Viability Study of Nylon-12 Carbon Fiber Filaments for Use ...

APPROVED: Vijay Vaidyanathan, Major Professor and

Chair of Department of Biomedical Engineering

Melanie Ecker, Committee Member Yong Yang, Committee Member Hanchen Huang, Dean of the College of

Engineering Victor Prybutok, Dean of Toulouse Graduate

School

VIABILITY STUDY OF NYLON-12 CARBON FIBER FILAMENTS FOR USE IN THE CONSTRUCTION

OF A POWERED LOWER BODY EXOSKELETON VIA FUSED DEPOSITION

MODELING BY MEANS OF COMPUTER SIMULATION

Michael Andrew Lown Joiner

Thesis Prepared for the Degree of

MASTER OF SCIENCE

UNIVERSITY OF NORTH TEXAS

May 2021

Joiner, Michael Andrew Lown. Viability Study of Nylon-12 Carbon Fiber Filaments for Use

in the Construction of a Powered Lower Body Exoskeleton via Fused Deposition Modeling by

Means of Computer Simulation. Master of Science (Biomedical Engineering), May 2021, 79 pp.,

14 tables, 31 figures, 2 appendices, 52 numbered references.

Members of the elderly population is disproportionately prone to experiencing mobility

impairment due to their aging bodies and as a result have frail bodies that are at a higher risk of

grave injury due to falling. In order to combat this assistive mobility devices such as

exoskeletons have been developed to help patients enhance their range of motion. With

additive manufacturing techniques, such as fused deposition modeling (FDM), becoming a more

mainstream form of design, the inclusion of lightweight polymers such as nylon 12 as primary

construction materials for these devices has increased. In this thesis computer aided design

(CAD) software was used to design a prototype lower body exoskeleton and simulation

software was used to give the device the characteristics of Stratasys’ nylon 12 carbon fiber FDM

material to verify it if could be used as the primary construction material for this device when

extruded from a FDM printer on either the XZ or ZX printing plane. From the simulations it was

found that the material printed along the XZ plane could create a device that could withstand

the weight of an average elderly male patient (200 lbs.) as well as the 35 lbs. of force applied to

the device by a linear actuation motor that would be used to extend and contract the

exoskeleton leg.

ii

Copyright 2021

by

Michael Andrew Lown Joiner

iii

ACKNOWLEDGEMENTS

I would like to give thanks to Dr. Vijay Vaidyanathan for being my committee chair and

for all of his unwavering support and guidance that he has shown me throughout both my

undergraduate and graduate careers. Thank you to Dr. Melanie Ecker for helping to guide me

through my research on polymeric materials and additive manufacturing as well as for her

support in my graduate studies. Thank you to Dr. Yong Yang for his support and insight

throughout this thesis’s process.

Additionally, I would like to thank Edward Gates for believing in my abilities and allowing

me to join him in his research and development of a novel lightweight lower body exoskeleton.

Lastly, I would like to thank my friends and family for all of the support and love they have

shown me throughout this process and my academic career.

If not for these individuals, along with many others, I would not be where I am today.

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TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ................................................................................................................... iii LIST OF TABLES ................................................................................................................................ vi LIST OF FIGURES ............................................................................................................................. vii CHAPTER 1. INTRODUCTION ........................................................................................................... 1 CHAPTER 2. BACKGROUND ............................................................................................................. 2

2.1 Mobility Impairment within the Elderly ................................................................. 2

2.2 Fused Deposition Modeling .................................................................................... 3

2.3 Nylon-12 Carbon Fiber Filaments ........................................................................... 6

2.4 Medical Exoskeletons ............................................................................................. 8 CHAPTER 3. DEVICE DESIGNS AND METHODOLOGIES ................................................................. 11

