Development of a functional hand orthosis for boys with Duchenne muscular dystrophy

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DEVELOPMENT OF A FUNCTIONAL HAND ORTHOSIS

FOR BOYS WITH DUCHENNE MUSCULAR DYSTROPHY

CLAUDIA HAARMAN

(BW-PDENG-002)

SEPTEMBER 30, 2016

EXAM COMMITTEE

prof.dr.ir. H.F.J.M. Koopman

ir. E.E.G. Hekman

dr.ir. T.H.J. Vaneker

dr.ir. D. Lutters

F. Tönis

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Development of a functional hand orthosis for boys with Duchenne

Muscular Dystrophy

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Summary

Duchenne muscular dystrophy (DMD) is a progressive disease causing a gradual loss of muscle strength. Around their twenties patients already lost most of their arm and hand function. In this research (part of Symbionics, STW funded research program) a new functional hand orthosis is developed that can assist users during daily tasks. This orthosis is wearable and needs to assist flexion and extension movements of the fingers. Wearable robotics (and hand orthotics in particular) have to achieve strict requirements as the devices will be worn on a permanent basis. User requirements were established during an intensive background study. These interviews and questionnaire resulted in a listed of 15 requirements from which 10 were considered the primary focus points. Another background study was conducted to relate important ADL tasks to grasps that are used in daily life. Together with other aspects this led to the system requirements. A hand orthosis design was established using a novel tape spring mechanism. The mechanism contains sliders and a sliding tape spring that are able to flex the fingers. Extension of the fingers was done with a passive constant force spring. The low profile around the fingers and hand together with the low weight due to remote actuation and the use of 3D-printed parts makes this design especially suited for daily usage.

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Chapter 2

General introduction

2.1 Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked progressive disease with an incidence of one in 3600-6000 male live births. [1] A mutation in the dystrophin gene causes a gradual loss of muscle strength starting at the lower extremities. Early signs include walking problems which become apparent around year 5. At the age of 10 most boys with DMD are already committed to a wheelchair. Progression of the disease is from proximal to distal. During their teenage years patients lose most of their arm function, whereas hand function is still present until their (early) twenties. Recent developments have led to an increase in life expectancy. [2] In order to maintain autonomy and a good quality of life the need for proper hand functioning is increased.

A typical timeline of a boy with DMD is displayed in Figure 2.1.

Figure 2.1: Typical timeline of a boy with DMD

2.2 Wearable robotics

Wearable robotics can be aimed at rehabilitation or assistance in daily tasks, commonly referred to as Activities of Daily Living (ADL). Robotics for rehabilitation do not have to be wearable as usage is limited to training sessions in a rehabilitation center or at home. Robotics designed to assist in daily tasks should be wearable in order to fulfill their function. They are also called permanent assistive devices as they should be worn on a permanent basis.

Orthoses are permanent assistive devices that are lightweight and unobtrusively support human function. A hand orthosis should restore lost hand function and/or enhance the remaining function where it can be detected, shares control where needed, and compensates for function when instructed.

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2.3 Project description

The Symbionics program is a unique collaboration between research groups, companies and foundations. The six projects within this program aim to improve assistive devices that are used to help people with disabilities. The focus is on providing adaptive support, meaning that the devices adapt to the user, and not the other way around.

Figure 2.2: Symbionics project

In project P1.3 an intuitive hand orthosis will be developed for boys with DMD. The hand orthosis will also be integrated with the arm device of the original Flextension project and has to be usable as a permanent assistive device.

2.4 Problem definition

As boys with DMD get older, their problems increase. Eventually their hand function is affected by the loss of hand muscle strength. Difficulty with the execution of activities of daily living lead to an increase in the demand of support. Lack of independence may adversely affect their quality of life.

2.4.1 Goal

A device that will support grasping and other activities of daily living while being as unobtrusive and lightweight as possible, may significantly improve the quality of life for boys with DMD. The goal of the project is therefore to develop a functional hand orthosis that can be used throughout the stages of Duchenne Muscular Dystrophy and supports and/or enhances the hand function of these users. As an extra benefit the user is stimulated to use the hand more, which can prevent further contractures.

Important research questions that will be answered during the research project are:

• How can the hand function of a DMD patient be supported with minimal construction? • How to ensure maximum comfort when using assistive technologies?

• How can the remaining hand function be optimally stimulated to prevent disuse? • How can the device be controlled in an intuitive and safe way?

In this design report focus is on the first two research questions. A prototype will be developed with the new insights and technologies established during the PDEng assignment.

2.5 Background

2.5.1 State of the art

For other target groups such as stroke patients or paraplegic patients several functional hand orthoses are commercially available or are under development, for instance the SEM glove (Bioservo Technologies AB) or SaeboGlove (Saebo, Inc.). Although the characteristics of the target group can be different (e.g. residual force, age, contractures, spasticity, type of support) the function of the assistive device is the same, namely:

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2.5. BACKGROUND 9

restoring hand function. Research in this field is more mature. In [3] we presented a structured overview of trends and technologies used in dynamic hand orthoses. From the 165 found devices in this search a few examples will be highlighted in the chapter about the mechanical design.

No devices are currently available on the market that are specifically aimed at improving hand function for Duchenne patients. Because the arm is affected prior to the hand, most devices only focus on restoring the functionality of the arm. Increased life expectancy of patients with DMD during the last decades have only recently led to increased efforts in re-enabling hand function. This is a new design area and is currently unexplored.

Figure 2.3: JACO robotic gripper

Typical assistive devices available for boys with DMD are robotic grippers. A joystick is operated by the user and controls the gripper in a master-slave configuration. There must be some residual hand function to control the joystick. An example is the JACO Robotic Arm, see Figure 2.3. Although this device is capable of assisting users during ADL tasks that normally would use one or two hands, no actual hand function is regained. It thus does not stimulate the use remaining hand function but encourages disuse which is a contradiction to the objectives of the device.

2.5.2 Target group

The target group is given below. The orthosis will be developed for users with DMD that: • Have limited hand function: Brooke scale grade 5 or 6 [4], see Table 2.1 below.

• Are in the early or late non-ambulatory stage of DMD according to the classification of Bushby [1]. • Have sufficient skill level to be trained using the device.

• Use a mobile arm support

• Have limited contractures of finger joints • Have measurable EMG activity of hand muscles

Table 2.1: Brooke upper extremity scale [4]

Grade Item

5 Cannot raise hand to mouth but can use hands to hold pen or pick up pennies from table 6 Cannot raise hands to mouth and has no useful function of hands.

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2.5.3 Stakeholders

The stakeholders of the project are listed in Table 2.2. As all stakeholders have different interests, their primary interests are also presented. During the development process their interests should be considered.

