairly simple devices that essentially had two “fingers.” That is, one finger was the opposable thumb, while the other finger was the index, middle, ring, and pinky fingers all moving in unison. This kind of functionality works well with a process such as grasping and picking up an object, but when more control is needed, the simplicity of two opposing fingers just isn’t enough. [7]
What can the hand do?
Our team's prosthetic hand is able to perform numerous functions. Instead of the stated “two finger” mechanical system, our bionic hand incorporates multiple facets in each finger so that each one is able to move independently. Also, we have incorporated an advanced wrist system that would be able to contort at multiple angles. Grip patterns are important, but being able to reach the area that one wants to grip is also a significant concern. Our prosthetic hand will be attached to the arm of the subject via a wrist with multiple functions. Should one need to reach under a chair to get something, or reach above to grasp an object higher up, the prosthetic can perform its required task.
The bionic hand can assume the “tripod” position by which it would be able to hold a writing utensil between its thumb and forefingers and write with it. The prosthetic is also able to produce circular motion where it can turn a knob or key. The hand will be able to grasp small objects such as a pin with ease by using only the tips of its forefinger and thumb. The hand will be able to wrap its fingers around an object, or extend the fingers to grab onto an object that is flat and long. Along with these specific examples, the bionic hand will be able to make use of its independent fingers to grip objects more tightly, or to apply pressure to different areas.
What other features does the hand equip?
One final feature of our bionic extremity is the ability to control the hand apparatus through the brain. There are currently crude methods of using brain to control machines, but when the machine that is being controlled is attached to the body, we would want as much control as we can get. We have created a fully functional system that will be controlled by the brain. The control that we are referring to is nervous control. That is, when the prosthetic hand feels something, the brain should receive that information and process it accordingly. With all of these functions, the user of our prosthetic hand will be able to conduct everyday life with less help from an outside aide.
Although our hand is not water resistant, it can be detached from the patient easily, allowing the hand to recharge. For example, if the patient wanted to take a shower, he/she can take off the hand and recharge it. After the patient is done, he/she can easily attach the hand back due to our intricately designed docking mechanism.
Back to TopPower
What is the hand's energy source?
The source of power for this prosthetic arm will be three 10-V lithium ion battery, an upgrade from the more common nickel-cadmium battery which was typically used in older battery-powered prosthetics. Generally, this energy source provides approximately twice the capacity of nickel-cadmium batteries and weighs about 25% less with 15% greater voltage [8].
Lithium ion batteries are light and small, reducing from the appearance of excess bulk and weight in the arm. The location of this 54x25x20.2 mm device is conveniently situated in the arm cavity where it is easily accessible for removing and recharging. Typically, this prosthetic arm will last over a span of twenty-four hours without losing charge, versus the older batteries that would only last for eight hours [9]. When not in use, the patient can easily charge the lithium ion battery for a few hours, after which the arm will be ready for use.
This battery is guaranteed to neither over-charge nor under-charge because it has a charge-discharge circuit that will automatically stop the device from charging once it reaches its maximum charge capacity [8]. Special requirements for this battery to be compatible with the prosthetic arm include an MC30 series Battery Adapter and a BC99 Fast Charger for full functionality [8]. The Battery Adapter will be used to connect to the prosthetic arm while the Fast Charger will ensure a minimal charge time.
With a more powerful energy source, the prosthetic arm will be able to perform more actions with greater strength when necessary. This arm has the strength to hold 10 kilograms for an entire day, a span of which the average human would tire only after a few hours [10].
The motors required to power the prosthetic arm are seven Custom DC Electric Servo Motors, which will accomplish all the required grip functions and tasks. These motors work at a total of 15.8-V, so the three lithium ion batteries will have enough power for these to perform without losing their initial strength.
Back to TopControl
How does the patient use the hand?