3.1 Design of Exoskeleton ........................................................................................... 11

3.1.1 Design of the Thigh ................................................................................... 12

3.1.2 Design of the Ankle ................................................................................... 13

3.1.3 Design of the Knee .................................................................................... 14

3.2 Force Modeling Simulations ................................................................................. 18

3.2.1 Force Modeling of the Thigh ..................................................................... 19

3.2.2 Force Modeling of the Ankle..................................................................... 21

3.2.3 Force Modeling of the Knee...................................................................... 23

3.2.4 Force Modeling of Leg .............................................................................. 28 CHAPTER 4. RESULTS AND DISCUSSION........................................................................................ 36

4.1 Simulation Results ................................................................................................. 36

4.1.1 Thigh Simulation Results ........................................................................... 36

4.1.2 Ankle Simulation Results ........................................................................... 38

4.1.3 Knee Simulation Results ............................................................................ 40

4.1.4 Leg Simulation Results .............................................................................. 46

4.2 Discussion.............................................................................................................. 52

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CHAPTER 5. CONCLUSIONS ........................................................................................................... 55

5.1 Experimental Conclusion ...................................................................................... 55

5.2 Future Avenues of Research ................................................................................. 56 APPENDIX A. ENGINEERING DRAWINGS OF EXOSKELETON ......................................................... 58 APPENDIX B. MATERIAL DATA SHEETS ......................................................................................... 69 REFERENCES .................................................................................................................................. 75

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LIST OF TABLES

Page

Table 3.1: Force Elements Applied to Exoskeleton ...................................................................... 19

Table 3.2: Mechanical Properties of Simulated Materials............................................................ 19

Table 3.3: Thigh Simulation Parameters ....................................................................................... 21

Table 3.4Ankle Simulation Parameters ......................................................................................... 22

Table 3.5: PO Knee Simulation Parameters .................................................................................. 25

Table 3.6: PC Knee Simulation Parameters................................................................................... 27

Table 3.7: PO Leg Simulation Parameters ..................................................................................... 29

Table 3.8: PC Leg Simulation Parameters ..................................................................................... 33

Table 4.1: Cumulative Results of Thigh Static Force Simulations ................................................. 38

Table 4.2: Cumulative Results of Ankle Static Force Simulations ................................................. 38

Table 4.3: Cumulative Results of PO Knee Static Force Simulations ............................................ 43

Table 4.4: Cumulative Results of PC Knee Static Force Simulations ............................................. 46

Table 4.5: Cumulative Results of PO Leg Static Force Simulations ............................................... 49

Table 4.6: Cumulative Results of PC Leg Static Force Simulations ............................................... 49

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LIST OF FIGURES

Page

Figure 2.1: FDM Simplification ........................................................................................................ 4

Figure 2.2: Layer Orientation and Force Direction ......................................................................... 5

Figure 2.3: Lattice Structures .......................................................................................................... 6

Figure 2.4: Molecular Structure of Nylon ....................................................................................... 7

Figure 2.5: Lightweight Hip Exoskeleton that Utilizes Compact Actuators for Movement.[Images courtesy of Giovacchini et al. [45]] ............................................................................................... 10

Figure 3.1: Full Exoskeleton Leg with Posterior Offset Knee ........................................................ 11

Figure 3.2: Full Exoskeleton Leg with Polycentric Knee ................................................................ 12

Figure 3.3: Exoskeleton Thigh ....................................................................................................... 13

Figure 3.4: Exoskeleton Ankle ....................................................................................................... 14

Figure 3.5: Posterior Offset Knee .................................................................................................. 16

Figure 3.6: Polycentric Knee ......................................................................................................... 17

Figure 4.1: Stress Plots for N12CF Thigh Printed on XZ Axis (A)Front view, (B) Right view, (C) Rear view, (D) Left view......................................................................................................................... 37

Figure 4.2: Stress Plots for N12CF Thigh Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left side view ......................................................................................................... 37

Figure 4.3: Stress Plots for N12CF Ankle Printed on XZ Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 39

Figure 4.4: Ankle Stress Results for ZX Printing Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view .................................................................................................................................. 39