Table 2.2: List with the stakeholders of the project

Stakeholder Primary interest

DMD patients Improve quality of life by regaining lost hand function Family/friends Make daily care easier

Orthotist Adjust device to fit customer needs Occupational therapist Improve quality of life

Rehabilitation physician Monitor hand function

Physiotherapist Improve hand function / stop progression

Health insurance company Keep costs low (low product costs + long lifetime) Patient interest groups/ patient

association

Improve quality of life for group of patients

Investor Earn money

Research group Gain knowledge on new mechanisms and solutions Engineer Gain knowledge on new mechanisms and solutions Producer Earn money, establish reputation

Supplier Earn money

Device trainers Train patients using device as best as possible Government Improve quality of life of citizens

2.5.4 Hand and finger anatomy and kinematics

2.5.4.1 Bones

From distal to proximal the bones that form the hand are: distal phalange, middle phalange and proximal phalange (for each finger). The thumb only has a distal and proximal phalange. The metacarpal bones are attached to the proximal phalanges. More proximal we find the carpal bones that provide a connection between the metacarpal bones and the two forearm bones (ulna and radius). Figure 2.4 display the bones of the hand.

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2.5. BACKGROUND 11

2.5.4.2 Degrees of freedom

The human hand has many degrees of freedom (DoFs). Finger joints are formed at the articulating surfaces of the bones. These joints are: The carpometacarpal joint (CMC), metacarpophalangeal joint (MCP), distal interphalangeal joint (DIP), and proximal interphalangeal joint (PIP). The MCP joint represents 2 DoFs and allows for flexion/extension and ad/abduction movements. The DIP and PIP joints are 1 DoF revolute joints and are only able to provide flexion/extension movements. Figure 2.5 shows a kinematic model of the finger. In this picture the metacarpal (MCP) joint, proximal interphalangeal (PIP) joint, and distal interphalangeal (DIP) joint are indicated.

Figure 2.5: Kinematic model of the finger with the joint movements indicated.

The range of motion (ROM) of the wrist consists of flexion-extension, radial-ulnar deviation and pronation-supination movements, see Figure 2.6.

The ROM of the wrist and hand joints (given as a deviation from their neutral position) can be found in Table 2.3. Per degree of freedom the maximum and minimum joint angles are listed.

Figure 2.6: Joint movements of the wrist

Table 2.3: Normal ROM for the hand and wrist joints (data ex-tracted from [5])

Segment Joint DoF Max (◦) Min (◦) Forearm Wrist Flexion-extension 80 70

Pronation-supination 80 85 Radial- ulnar deviation 20 30

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Segment Joint DoF Max (◦) Min (◦) Thumb CMC Palmar abduction 50 0

Radial abduction 20 0 MCP Flexion-extension 45 0 IP Flexion-extension 100 0 Index, middle, ringe, pink MCP Flexion-extension 83 30

Abduction-adduction 25 20 PIP Flexion-extension 101 0 DIP Flexion-extension 73 0

2.6 Report outline

In Chapter 3 the user requirements are described. These can help to identify problematic areas and focus points for the design through interviews, observational studies, a questionnaire and a focus group. These use requirements are then ranked based on importance. In Chapter 4 a translation from user requirements to system requirements is made. Analysis of grasps used in daily life lead for example to requirements about the useful range of motion for the finger joints. The mechanical design of the hand orthosis is presented in Chapter 5 followed by the technical evaluation of the force transmission system in Chapter 6. At the end of this chapter a short review of the design relating to the system requirements is given. Finally, in Chapter 7 the general conclusion is found with some discussion points and recommendations.

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Chapter 3

User requirements for a functional

hand orthosis

Thinking that an engineer or healthcare professional is perfectly able to identify the needs of potential users is a common pitfall. The hand orthosis benefits from an increased effort to identify user needs because the end product will better match their expectations and fits their wishes and desires. Setting user requirements without consulting the user is something that should be avoided. Without the consent of the most important stakeholders the product is likely to end up unused. Disuse of the product together with the inability of the hand to perform an increasing number of ADL tasks promotes disuse of the affected hands. The consequences of disuse can be dramatic: faster degeneration of muscles, and other secondary complications of inactivity (such as an increase of contractures and a lower active ROM), eventually resulting in social isolation [1] [6]. The goal of this chapter is to analyze user needs.

3.1 Background

Activities of daily living can be categorized according to different themes. Some themes are more important than others for boys with Duchenne in the early or late non-ambulatory stage. This has to do for example with their physical capabilities (e.g. wheelchair-bound, cardiac and respiratory problems), external circumstances (e.g. not being able to live without supervision of a (professional) caregiver) and/or age. People with muscle weakness are typically elderly and they have very different demands from an assistive device that supports their hand function.

A literature search revealed several studies that can help to identify problematic tasks for boys with DMD. The functional capabilities of our target group are described in [7], the difficulty in execution of certain ADL tasks is described in [8]. The relation between ADL tasks and their associated grasps is the focus of [9]. The first two studies are focused on boys with DMD. Bartels et al. assessed the upper limb function of boys with DMD with the Motor Function Measure (MFM) [7]. The MFM includes six tasks that are related to upper limb function, but these don’t resemble functional (ADL) tasks. Fujiwara et al studied the difficulty that boys with DMD have when performing ADL tasks according to different themes based on the Functional Independence Measure. [8] From these results only a broad overview of problematic areas for boys with DMD is obtained because the difficulty is assessed based on general themes and not specific tasks. The third study is about the identification of grasps related to ADL tasks. Bullock et al performed a grasp analysis and classification during frequent manipulation tasks [9]. However, their target group consisted of healthy subjects.

We are interested in the tasks that apply to young adults with muscle weakness, and what they consider important tasks.

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3.2 Methods

First an exploratory study with two participants is described to get familiar with the potential users and examine the characteristics of the target group. Then a questionnaire about the importance of different ADL tasks is presented. Finally the outcome of a focus group meeting are reported. From these results the user requirements are identified and a ranking is determined.

3.2.1 Exploratory study with users: Interview and observational study

Two users are asked to participate in an interview to map the difficulties and problems during daily activities due to a decreased ability to move arms, wrists and fingers. This exploratory study is focused on boys with DMD in a late non-ambulatory stage of the disease. This means that they are already in wheelchair for a long time. In individual sessions of about one hour participants are asked about social participation, daily activities, physical conditions, use of assistive devices, change in execution of activities and pain/stiffness. They are also asked about the importance of tasks in themes such as study/work, domestic, transportation, health and relations. Finally, a short observational study is conducted where in a domestic situation the activities are observed.

3.2.2 Questionnaire

Janssen et al [10] studied the activity limitations of boys with DMD using the Capabilities of Upper Extremity questionnaire (CUE) [11] and ABILHAND-plus questionnaire (extended version of the ABILHAND questionnaire) [12]. Basic Upper Extremity mobility activities can be measured with the CUE and complex activities with the ABILHAND-plus.

We are not only interested in the (remaining) capabilities, but also want to know what the users think are important ADL tasks. Therefore, both the CUE and ABILHAND-plus questionnaires are combined and extra ADL tasks are added that we thought to be important as well for boys with DMD. In this new questionnaire, the user is asked to grade all ADL tasks based on what he thinks is an important task to be able to execute independently (with the help of the new assistive device). Each task is given a grade from 1-5 where 1 is not important at all and 5 is very important.