The control system for our prosthetic device is through the use of the registering electrical impulses which travel through the median nerve located in the forearm. This type of control system is relatively new compared to the EMG system but it provides a more complete control system [11]. Peripheral Neural Control requires an installation of a 4mm x 4mm Carbon Nanotube-coated electrode array to be surgically implanted into the median nerve of the patient [12]. When the patient loses his/her limb, the neural pathways which once connected the missing limb to the peripheral nervous system are still in use. When the patient attempts to move a “ghost” limb, electrical impulses are still sent along these efferent pathways even though there is no limb to move. [13]
How does the peripheral neural control system function?
The electrode array senses when these impulses occur and, depending on location in the nerve through which the impulse traveled, activate motors to move the hand in a fashion that would have been obtained through that of a natural hand. This allows for a variety of movement such as the rotation of the wrist, the opening and closing of the digits, and the rotation of the thumb. The added coating of Carbon Nanotubes improves the neuron-electrode interface. Allowing for the electrodes to both send and receive signals with higher sensitivity increases it's performance a thousandfold. [14] There are two metal plates on the controller which connect the pateint's upper arm to the nerves, which are then connected to the electrode array. The neural controller interface is placed at the end of the bionic hand in order for this work.
What are the benefits of this system compared to the MyoElectric system?
This type of control system also allows for afferent signals from touch sensors in the hand to be received by the peripheral nervous system [5]. It gives the user an almost natural feel to the prosthetic limb. With it, the patient could feel objects to determine if they're hot, cold, or possibly harmful to the patient. [15]
Back to TopEconomics
Our team’s goal was to create a bionic hand with an estimated cost of $15,000-$18,000, including the electrode implants. Though the price of our prosthetic hand is higher than that of the i-LIMB, the most proficient prosthetic hand there is today, ours consists of better technology and materials.
How much do each component cost and why?
Our hand is made up of seven custom-made DC electric servo motors for the fingers and one rotational DC servo electric motor for the wrist. DC electric servos and DC rotational motors generally cost between $80-$120 each, totaling up to an estimated cost of $640-$960 for all eight motors.
The aluminum alloy backbone of the hand will be covered with a layer of plastic to prevent the metal from oxidizing. The plastic also reinforces the frame of the hand, because of its rigidity and sturdiness. Aluminum alloy costs $1,300/tonne* and plastic costs about $0.66-$4.55 for less than 10,000 lbs [16][17]. Since aluminum alloy and plastic are fairly cheap, the total cost of both should be no more than $1000.
The estimated cost for neurostimulator electrode implants should be around $7,000-$10,000, but since we’re using carbon nanotube coated electrodes, the cost may be higher by a few thousand [18]. Though the carbon nanotube coated electrodes may be more costly, they're more efficient because of their higher sensitivity to nerve impulses.
Although FILMskin is not yet available, we estimated that it would possibly cost around $4,500 per kilogram, though it might be more. It’s a bit pricey because it mostly consists of carbon nanotubes, but the FILMskin will be beneficial in the end.
Rechargeable lithium ion batteries cost around $80 each, and should be replaced every year or so. Though the owner of the bionic hand is required to buy new lithium ion batteries every now and then, it is favorable in the long run because of their high capacity [19]. The higher the capacity, the longer the arm will run before needing to be recharged. In order for the special lithium ion batteries to be recharged, it is necessary to buy an MC30 series Battery Adapter, which costs another $80.
Overall, the total cost of our arm should be an estimated $13,300-$16,600. If we include the price of manufacturing and production, we estimated that the total cost of the bionic hand should be within the range of $15,000-$18,000.
How long is the lifespan of the hand?
The average lifespan of a prosthetic device is about 3 years [20]. Another one of our team's goals was to lengthen the lifespan of our bionic hand to at least 5 years. We are fairly confident that our bionic hand will be able to do so, because we used the newest and most durable technology. We effectively increase the lifespan of the bionic hand by investing in lower maintenance components. The hand must also be maintained and used carefully, in order to utilize it to its highest extent.
*1 tonne = 1000 kilograms
Back to TopBibliography
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