Figure 4.5: Stress Plot for Ti6Al4V PO Knee with N12CF Thigh and Shin Printed on XZ Axis ....... 41

Figure 4.6: Stress Plot for Ti6Al4V PO Knee with N12CF Thigh and Shin Printed on ZX Axis ....... 41

Figure 4.7: Stress Plots for N12CF PO Knee Printed on XZ Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 42

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Figure 4.8: Stress Plot for N12CF PO Knee Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 42

Figure 4.9: Stress Plot for Ti6Al4V PC Knee with N12CF Thigh and Shin Printed on XZ Axis ........ 44

Figure 4.10: Stress Plot for Ti6Al4V PC Knee with N12CF Thigh and Shin Printed on ZX Axis ...... 44

Figure 4.11: PC Knee all parts N12CF printed on XZ axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view......................................................................................................................... 45

Figure 4.12: PC Knee all parts N12CF printed on ZX axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view......................................................................................................................... 45

Figure 4.13: Stress Plot for N12CF PO Leg printed on XZ Axis with Ti6Al4V Knee (A) Front view, (B), Right view, (C) Rear view, (D) Left view ................................................................................. 47

Figure 4.14: Stress Plot for N12CF PO Leg printed on ZX Axis with Ti6Al4V Knee (A) Front view, (B), Right view, (C) Rear view, (D) Left view ................................................................................. 47

Figure 4.15: Stress Plot for N12CF PO Leg printed on XZ axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 48

Figure 4.16: Stress Plot for N12CF PO Leg Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 48

Figure 4.17: Stress Plot for N12CF PC Leg Printed on XZ Axis with Ti6Al4V Knee (A) Front view, (B), Right view, (C) Left view ......................................................................................................... 50

Figure 4.18: Stress Plot for N12CF PC Leg printed on ZX Printing Axis with Ti6Al4V Knee (A) Front view, (B), Right view, (C) Left side view .............................................................................. 50

Figure 4.19: Stress Plot for N12CF PC Leg printed on XZ Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 51

Figure 4.20: Stress Plot for N12CF PC Leg Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 51

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CHAPTER 1

INTRODUCTION

The purpose of this thesis is to determine the viability of Nylon-12 carbon fiber

filaments as a material to be used in the construction of a novel powered lower limb

exoskeleton prototype. To conduct this study, SolidWorks by Dassault Systèmes was utilized for

both the design and static force simulations. The material chosen to be simulated for this

project was Stratasys’ FDM N12CF filament as it is one of the few forms of this material

available on the market. A unique characteristic of the material is that it has two different

tensile and compression moduli depending upon the axis on which it is extruded. As such, two

force simulations were conducted on the various parts of the devices to see if they could be

constructed using both forms of the material.

The aim of this thesis is to expand the knowledge of exoskeleton technology and nylon-

12 carbon fiber material by answering the following questions:

1) Can nylon-12 carbon fiber filaments be used to construct load bearing components of a powered lower body exoskeleton?

2) Can the device constructed be used by an elderly patient of an average weight of 200 lbs.?

2

CHAPTER 2

BACKGROUND

2.1 Mobility Impairment within the Elderly

The elderly population is one of the fastest growing groups in America; because of this,

new, cost-effective caretaking methodologies must be developed in order to treat many

ailments that the elderly face [1]. According to a 2018 survey conducted by the CDC, 15% of

Americans have some sort of impairment that affects their mobility with a majority of reported

cases affecting seniors (aged 65+) [2]. Disruption of gait within the elderly population is a

common occurrence attributed to deterioration of the brain and the musculoskeletal system

causing the body to become frailer. This increase in frailty has been associated with decreases

in overall gait speed for elderly patients and an increase in fall occurrences while walking [3-5].

Falls have been found to result in intense physical trauma for elderly patients that have been

admitted to hospitals and account for a large percentage of deaths resulting from unintentional

injury [6-9].