46 potential users are invited to participate from which 39 (partially) filled in the questionnaire. The results are used to make a selection of the most important ADL activities that the device should enable (or not impede if that activity is still possible).

3.2.3 Focus group meeting

The goal of the focus group meeting is to get user feedback during the design phase. Four users in the early to late non-ambulatory stage of the diseased and their caregivers are invited for this meeting. During the session users are asked about their problems in daily life; Their needs with respect to hand function are discussed. The focus group meeting was scheduled for 2 hours. Four users from different stages of the disease are asked about their upper extremity range of motion, daily activities and usage of assistive devices.

3.3 Results

The results of the exploratory studies (interview and observational), questionnaire and focus group meeting are reported in this section. Based on the answers and observations obtained during the exploratory study problematic areas during ADL tasks are found. The results of the questionnaire are displayed as two graphs and used to identify tasks that are perceived important. The focus group meeting highlight the upper extremity range of motion, daily activities and assistive devices.

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3.3. RESULTS 15

3.3.1 Exploratory study (Interview)

Two boys are approached that satisfy the inclusion criteria for the interview and observational study. They are Kevin (23) and Pascal (20). Kevin is living in an assisted living facility, Pascal is living with his family. A short summary of the interview is given below.

Kevin has a modified wheelchair with a robotic gripper. He does not have an arm support because the level

of support became too low. Kevin has difficulties moving his arm forward. The fingers of his right hand suffer less from contractures than his left hand due to the use of the joystick. Educational level: VMBO-T. He plays wheelchair hockey (only training). Kevin is in a wheelchair since he was 12 and goes with a taxi to a day-care center where he builds websites with a friend. Kevin is not frequently outside, because the stiffness of his fingers increase when it is cold. He has no pain in his limbs and joints, except after a large physical exertion.

Key requirements that came up during the interview are in the social, transportation and leisure domain. He wants to be able to keep using the computer (keyboard plus specialized computer mouse), keep using the wheelchair with joystick. Also, he is not able to see the screen of his phone because he cannot raise his hand high enough and cannot bend his head downward. Because his house is adapted to his needs, he is relatively able to live independent. Still he needs help with getting up from bed, dressing, and personal hygiene. Also eating is difficult as he cannot cook himself and bring food to his mouth. This is due to the lack of arm and hand function (providing enough grip force to hold an object).

Pascal lives with his parents and uses a modified wheelchair. He is in a wheelchair since age 12 and plays

hockey (training and matches). The ability to use arms and hands in functional tasks is still present. Pascal goes to school (ROC). In the social domain he is very active. He has a lot of friends and regularly goes outside. One of his hobbies is photography.

Contractures are limited and also the stiffness of his finger joints is low. He does not suffer from pain in his limbs and joints. Because his hand function is relatively good he has not experienced many changes in his activity patterns. He does make compensatory movements with his shoulder to sway his arm forward. Pascal has trouble opening with lifting heavy objects (lack of strength in his arm).

3.3.2 Exploratory study (Interview)

Four pictures of Kevin and Pascal, taken during the observational study are shown in Figures 3.1 and 3.2.

The range of motion of Kevin’s fingers is compromised. In the top right picture it is shown that he is not able to fully extend his fingers. Full flexion of his index and middle finger is almost the same as his resting position. Contractures are present in almost all fingers. Wrist function very limited as well.

The range of motion of Pascal’s fingers is approximately equal to healthy subjects. Also, the force left in his hands is sufficient to perform most ADL tasks. He does experience some hyper-extension of his finger joints (see the bottom right figure) and makes compensatory movements with his shoulder to sway his arm forward.

3.3.3 Questionnaire

The results of the questionnaire can be found in Appendix A. Users are divided in four groups based on their age: 5-10y (n = 10), 10-15y (n = 19), 15-20y (n = 6) and 20-40y (n = 11). To illustrate the possible differences with other stakeholders, the questionnaire was also filled in by two engineers and two healthcare professionals. Their results are plotted in this graph as well.

From these results it can be seen that there is a large spread in rating between the groups. Other remarks regarding the ratings can be made. The healthcare professionals and oldest users (20-40y) gave the lowest scores. The engineers gave the highest scores. Tasks involving social interaction with their environment (such as using the touchscreen of a mobile phone, controlling the joystick of a wheelchair, game controller, elevator buttons, emergency button, keyboard and computer mouse) are very important for all user groups.

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Figure 3.1: Pictures of Kevin during the observational study. Top left: overview; Top right: Maximum extension; Bottom left: maximum flexion; Bottom right: Control of remote control

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3.3. RESULTS 17

Figure 3.2: Pictures of Pascal during the observational study. Top left: overview; Top right: Using game controller; Bottom left: Writing; Bottom right: Maximum extension during closing of DVD case

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Boys between 15 and 20 were best able to display their needs with respect to important ADL tasks because they still have some remaining hand function left. For older participants we experienced it was harder for them to recall what they think is important in daily life because they cannot perform those tasks anymore. On average their scores are lower. In Appendix B the results of the questionnaire for the ages 15-20 and 20-40 are shown again. The tasks that score on average 3.5 or higher are highlighted.

Besides the tasks associated with social interaction other important tasks include money handling (to go shopping independently), writing, washing, and using a spoon. It is remarkable that other tasks involved with eating are considered less important.

For the relevant tasks (score >3.5) it is desired that the orthosis should enable these tasks. In the next chapter a grasp analysis is conducted where the tasks are related to essential grasps.

3.3.4 Focus group meeting

Four boys participated in the focus group meeting.

Table 3.1: Participants of the focus group meeting

User Accompanied by Age Hobbies

Sam Mother 13 Wheelchair hockey

Dennis Mother 21 Watch football, likes to cook Tom Mother 21 Wheelchair hockey

Tim Caregiver from assisted living facility 22 Wheelchair hockey

3.3.4.1 Upper extremity range of motion

Both Sam and Tom can still raise their arms. Dennis and Tim are unable to move their arm and wrist and only have functionality of their fingers left; Tim can only move his finger a bit. Dennis primarily moves his thumb, index finger and middle finger of his right hand.

3.3.4.2 Daily activities

When asked about their daily activities all boys gave a description of a typical day.

Sam: He starts his day with school, gets home at three o’clock and makes his homework. After this is done

he watches TV and plays with the Playstation. He cannot dress himself, but is fine with that. At school he uses a computer for most of the tasks that involve writing. Someone helps him packing his bag. In a taxi he is transported to the wheelchair hockey training. In bed he is able to reach the alarm clock, but cannot rotate himself. His mother helps him with this.

Tom: Tom gets up with help from his mother. Dressing of his is shirt/sweater is mostly done by himself.

Tom does volunteer work two days a week. He helps in the library with lending of books. He plays wheelchair hockey a lot.

Tim: In the morning a caregiver helps him to get out of bed and dress. They also help with eating. Tim’s

daytime activities mostly consist of photoshopping and video editing. Besides that he is a frequent gamer. He plays wheelchair hockey from a special wheelchair and is unable to hold the hockey stick.

