• mechanism design
• prosthetic thumb
• robotic hand
• robotic thumb

The replacement of a missing human hand with a prosthetic has become a definitive need recently. The prosthetic hand shall serve as a proper replacement both in terms of cosmetic and functional aspects. The functional aspects of the prosthetic hand have seen a range of developments and innovations in the past 40 years from passive wooden hands to multifunctional electronic hands. This research is still ongoing as the need to effectively compensate for the functionality of the human hand still exists.
The thumb plays a prominent role in the hand allowing it to perform a variety of tasks. It is also considered the evolutionary paradigm shift in terms of limbs functionality. Hence, there is a dire need to restore and improvise the functionality of the thumb in the prosthetic hand. The difficulty arises in the design of thumb due to its unique geometry and movements compared to the other fingers. The thumb can change its orientation from opposable to lateral position depending on the need. The thumb also performs the flexion/extension movement like the other fingers, but the range of motion is greater, as this motion can be coupled with the thumb abduction/adduction. Therefore, the thumb has two separate axes of rotation and the challenge in the prosthetic thumb is to obtain both these movements maintaining the dexterity but not compromising with other geometrical concerns such as weight and size of the thumb. Hence, efforts are made to perform both these movements with a single actuator by realizing a mechanism.
The design of the mechanism has been approached by setting a set of prerequisites that define the boundaries to explore design inspiration. The prerequisites have been divided in terms of quantitative and qualitative parameters. The primary difference in between them is that the quantitative parameters can be measured directly, and the quantitative parameters need a different mode of measurement. The quantitative parameters are
• Size- The size of the prosthetic thumb must be comparable to the standard human hand and is considered as a very important prerequisite
• Weight- Weight is an important feature in terms of user comfort and shall be made to be as close to the human hand
• Grip force- The grip force determines the level of usage permissible with the prosthetic limb. Hence is kept as a feature that needs to be matched with the human hand as close as possible
• Speed- speed and grip force cannot be realized at the same time as they can be considered as tradeoffs. As we increase one, the other is compromised. Therefore, the speed of the thumb shall be less important than the rest of the prerequisites.
• Nonback-drivability- This feature enables to hold the position and allows to maintain the force without the continuous supply of power. This is an essential feature in the prosthetic as it can help with fatigue and power concerns.
The quantitative parameters are
• Adaptability- this feature determines the adaptable nature of the thumb with respect to the holding object. This cannot be directly determined. Hence, it is realized as a resultant feature of an adaptive grasp mechanism. This feature is not considered as a very important feature.
• Complexity- the level of complexity employed in the prosthetic thumb determines the level of innovation. When the need to encapsulate both the flexion/extension and abduction/adduction movements into one single mechanism, the complexity arises. Therefore, this feature is considered, yet kept as a less important feature.
• Reliability- The reliability of the prosthetic thumb reduces with increases with increase in the number of parts employed in the mechanism. The more the number of parts in the mechanism, the more risk of them failing and resulting in a critical failure. Therefore, this feature is considered very important
• Dexterity- As already proclaimed, the most intriguing feature of the thumb is its dexterity, and it can be measured by checking the number of grasps achievable.
Keeping these design prerequisites, a different state of the art prosthetics was studied and compared. The comparison was made on the basis of mechanisms used and its resultant performances. The state of the art was divided as commercial prosthesis and research prosthesis. The commercial prosthesis was a design that has been successful and has been engaged with the market that gives definitive feedback. The research prosthesis explores different innovations and can give scope for the betterment. These hands were compared by type of mechanism, type of actuation technique and evaluated against the design prerequisites proclaimed previously by using quality function deployment in House of Quality.

• Ilimb
• Bebionic
• Vincent
• Manus
• Smarthand
• Vanderbilt
• SSSA-MyHand.
Using the data obtained from comparison from the House of Quality four designs were proposed. The designs were classified as semi-passive and passive abduction mechanisms. The semi-passive designs have an abduction/adduction movement assisted by the actuator in only one direction. The opposite direction shall be realized manually. The passive abduction mechanism operates independently without any assistance from the actuator and only relying on the external manual force.
Design 1- The design utilizes a worm gear mechanism to perform the flexion motion. Due to the use of the worm gear mechanism, the nonback-drivability is an inherent property of the mechanism of flexion/extension. The abduction/adduction mechanism is performed by a gear that engages with a toothed abduction block when the thumb performs a hyperextension and undergoes abduction mechanism. The Cad model for the mechanism was presented and the meshing of the gears with precision was a huge problem and the design was not presented further.
Design 2- The design has a similar feature as to design 1 and performs exactly in the flexion/extension movement. The abduction/adduction movement is realized by a friction wheel attached as an extension to the back of the thumb and engages with abduction block when the thumb performs hyperextension. The rotation of the motor continuous the abduction movement in one direction. The nonback-drivability of the abduction/adduction is ensured by employing a ratchet and pawl mechanism. The opposite return of the thumb is performed by a hyperadduction and engages a snap mechanism that returns the thumb to its primary opposition position
Design 3- The design is a passive abduction/adduction mechanism. It uses the same worm gear mechanism for flexion but for the abduction/adduction movement it uses a button trigger that releases the position and allows it to adjust the desired position. This trigger can be accessed manually of also using an external constraint such as pressing against an object. The disadvantages of the mechanism were the amount of friction that developed and trying to fix an accessible position for the trigger button.
Design 4- This mechanism keeps the flexion mechanism same as previous but for the abduction uses a pin that engages and disengages due to an external force. The mechanism is very simple yet, effective and complies with all the prerequisites efficiently.
The design 4 was manufactured to scale to test further its effectiveness and has been able to retain it. The thumb in overall complies exactly with the size and weight constraints of the human thumb. The thumb weights 58.6 g including motor and gear reduction mechanism, excluding the cover and other electronics. This is close to 1/5th of the weight of the natural human hand. The reliability fares high too due to its simplistic mechanism and a minimal number of parts. The thumb exhibits 22.7 N of force during flexion.
The design 2 even though has some disadvantages due to the stresses developed can be further developed and presented an optimal solution for the prosthetic thumb. This work shall be carried out in the future.