To reduce the instances of injury caused by falling, many types of assistive mobility

devices (AMDs) have been developed, including canes and wheelchairs. In recent years,

research focusing on medical exoskeletons as AMDs has come to the forefront of fields such as

biomechanics and rehabilitation therapy. Unfortunately, such a device is expensive to

manufacture and use of heavy materials and bulky movement actuators can limit a patient’s

movement and overall time using it [10,11]. As such, researching more cost-effective

manufacturing methodologies and the incorporation of sturdy, lightweight materials are

essential for furthering the development of assistive mobility exoskeletons.

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2.2 Fused Deposition Modeling

Additive manufacturing (AM) is a manufacturing technique that utilizes computer aided

design (CAD) software in order to construct devices by adding specific material(s) layer by layer.

AM has three main processing techniques, liquid-based, solid-based, and powder-based. Each

of the processes have different AM techniques in order to fuse the materials together. For

liquid processes, such as UV lithography, an external energy source is used to polymerize a

reservoir of liquid polymer resin to produce a final solid object. Solid-based techniques, such as

fused deposition modeling (FDM), utilize either pre-polymerized resin or soft metal materials

that are heated up and extruded on to the printing plane, whereas powder-based techniques

usually utilize a form of laser sintering in order to melt the materials together. The rise of AM

within recent years has resulted in its use in industries such as aerospace, automotive, and

medicine as it allows for quick prototyping and the use of complex geometric designs [12,13].

FDM, shown in Figure 2.1, is a 3D printing technique in which a continuous polymer

filament is melted down, passed through an extruder head, and deposited onto a surface in

order to generate a structure. Unlike other AM methods, there is no outside light source

(UV/laser) that is needed in order for the filament to fuse to itself. Advantages of FDM include

compact machine size, low maintenance cost, and wide variety of available materials [14].

While widely used by both large companies and hobbyists, this method does not currently have

an international standard set by ISO, so the quality of a product is measured by the individual

manufacturing device to ensure that the product meets customer requirements. As with all

forms of AM, FDM is limited in what can be produced. Due to the compact size of most

machines, FDM printers are not able to manufacture large complex components. As such, it is

4

important to scale the parts that are to be manufactured in order to ensure that it is within the

parameters of the printer [15].

Figure 2.1: FDM Simplification

FDM is able to utilize a number of different material types, including soft metals and

ceramics, but this paper will focus primarily on how it can be used to manufacture polymer

structures. Polymers make up a majority of the materials consumed in AM. Due to their low

cost and light weight, they are ideal for prototyping and final production depending on the

application of the device. Some of the polymers that are widely used in FDM include: polylactic

acid (PLA), acrylonitrile butadiene styrene (ABS), and nylon. When printing using polymers, it is

vital that there is sufficient bonding between the layers of the polymer or the device is likely to

fail. A number of factors can affect the bond between the layers, including: the temperature of

the printing environment, fabrication pattern, layer thickness, filament diameter, printing

orientation, and the rate at which the polymer cools after extrusion [16,17]. Of the mentioned

factors, print orientation and the pattern used to print a part’s structure are particularly

5

important as they can determine how overall forces affect the structure of the manufactured

item. Figure 2.2 demonstrates how forces usually interact with a printed part. When the layers

are perpendicular to an applied tension force, the structural failure is more likely to occur

because of delamination as the bonds between layers are points where high amounts of stress

accumulate. Whereas when the layers are oriented in a direction parallel to an applied tension

force, that force will need to apply enough energy to break the bonds of the polymer chains in

order for mechanical failure to occur.

Figure 2.2: Layer Orientation and Force Direction

The pattern in which a part is printed can also affect how forces are distributed within

an object as they can determine the overall structure of the part as well as the amount of

material that is used for construction. The introduction of intricate structures such as lattices,

shown in Figure 2.3, allow for an object to be as hollow as possible to reduce overall weight

while also allowing for better flexibility within the structure. The various walls of a lattice

structure cause an incoming force to dissipate and be distributed amongst themselves reducing

the maximum stresses that can be applied to a singular point within a part. Size and density are

6

key to the effectiveness of this style of structure as smaller lattices allow for more flexibility and

a larger number allows for a greater dispersion of force throughout the device.