Dennis: Every morning Dennis is helped out of bed by a caregiver. This person also helps him with small

medical interventions. Four days a week he is at home working on his computer and watching football and tv shows about cooking. He is not able to cook himself, but thinks a lot about recipes. The social aspect is important for Dennis and regularly meet with friends. He uses a computer mouse to control his mobile phone.

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3.4. USER REQUIREMENTS 19

3.3.4.3 Assistive devices

The participants are also asked about previous experience with assistive devices and their expectations about assistive devices (in particular focused on the hand).

Sam: Used an foot splint in the past to align his foot. Due to feelings of discomfort he stopped using

this splint. Sam’s greatest desire is to keep gaming. Also, he has some issues with scratching the back of his head and holding a knife. There are no specific functions he wants to regain. Sam does not need a joystick-controlled wheelchair at the moment but uses E-motion wheels.

Tom: Also Tom used night splints for his feet in the past; Pressure of the splints against his skin disturbed

his night’s rest. Tom’s dream is to ride a car.

Tim: Tom uses night splints for his hands to keep functionality of his finger. He is afraid that without finger

function it will be impossible to control his wheelchair. His lightweight splint is comfortable. He also has a large splint (around his upper arm) but that one is hurting due to the weight. Tim thinks his mobility greatly improves if he is able to move his arm (his wrist is very stiff). His wish is to drive a car. Tim uses his phone through the wheelchair. Using his own hands will increase his sense of independence. He has a robotic gripper but the power supply of the wheelchair is too limited to frequently use this device.

Dennis: He has a night splint to keep his hands open but is unable to wear the splint for a long time because

it hurts at the fingers and wrist. He thinks it is very important to keep the hand open so he is able to control wheelchair with the joystick. His desire is to being able to scratch his head, move his hand forward and backward and keep gaming. He is afraid to go out alone, because he doesn’t feel secure. Dennis uses his robotic gripper for drinking and feeding tube. Dennis used the Armon arm support in the past, but this device quickly failed due to misuse.

3.4 User requirements

User requirements are extracted from the interviews, questionnaire results and interaction with potential users and professionals as described in the sections above. These are:

• Support remaining hand function of user during important activities of daily living including

flexion and extension.

• Using the device should be safe. It should not lead to dangerous situations for the patient or his surroundings.

• Reliable functioning of the device should be insured to support hand functioning, increase safety and maximize compliance.

• Easy donning/doffing will improve patient independence and increase their quality of life. • Wearing the device should be comfortable to maximize patient compliance.

• Aesthetics are important because a nice looking orthosis has a higher chance of being accepted by the user.

• The orthosis should be easy to use. Intuitive usage of the orthosis will increase the acceptance rate. • The orthosis should support the wrist.

• Because the orthosis will be worn all day, it should be lightweight and compact. • Low visual attention should be necessary to use the device.

• The device should be non-disturbing to its surroundings.

• Easy maintenance is essential for the patient, because then the downtime is minimized.

• Low costs are also functional characteristics of the device that cannot be ignored, because insurance companies normally only reimburse limited funds.

Seven users (6 professionals and 1 boy with DMD) are asked to divide the numbers from 1 to 15 among the selected needs, where 1 is the most important need and 15 is the least important need. Based on the user ranking several primary and secondary needs are identified, see Figure 3.3. The mean scores of all users are computed per need.

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General remarks about the differences found between professional users and patient: • The patient values reliability, compactness, weight, support and safety.

• Clinicians don’t value compactness and weight to the same extent. • Extension of fingers is less important than flexion of fingers.

During the first development cycle only the primary needs (indicated in red) are taken into account. Not included in this evaluation was the need that evaluates the ability to interact with the (active) mobile arm support. This will be considered in a later stage.

Figure 3.3: Mean scores computed after the ranking of each user requirement by the seven users with standard deviation. The lowest scores are highlighted in red (primary needs).

3.5 Conclusion

Based on the results of the interviews, questionnaire and focus group the user requirements for the hand orthosis were established. The focus points during the rest of the design should be on the primary needs of the users. These are: support flexion of fingers, support extension of fingers, safe, reliable, easy donning and doffing, comfortable, ease of use, wrist support and lightweight and compact. In the next chapter these user requirements will be converted into system requirements.

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Chapter 4

System requirements for a functional

hand orthosis

4.1 Introduction

The ability to perform a desired task using hands and fingers will decrease as the disease of Duchenne progresses. In order to regain functional performance of their hands users can benefit from a permanent assistive device.

The active orthosis that is developed in this project should assist finger movements and provide additional forces to interact with the environment. In this chapter user requirements are converted into system requirements. First a background study is presented in which the selection of ADL tasks of the previous chapter are converted into grasps and corresponding joint angles. Also, wrist motion during ADL tasks are determined. Then the comfort of hand orthosis is investigated. By relating comfort to physical and psychosocial properties the concept of comfort is further specified.

After this background studies a description of the system topology and a functional analysis is given. The user requirements are related to the system requirements. Several products aspects can have an influence on these system requirements. The last paragraph identifies the relations so during the development process choices on product level can directly be related to the user requirements.

4.2 Background

To be able to convert the user requirements into system requirements, the results of the task importance rating results from Chapter 2 are further analyzed. This is done in the grasp analysis below. Then a literature search for the useful range of motion of the wrist is presented. Also, properties of the system that relate to the comfort of hand orthosis are further investigated. Understanding of which properties contribute to comfort will help to further specify the system requirements.

4.2.1 Grasp analysis

From the user requirements analysis a selection of ADL tasks is made that the device should enable (or not impede if that activity is still possible). Performing important ADL tasks require certain movements to be executed by the user. Grasp types can be used to describe this hand use. In literature a distinction is made between power grasps and precision grasps. With a power grasp, the emphasis is on stability and security. A precision grasp is more focused on dexterity and sensitivity. [13] [14]

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For a functional hand orthosis the device should enable essential grasps in order to accomplish these ADL tasks. If too few grasps are permitted there might be too limited functionality. Too many grasps however can lead to unnecessary extra complexity of the hardware (or software).

Based on the grasp taxonomy by Feix [14] and the selection of most important tasks (see previous chapter) the grasp type of the dominant hand is determined. It should be noted that there are can be more grasps that can be used to execute one task. In our analysis we identified only one of the possible grasps.

For our first prototype we ignored tasks that involve water; Also, we only looked at the dominant hand (in case of a bimanual task). The selected grasps, see Figure 4.1 are Large diameter (A), Small diameter (B), Adducted thumb (C), Prismatic 2 finger (D), Precision disk (E), Writing tripod (F) and Parallel extension (G). For example the precision disk can be used to control the computer mouse, with a writing tripod grasp

the user is able to write.