Figure 2.3: Lattice Structures

Other aspects that have been found to affect the mechanical properties of a polymer

include the type of filler that is mixed with the polymer to form the filament, the ratio of filler

to polymer, and the direction of the filler that is placed in when the filament is extruded [18-

21]. When introducing a filler material into a polymer matrix, it is important to understand the

ways in which the two components interact with each other chemically as this can affect the

bond strength between them.

2.3 Nylon-12 Carbon Fiber Filaments

Nylons are a family of synthetic thermoplastic polymers composed of a chain of carbon

atoms that are linked together with amide links (chains linked together by NH groups). These

polymers are heavily used in manufacturing of plastic shells and textiles due to their tensile

strength and elastic properties. Nylon-12, also known as polyamide-12, is a 12-carbon chain

nylon polymer, as shown in Figure 2.4, that is produced either from the polycondensation of ω-

7

aminolauric acid or from ring opening polymerization of laurolactam monomer rings [22]. In

comparison to other nylon compounds such as nylon-6 and nylon-66, nylon-12 has similar

tensile and compression properties but the lowest melting point of any nylon

(178°C:220°C:264°C). These differences are due to the length of the molecule’s carbon chain as

the shorter chains allow for a tighter and more compact polymer structure which limits the

movement of the branches causing more energy to be required to affect the structural integrity

of the solid.

Figure 2.4: Molecular Structure of Nylon

Nylon-12 carbon fiber (N12CF) is a polymer filament used in FDM manufacturing. This

composite material is created by mixing a nylon-12 resin with fragments of carbon fiber filler.

This filler can be distributed within the matrix either in a specific pattern or at random in order

to reinforce the material’s mechanical properties [23,24]. The specific N12CF material that is

being studied in this thesis has a filler comprised of chopped carbon fiber threads distributed at

random orientations within the polymer matrix at a weight percentage of 35%. Studies

conducted to evaluate the mechanical properties of parts constructed from FDM-printed nylon-

12 have shown that the parts have different strengths depending upon the axis (XYZ) that the

part was printed on [25,26]. Parts printed in the X-axis had better tensile strength while parts

printed in the Z-axis could withstand compression better. Flexural properties of N12CF have

8

also been studied and found that the degree of flex that a part has is dependent upon fluid

absorption. N12CF has been found to have the lowest fluid absorption rate of any current nylon

filament and, when dried, a N12CF part has the same properties as one that has never been

exposed to fluid [27-29]. Hu and Hossan [30] simulated a gear pair constructed from N12CF

using computerized software and found that due to the difference between the tensile and

compression strengths, material failure can occur at areas of tension, stress concentration, and

compression stress concentration when the gear teeth come in contact with one another. Thus,

areas where contact between two or more parts is made must be reinforced to ensure that a

device constructed from N12CF does not fail. More manufacturing of FDM printed N12CF parts

must be conducted in order fully understand the capabilities of this material, as its use in FDM

is fairly novel.

2.4 Medical Exoskeletons

Medical exoskeletons are a type of external orthotic device that a patient can wear in

order to supplement a limb and allow for a greater range of movement. There are two main

categories of orthotics: powered and unpowered. Unpowered orthotics, as the name implies,

do not require an external power source in order to function and assist a patient. These types

of orthotics range from basic support braces to more advanced mechanical joints that utilize

simple machines such as springs and pulley systems in order to facilitate movement. Powered

orthotics are similar to their unpowered counter parts in the sense that they are meant to

support and augment a user’s movement. However, powered orthotics require an energy

source to power the machinery that allows the device to perform a desired task such as

extending a patient’s leg.