Figure 4.1: The selected grasps for the hand orthosis. A) Large diameter B) Small diameter C) Adducted thumb, D) Prismatic 2 finger E) Precision disk F) Writing tripod G) Parallel extension

For each grasp that is selected the MCP, PIP and DIP angles of the index finger are measured with a goniometer. The index finger orientation for each grasp is displayed in Figure 4.2. The MCP angle (αmcp)

varies from 10 to 60◦ flexion, the PIP angle (αpip) from 10 to 70◦ flexion and the DIP angle (αdip) from 0 to

30◦. The DIP joint is only slightly flexed. Active flexion of the DIP joint might therefore not be necessary. The contribution of each digit to the total grip force decreases from radial to ulnar digits. Thumb contributes approximately 50% [15]. The maximum fingertip forces during ADL tasks found in literature range from 6.3N [16] to 10.5N [17]. In our research 6.4N was considered sufficient.

4.2.2 Wrist motion analysis

The wrist is considered a 2 DoF joint that allows flexion-extension and radial-ulnar deviation movements. Pronation-supination movements arise from the forearm. The minimal motion requirements during daily activities found in literature show a large variation. Accordint to Ryu et al. the useful wrist angles are 70% of the maximum range of motion [18]. According to studies by Palmer and Nelson, these angles can be lower (Table 4.1).

Actuation of the wrist is assumed to be necessary and for proper functioning of the hand orthosis during ADL tasks. The motion requirements of the wrist are determined to be 20◦ _{flexion and 20}◦_{extension. One}

should take into account that compensatory movements of the elbow and shoulder can make up for a limited ROM in the wrist.

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4.2. BACKGROUND 23

Figure 4.2: Index finger position during different grasps with the MCP joint as the origin. The finger segments are indicated with solid lines. For the first grasp the joint angles of the MCP, PIP and DIP joint are indicated.

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Table 4.1: Useful range of motion of the wrist according to studies of [19], [20] and [18]

Palmer Nelson Ryu Flexion 5◦ 28◦ 40◦ Extension 30◦ 37◦ 40◦ Radial deviation 10◦ 12◦ 10◦ Ulnar deviation 15◦ 27◦ 30◦

4.2.3 Comfort

One of the user requirements is related to comfort. Although research has shown that comfort of body-worn devices appears to influence the usage of these devices, most of the engineering effort is still put into the functionality of the device and less into comfort aspects. [21–23] Hand orthoses are capable of restoring lost hand function due to the degeneration of muscles. As these devices have to be worn all day, comfort is an important aspect.

Generally speaking, the term comfort is associated with feelings of wellbeing and happiness, whereas discomfort is the term used when talking about aspects that have a negative impact on the state of wellbeing such as pain, tiredness, soreness, numbness, etc. [24]. Vink et al. [25] combined the findings of ten papers into a new comfort model (Figure 4.3). The interaction (I) between person, task and environment causes internal human body effects (H). The perceived effects (P) are affected by expectations (E). The result of these effects can either be: comfort (C), not feeling anything (N) or feelings of discomfort (D). Discomfort can in its turn lead to musculoskeletal complaints (M). As can be seen from the model, comfort is not only related to physical properties of the system, but also affected by expectations. This is something that should be taken into account, as (dis)comfort may lead to lower wearing times of the orthosis thus decreasing the efficiency.

Figure 4.3: Comfort model of Vink

Many aspects of the device’s design influence comfort of Duchenne patients when a hand orthosis is used. Comfort descriptors related to orthotics/prosthetics include friction force, pressure points [26] and temperature [27–30]. Other descriptors may for example be: easy in use, fits the hand, task performance, reliable, styling/looks. [31] Temperature, pressure and friction are considered major contributors to orthosis comfort. In the orthosis design these descriptors should be taken into account.

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4.3. SYSTEM SPECIFICATIONS 25

4.3 System specifications

The organization of components in a system, or topology, is presented below and shows the functional parts. From the topology a functional analysis is created by establishing relations between the functional parts of the device. Each relation equals a function that the device should fulfill. For each functional part (also called system element) several aspects are considered important to accomplish the user requirements. These are listed in the paragraph system requirements.

4.3.1 System topology

The system consists of six main functional parts: force transmission, interface, actuation, control, sensing and electrical system. They are represented in the diagram of Figure 4.4. Each functional part can be further divided. The force transmission for example contains a finger and wrist mechanism. The interface consists of a hand interface, arm interface, finger interface and casing. In order to control the device, a control algorithm should be implemented on a microprocessor. The sensing part consists of a physiological signal sensor and kinematics and/or kinetics sensor. Lastly the electrical is made up of wire, a power supply and on/off switch. In this design assignment the focus will be on the force transmission mechanism of the finger, the hand interface, finger interface and the actuation.

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4.3.2 Functional analysis

In the functional view of Figure 4.5 the system elements of the topology are connected. Each arrow represents a function that the orthosis should fulfill.

The system interacts with the human (Fig. 4.5 orange box). The desired state of the user is measured by the physiological signal sensor. This information is transferred to the microprocessor. Based on the difference between the desired state and current state of the user the control algorithm computes the output signal that is then sent to the actuator.

The actuation mechanisms are attached to the hand and fingers. Forces are transmitted to the user through different interface in order to move the hand and fingers to the desired state.

Figure 4.5: Functional view of the system where the functional elements of the system (Figure 4.4) are connected by functions.

4.3.3 System requirements

Primary user requirements that are established in Chapter 2 are related to the device aspects. Requirements for each of these aspects are established. Characteristics of the functional system elements influence these device aspects. Two examples are given below.

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4.3. SYSTEM SPECIFICATIONS 27

Characteristics such as dimensions of the force transmission mechanisms (finger and wrist), interfaces, actuator and casing influence for example the device aspect height of the system. Which in turn adds to the user requirement compactness.

The user requirement safety relates to the system requirements maximum joint velocity, active ROM, maximum fingertip force, skin pressure and electrical safety, etc. Finger tip force for example can be linked to the product aspects: actuator nominal torque, type, operating voltage, and force transmission mechanism finger moment arm.

In this way a direct relation between functional elements of the system and user requirements is obtained. The most important system requirements and their values are listed in Table 4.2. If possible a reference is given.

Table 4.2: Main system requirements for the hand orthosis

Device aspect Requirement Reference

Kinematics Actuated DoF per finger 1 [32] Type of support Thumb, index, middle

finger

Grasp analysis

Wrist support

Active ROM fingers MCP F/E [60°, 10°] Grasp analysis PIP F/E [70°, 10°] Grasp analysis DIP F/E [0°, 0°] Grasp analysis Active ROM wrist F/E [20°, 20°] Wrist motion

analysis Accuracy (end-point error) < 1°

Forces Fingertip force > 6.3 N [16]

Geometry Height above fingers dorsal < 10 mm Height above hand dorsal < 15 mm Circumference arm < 20 mm

Surface restrictions Leave fingertip area free [33] Leave palm of hand free

Safety Fingertip force < 9.5 N 150% of max fingertip force Weight body mounted part < 300 g [34] [35] [36]

[37] Donning time < 2 min

Doffing time < 1 min Joint velocity < 30°/sec

Active ROM < 80% of passive ROM

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Chapter 5

Mechanical design of a functional

hand orthosis

5.1 Introduction

Duchenne muscular dystrophy (DMD) is a neurological disorder mostly affecting boys and causes a gradually loss of muscle strength starting at the lower extremities. Due to this muscle weakness support of the fingers (and wrist) during activities of daily living (ADL) tasks is necessary.