9

In recent years, the medical field has begun to adopt Additive Manufacturing techniques

to design and manufacture various types of medical devices. In recent years, researchers have

started to investigate if these methods can be used to manufacture orthopedic devices to treat

mobility impairments within a patient’s leg [31-37]. One such device that has been researched

is unpowered ankle-foot orthoses (AFO). By utilizing 3D foot scanners, researchers have been

able to construct models of patient-specific devices and construct them via FDM.

Unfortunately, due to the constraints of this printing method, the orthotics are not able to have

complex geometric shapes [38,39]. Utilizing other AM techniques such as selective laser

sintering (SLS) allows for the generation of more complex devices but also increase the time

required to manufacture the devices.

The use of lower limb powered orthotics as devices for mobility rehabilitation has

become an area of great interest for researchers [40], and a number of devices have been

developed for patients with ailments caused by failing posture and stroke [41-43]. As

mentioned previously, the use of bulky components in medical exoskeletons has the potential

to limit a patient’s mobility. A review conducted by Sanchez-Villamaen et al. [44] reported that

the incorporation of compact actuators is essential in future research as their light weight and

small size remove most of the bulk that makes up modern exoskeletons, and, when used in

combination with lightweight materials, would allow for the construction of more compliant

devices. Studies that designed exoskeletons that utilized these two attributes, shown in Figure

2.5, have reported that the subjects’ range of motion incurred little to know interference with

the devices equipped [45,46]. The utilization of FDM techniques to create lightweight

components for a gait training exoskeleton has been reported in a study conducted by Jin et al.

10

[47]; while the constructed components were attachment points for the test subject and not

key joints for the exoskeleton, this alongside the use of AM to construct full AFO devices does

show the potential for FDM to be used in the construction of an active lower body exoskeleton

made of polymer components.

Figure 2.5: Lightweight Hip Exoskeleton that Utilizes Compact Actuators for Movement.[Images

courtesy of Giovacchini et al. [45]]

11

CHAPTER 3

DEVICE DESIGNS AND METHODOLOGIES

3.1 Design of Exoskeleton

As the legs of this device (Figures 3.1 and 3.2) are expected to be constructed via fused

deposition modeling, each part was designed to fit within a printing space of 12 in3 while also

allowing for a tolerance of 0.03 in. to account for any dimensioning errors that could occur. The

following sections will describe the design of each of the three joints (hip, knee, and ankle) and

how the parts are to interact with one another.

Figure 3.1: Full Exoskeleton Leg with Posterior Offset Knee

12

Figure 3.2: Full Exoskeleton Leg with Polycentric Knee

3.1.1 Design of the Thigh

The hip connection of the device shown in Figure 3.3 is composed of two pieces: an

upper motor housing and a lower connection shank that acts as the femur of the device

connecting the hip to the knee. Within the housing will be a harmonic gear set that is meant to

greatly multiply the force of the hip motor and allow for the patient to move their leg. On the

rear side of the motor housing is an attachment point for a linear actuation motor (LAM), which

will act as the unit’s hamstring and quadricep for full extension and contraction of the leg. The

bottom half of the motor mount is a hollow shaft that the connective shank can be inserted

during assembly. The shank is a singular 10 in. long shaft with a 0.688 in. hole at the bottom in

which either a needle bearing and or bolt will be inserted depending on which knee the leg will

13

connect to. The body of the shank has also been hollowed out to minimize the weight of the

thigh and allow for storage of electrical components.

To assemble this portion of the exoskeleton, the thigh shank is inserted into a hole at

the bottom of the motor housing and locked in place with a clevis pin. Multiple holes for the

clevis pin were made along the length of the shank to allow for height adjustability of the

device to better accommodate to the needs of the patient that is using it.