A device that will support grasping and other activities while being as unobtrusive and lightweight as possible, may significantly improve the quality of life of the users. A functional hand orthosis is developed that helps boys with DMD during important ADL tasks.

5.1.1 Current solutions

Precise control of many DoFs or joint torques to support finger movements leads to complex and bulky designs. Such devices are therefore mainly used for rehabilitation purposes or virtual reality and haptic feedback. ([39] [40] [41] [42])

Since tight constraints are placed upon the device, for instance in terms of size and weight, it is essential to find a good balance between complexity of the hardware and functional performance. Under-actuation is a solution to this problem.

In recent years many devices have been developed that did not follow the approach of traditional exoskeletons with numerous rigid mechanical parts, see Figure 5.1 for examples. They typically consist of glove-like designs using cables with remote actuation [37] [43] [44] or soft pneumatics [45] [46] [47] [48]. Their advantages are numerous: increased compliance (beneficial for human-robot interaction safety), a larger number of DoF (although not actively controlled) and lower material and manufacturing costs.

Under-actuation is used in the Soft Extra Muscle (SEM) glove from Bioservo [37] (Figure 5.1, middle). Thin cables inside the seams of the glove allow for flexion movements when tensioned. Actuators are remotely placed and control the Bowden cables. Fabric around the fingertip decreases the sensation during object manipulation and cable forces result in high compressive forces on the joint surfaces. Also, the cables run through the palmar area of the hand which might compromise gross grip.

High compressive forces in the finger joints are avoided by using soft actuators at the dorsal side of the hand as in the Soft robotic glove [45] or Power Assist Glove [48]. These actuators flex the finger when their elastomeric chambers are inflated, (Figure 5.2). More space is available at the dorsal side of the hand, and the palmar area is not restricted. The amount of force that can be applied however is limited due to the

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Figure 5.1: Overview of three alternative designs. Left: Soft robotic glove [45], middle: SEM glove [37], right: Three-layered sliding spring mechanism [49]

configuration of their soft actuator. In order to flex the elastomeric material has to expand. A high stiffness of the walls leads to a low force transmission ratio, but a low wall stiffness compromises the maximum force.

Figure 5.2: Working principle soft robotic glove [45]

Instead of air pressure or cables also leaf springs can be used to transfer forces to the fingers [34] [49]. A three-layered sliding leaf spring configuration is installed for this purpose (Figure 5.3). The fingertip is flexed when the middle leaf spring is pushed distally with push-pull Bowden cables. The inner and outer leaf springs keep the relative position of the sliders and make sure the middle leaf spring is able to transmit forces. This mechanism has a low profile but is unable to convey high forces. Other disadvantages include the fixation of the lower leaf spring. When the fingers are flexed, this structure prevents further flexion as the knuckles touch the spring.

5.2 Design approach

As the literature search above displayed, several design approaches can be followed. The system consists of several subsystems as was explained in Chapter 3.4. For the subsystems force transmission and actuation several design approaches are investigated and listed in the paragraphs below. The options are shown together with their main advantages and disadvantages. Possible improvements are mentioned for each solution. Based on the investigation a decision is made. The system overview is presented in paragraph 4.3 below.

5.2.1 Subsystem force transmission finger

Different force transmission mechanisms are investigated. These mechanisms should be able to apply forces to the finger to flex and extend them. For flexion the maximum force that should be applied to the fingertip is 6.4 N. For extension less force is needed. The weight should be lower than 300 gr. and the height above the fingers <10 mm. Also shear force applied to the skin are of great importance.

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5.2. DESIGN APPROACH 31

Figure 5.3: Working principle three-layered spring mechanism [34]

The mechanisms that are selected are: Bowden cable, push-pull cables in different versions, hydraulics and linkages (see Figure ??). Based on previous experience, literature investigations and experimental work the advantages and disadvantages of the selected systems are given in Table 5.1. Also their potential for improvement (to meet our requirements) is listed below followed by the conclusion.

Table 5.1: Advantages and disadvantages of the selected force transmission mechanisms for the finger

Type of force

transmission Advantages Disadvantages Bowden cable Limited number of parts Uni-directional

Low profile on the dorsal side of the hand

High shear forces (force direction)

No joint alignment needed High cable forces Low weight of force transmission

mechanism

Cables running through palmar area

Remote actuation possible Maintain cable tension Creep

Push-pull cable Limited number of parts Large cable forces

No joint alignment needed Large shear forces (force direction) Easy maintenance Maintain cable tension

Low profile dorsal side hand Creep Low weight of force transmission

mechanism

Balance between load efficiency and play

Remote actuation possible Compensation for finger length change needed

Bi-directional Push-pull mechanism

(sliding spring)

Low profile Should be placed exactly above remote center of rotation finger joint Low weight No finger length change

compensation

Bi-directional (act-act) Shear forces on finger segments Push-pull mechanism

(monolithic)

Compact Relatively large profile in extended position

Lightweight Large shear force on metacarpals Safe Imposes fingertip trajectory

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Type of force

transmission Advantages Disadvantages No backlash Limited rotation

No lubrication required Dependence on material properties Easy fabrication and assembly Nonlinear motions

Bi-directional Difficult design Reduced friction

Reduced wear

Hydraulic Low profile Limited ad/abduction stiffness Safe Rotation and length change

compensation not at same location: unwanted shear forces

Limited number of interfaces with finger

High pressures needed (~3.5 bar to get 8N at fingertip)

Remote actuation possible Uni-directional (if not used vacuum) Lightweight (if used with air) Unable to adapt to different joint

stiffness

Compliant Difficult to control position Linkage Perpendicular force on the fingertip Size

Weight Backlash Play

Linkage center of rotation has to be aligned with joint axis

Figure 5.4: A) Bowden cable force transmission systems B) Push-pull (cable) force transmission system C) Push-pull (monolithic) force transmission system D) Push-pull (sliding spring) force transmission system E) Hydraulic force transmission system F) Linkage force transmission system

Bowden cables are a simple solution, but can only pull. One possible improvement is to add a (passive) elastic element for bi-directional usage. Also, decreasing the friction of the cable inside the cable housing

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increases efficiency of force transmission. By increasing the moment arm the torque needed to rotate the joints is decreased. Compliance can be added by using a series elastic element. High compressive forces on the finger joints are always present with this solution. Also the fingertip will always be covered thus lowering fingertip sensation.

Push-pull cables can be optimized by tweaking the cable configuration with respect to the cable housing. In this way the right balance between friction and play can be found. In either case the force transmission efficiency stays very limited. Series elastic element can be added for compliance.

With the sliding spring push-pull mechanisms the center of rotation is fixed and there is no compensation for the surface change of the finger. If a system is mounted on the dorsal side of the finger and pushed to cause flexion, the surface length changes. If the mechanism follows the contour of the skin it needs to elongate, otherwise it will touch the skin and limits further flexion. One possible improvement would be to compensate for this length change (Figure ??).

Figure 5.5: Finger surface length change when moving from extended position (red box) to a flexed position (black dashed box). The surface length (blue line) changes. Mechanism mounted on dorsal side of the hand (green line). The force application point (arrow) changes. ??

The monolithic variant of the push-pull mechanism can be improved by increasing the range of motion for example by using a deployable structure and converting this with the pseudo-rigid body modeling approach [50] into a monolithic structure.

The hydraulic force transmission system consisting of soft fiber-reinforced actuators can benefit from improving the efficiency such that the operating pressure can be lowered. Also a compensation for the surface length change when operated from the dorsal side is advantageous.

For the hand orthosis the sliding spring push-pull mechanism was considered the solution with the most potential and was selected for further investigation.

5.2.2 Subsystem interface

If forces are distributed over a larger area, the pressure on the skin is lowered. The surface area of the interfaces should therefore be as large as possible without interfering with the range of motion. Also the best location for the device-skin interfaces should be considered. Due to the composition of the tissues below the skin, some locations are more suitable for force application than other. For example the dorsal surface of the hand consists of thinner skin with less subcutaneous tissue than the palmar surface. It is therefore not able to withstand much compression force. However, the volar surface of the arm is able to tolerate high loads, because the muscles below the skin lower the peak pressures drastically.

Besides the location, also the direction of force application is of interest. In general, normal forces are better tolerated than shear forces. Shear stress occurs with joint movement, changes due to muscle contraction but also when the orthosis shifts due to movement.

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Several options are investigated to distribute the pressure and cause the least interference with the joint movements of the wrist and fingers. A few of them are shown in Figure 5.6 below.

Figure 5.6: Splint designs A-D

Design A consists of a custom-made splint that is clamped around the hand between the thumb and index finger, see Figure 5.6. The splint extends beyond the wrist, supporting this joint. Design B is another option where the hand is secured to the splint by a Velcro strap. Elastic bandages have the potential to apply high amounts of stress and may lead to constriction in the vascular and lymphatic circulation [51]. The straps around this part of the hand do not adversely affect the user.

Since the transverse arc of the hand is different when the hand is in open or closed position, see Figure 5.7, and the plastic shell does not adapt to this shape, this may feel uncomfortable. A solution is to leave the medial dorsal and palmar side of the hand uncovered, as can be seen in Design C (Figure 5.6).

In Design C no space is left for the thumb actuation slider module. Therefore Design D is considered a better alternative. It should be noted that in the final design of the orthosis the wrist support should be actuated. For now only a passive support is considered.

5.2.3 Subsystem actuation

The main requirements of the actuator are that it should be able to apply a force between 10 and 50N (depending on the efficiency of the force transmission mechanism of the hand) and able to translate the sliding spring over a distance (s) of approximately 30 mm. The single stroke time (t) is approximately 3

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Figure 5.7: The transverse arc of the hand (indicated with black line above the hand) in open and closed position. Image from [52]

sec. According to actuator selection tools such as found in [53] and [54], a number of actuators can achieve the desired stroke and output force range. These are: Hydraulic and pneumatic (cylinder or soft actuator), electric/magnetic actuators and electro-active polymers.

The most frequently used actuators in hand orthotics and prosthetics include mechanical actuators (e.g. hy-draulics or pneumatics), and electric/ magnetic actuators (e.g. electromagnetic or piezoelectric) [3]. For these actuators an overview with their advantages and disadvantages is presented below.

Hydraulic and pneumatic systems are able to generate high forces, but are relatively complex, heavy and noisy. If cylinders are used they may suffer from fluid leakage caused by seals. Also the working pressure can become very high in order to achieve the desired forces and special compressors are needed for power.

Electric or magnetic actuators are reliable, easy to control and have a high work capacity. The disadvantages are the high impedance and many moving parts that are prone to wear.

5.2.3.1 Remote actuation transmission system

To enable remote actuation of the force transmission mechanism of the hand, another transmission system is required depending on the choice of the actuator.

A simple, lightweight and practical solution is to use Bowden cables as transmission system between the force transmission system of the hand and the actuator. Inside a flexible housing the cable tension is controlled by a remote actuator. Similar options are found in literature [37] [43]. Disadvantages include a high static friction, wear and non-linear behavior [55].

Hydraulic and pneumatic cylinders have to deal with the trade-off between either high leakage or seal friction. Furthermore, these systems are often operated by a pressure source and valves. As a result they are non-back drivable. They also require a lot of space on the back of the hand especially when combined with the force transmission mechanism of the hand.

Another solution would be to use a closed hydraulic system with rolling diaphragm cylinders, see Figure 5.8 [56]. Low pressure seals (limit approximately 10 bar) are used to enclose the volume of fluid. Rolling diaphragms suffer from lower friction forces and fluid leakage is decreased. The solution requires small cylinders to be mounted on the dorsal surface of the hand that are connected to a remote actuator through flexible hoses. With a supply pressure of 3 bar and a cylinder diameter of 8 mm an output force of 15N can be achieved in theory. In practice, however, miniaturization increases the stress on the seal of the rolling diaphragms. Besides, the rigid structures add complexity to the system and increase the size. Also, an electric/magnetic motor is still needed to move the remote piston.

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Figure 5.8: Rolling diaphragm cylinder by Whitney et al. [56]

5.2.3.2 Conclusion

Electric actuators will be used in the orthosis design. They are able to achieve the desired stroke and force and easy to implement. It would be desirable to use one power source to actuate all equipment of the wheelchair. Since electric wheelchairs already have a battery on board, the electric actuators of the hand orthosis can be connected to this power source. Bowden cables are used to convert the motion of the electric actuators into a motion of the force transmission mechanism of the hand as they are lightweight.

5.3 System design

In this research an assistive device for the hand is developed. An overview of the system is presented in Figure 5.9.

The under-actuated mechanism is able to flex the finger of the user by a novel tape spring mechanism operated by cable tension. It is decided to place the force transmission mechanism at the dorsal side of the hand, since it does not obstruct object manipulation while enabling finger tip sensation. This provides a clear advantage over glove-like devices. Two sliders, connected to the hand and the proximal segment, serve as a guide for the tape spring. The tape spring is rigidly connected to the end block which is attached to the middle segment of the finger. Contrary to the design of [34] more force can be applied to the finger. Also, larger flexion angles are possible.

Flexion of three fingers (thumb, index and middle finger) is supported by three actuators that are remotely placed. Extension of the fingers is passively supported by constant force springs. The mechanism is able to move the fingers through a large range of motion.

Key features of the design are summarized below.

• The device consists of a low-profile and lightweight force transmission mechanism. A tape spring and two sliders flex the finger by pushing on the middle phalanx from the dorsal side.

• Bowden cables control the position of the tape spring mechanism.

• The slider on the dorsal surface of the hand is connected to a cable that pulls the tape spring distally. By controlling the cable force the position of the tape spring is adjusted.

• The end-block, connected to the middle phalanx, is extended beyond the DIP joint to prevent hyper-extension.

• Constant force springs with a low stiffness provide extension of the fingers if the tension of the Bowden cable is released. The extension force is only needed for returning the fingers to a neutral position. • The mechanism is controlled through motors that are placed away from the hand, thus reducing the

weight on the hand.

• The interface between hand and device is equipped with soft liner material in order to reduce the pressure.

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5.3. SYSTEM DESIGN 37

Figure 5.9: Overview of the hand orthosis

• Soft Velcro straps allow easy donning of the device.

• Flexion and extension movements of the wrist are actively supported by the orthosis. Without wrist support no functional hand movement can be established.

In the paragraphs below all features are explained in further detail.

5.3.1 Force transmission mechanism finger

It was decided to use a Bowden cable force transmission to allow for remote actuation. This requires another force transmission mechanism on top of the hand to convert the pulling force from the cables into a pushing force that flexes the finger.

Figure 5.10: Two-link serial chain representing the finger with the force transmission mechanism mounted on top

A schematical representation of the hand with the force transmission mechanism is presented in Figure 5.10. In this picture the metacarpal (MCP) joint, proximal interphalangeal (PIP) joint, and distal interphalangeal (DIP) joint are indicated. The tape spring is rigidly connected to the end block.

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A close-up of the force transmission mechanism is seen in Figure 5.11. Here the different elements of the mechanism are highlighted. In the paragraphs below all of these elements will be further explained.

Figure 5.11: Close-up of the force transmission mechanism mounted on top of the fingers.

5.3.1.1 Tape springs

Tape springs are thin metallic strips with a curved cross-section, called transverse curvature. In Figure 5.12 a typical configuration is depicted, consisting of a cylindrical shell of uniform thickness t and transverse radius of curvature R, subtending an angle α.

Transverse curvature: The transverse curvature all along its length is created during the manufacturing

process by heat-treating spring steel while held in a concavo-convex shape. It gives the deployed tape a longitudinal structural stiffness that enables it to maintain the straightness.

Longitudinal curvature: When bending the tape spring a longitudinal curvature is imposed. θ is the

relative rotation between the ends of the tape spring.

Figure 5.12: Tape spring indicating transverse and curvatures

Two types of bending are distinguished: opposite-sense and equal-sense bending (see Figure 5.13). Opposite-sense bending (bending moment M > 0) occurs if the longitudinal and transverse curvatures are in the

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5.3. SYSTEM DESIGN 39

opposite sense. Equal-sense bending (M < 0) induces longitudinal and transverse curvatures that are in the same sense.

Figure 5.13: Opposite-sense (top) and equal-sense (bottom) bending of tape spring

The deployment dynamics of tape springs are described by Seffen et al. [57]. In this research the moment-angle relationship of tape springs was empirically determined and a model was derived. For small angles, there is a linear relationship between angle θ and the moment M. For larger angles the behavior depends on the direction of bending: If the moment M is positive the cross-section flattens, after which the center snaps (from A to B, Figure 5.14). Beyond this peak, the moment M₊∗ remains approximately constant (B to C). If the moment is negative, at point F asymmetric torsional folds appear near the ends due to a flexural-torsional deformation mode. From F to G the amplitude of these folds increase. Beyond point G the moment M₋∗ remains approximately constant.

It can be seen from Figure 5.14 that the maximum positive bending moment is larger than the maximum negative bending moment. This feature is deployed in the force transmission mechanism of the hand orthosis. The energy needed to bend the tape spring in opposite-sense is higher, therefore high forces can be transmitted without a chance of buckling. In Figure 5.15 buckling of the tape spring has occurred (dashed line). In green the dangerous area is shown. This is where buckling might occur. Then a negative curvature is visible (green line). But the shape of the tape spring can prevent this if the opposite-sense bending moment is below the peak moment Mmax

+ .

5.3.1.2 Rollers

In the force transmission mechanism equal-sense bending of the tape spring is deployed, which is difficult to control as the moment is non-linear when moving from a neutral position to a flexed position. The moment peaks F and A (Figure 5.14) can be removed by flattening the tape spring. The forces that are applied are indicated in Figure 5.16.

The flattened tape spring in theory also allows for opposite-sense bending, as both peaks are removed. However, because the tape spring is attached to a jointed structure (the finger) the tape spring is forced to follow one direction of bending which is equal-sense bending. Flattening of the tape spring therefore has no influence on the chance of buckling.

Two miniature ball bearings, 1.5mm ID, 4 mm OD, 1.2 mm width, (S681X, Neita Techniek B.V.) are mounted alongside the tape spring. A third miniature ball bearing, 1.5 mm ID, 4 mm OD, 2 mm width, (681XZZ, Neita Techniek B.V.) is mounted above the tape spring. The housing is constructed from one piece of 3D-printed PLA, to reduce the risk of failure and simplify the assembly. A close-up of the roller configuration is shown in Figure 5.17.

The total height of the mechanism is 8.5 mm and the width is 17 mm. The clearance is sufficient to prevent the roller and skin from touching.

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Figure 5.14: Moment-angle relationship of tape spring [57]

Figure 5.15: Buckling of tape spring indicated with the red dashed line. The green line shows the negative curvature of the tape spring.

Development of a functional hand orthosis for boys with Duchenne muscular dystrophy

DEVELOPMENT OF A FUNCTIONAL HAND ORTHOSIS

FOR BOYS WITH DUCHENNE MUSCULAR DYSTROPHY

CLAUDIA HAARMAN

(BW-PDENG-002)

SEPTEMBER 30, 2016

EXAM COMMITTEE

prof.dr.ir. H.F.J.M. Koopman

ir. E.E.G. Hekman

dr.ir. T.H.J. Vaneker

dr.ir. D. Lutters

F. Tönis

Development of a functional hand orthosis for boys with Duchenne

Muscular Dystrophy

Contents

Chapter 1

Summary

Chapter 2

General introduction

2.1

Duchenne Muscular Dystrophy

2.2

Wearable robotics

2.3

Project description

2.4

Problem definition

2.4.1

Goal

2.5

Background

2.5.1

State of the art

2.5.2

Target group

2.5.3

Stakeholders

2.5.4

Hand and finger anatomy and kinematics

2.6

Report outline

Chapter 3

User requirements for a functional

hand orthosis

3.1

Background

3.2

Methods

3.2.1

Exploratory study with users: Interview and observational study

3.2.2

Questionnaire

3.2.3

Focus group meeting

3.3

Results

3.3.1

Exploratory study (Interview)

3.3.2

Exploratory study (Interview)

3.3.3

Questionnaire

3.3.4

Focus group meeting

3.4

User requirements

3.5

Conclusion

Chapter 4

System requirements for a functional

hand orthosis

4.1

Introduction

4.2

Background

4.2.1

Grasp analysis

4.2.2

Wrist motion analysis

4.2.3