Figure 3.3: Exoskeleton Thigh

3.1.2 Design of the Ankle

Like the hip described above, the ankle shown in Figure 3.4 is also made of two pieces, a

motor housing that acts as the ankle and a shin shank that supplements the patient’s tibia and

connects to the knee. A harmonic gear set is housed within the ankle and connects to a

removable insole that is to be placed within a patient’s shoe. When powered by the ankle

motor the gears rotate the insole to provide flexion and extension of the user’s ankle and allow

14

for more natural and comfortable movement. The upper portion of the ankle is a hollow shaft

in which the shank can be inserted during assembly of the joint. The shank is a 10.5 in. long

shaft with a 0.688in hole at the end in which a needle bearing and bolt can be inserted when

attaching to the knee. This shank also has triangular shaped protrusions that extend from the

rear on the left and right sides, these structures are the anchor point for the LAM to attach.

To assemble the joint, the shin is inserted into the hollow end of the motor housing and

secured with a clevis pin. Like the thigh shank, the shin has multiple holes along its length for

the clevis pin to be placed to allow for height adjustment for the patient.

Figure 3.4: Exoskeleton Ankle

3.1.3 Design of the Knee

The knee is one of the most critical joints in the human body in terms of mobility and

stability. It is composed of three bones (the femur, tibia, and patella) that are connected

15

together with four ligaments, the anterior and posterior cruciate ligaments (ACL & PCL) and the

medial and lateral collateral ligament (MCL & LCL), to keep the joint stable by applying constant

tension to the bones. Three tendons (the quadriceps tendon, patellar tendon, and hamstring

tendon) connect the bones to muscles that allow for the knee to move when contracted. During

flexion or extension of the knee the femoral head rotates about and translates across the

surface of the tibia. This movement is especially important the swing phase of a patient’s walk

cycle as it, along with flexion of the ankle, are what prevents the foot from striking the ground,

which would cause the patient to trip. With deterioration of the knee joint, it is important to

design a hinge joint that can guide a patient’s leg along a natural path that allows for optimum

flexion and extension of the knee while wearing an orthotic device as the shape and placement

of the artificial knee joint can affect the geometry of the patient’s joint [48].

Most of the knee designs used in orthotics today fall into one of three categories: single

axis (rotating or locked), posterior offset, or polycentric (multiple rotation axes), each used to

treat different issues that affect a patient’s knee such as hyperextension or lack of mechanical

stability [49]. For this exoskeleton two simple knee joints were designed, a posterior offset knee

(PO) and a polycentric knee (PC) that, when paired with a LAM, would act like a stance

controlling orthotic (SCO) knee. SCOs are a form of a knee ankle foot orthotic (KAFO) and are

designed to stabilize a patient’s leg to compensate for muscle weakness or deterioration within

the upper leg. The rationale for the two knees was to determine which style of knee would

allow for a more natural movement of the leg and cause the least amount of stress buildup to

occur along the length of the leg.

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3.1.3.1 3.1.3.1: Posterior Offset Knee

The PO knee, shown in Figure 3.5, is composed of two nearly identical sockets that rest

atop one another. As the name implies, the axis on which the joint rotates is located on the rear

side of the leg, as opposed to a human knee’s axis of rotation which is located close to the

anterior side. To facilitate this movement, the two parts have cylindrical protrusions that

extend from the rear that can slide together and connected via a carriage bolt to form a hinge

joint. Within this hinge, a needle bearing is housed that rotates about the shaft of the

connective bolt allowing for smooth rotation about the rear axis of the knee.

Figure 3.5: Posterior Offset Knee

This design was created to maximize the stability of the patient’s knee while standing.

To allow for this, the top and bottom halves of the joint lock together while the LAM is fully

extended when the patient is in an upright position. This locking mechanism is also to prevent

hyperextension of the joint. To limit the sway of the shin and thigh shanks in the medial and

17

lateral directions, the walls of the sockets lock the parts in place. The posterior cylindrical

protrusions also act as safety stops to prevent the two halves of the knee swaying in these

same directions.

3.1.3.2 Polycentric Knee

The PC knee, shown in Figure 3.6, is made of a single rectangular-shaped piece that has

been hollowed out to allow for the insertion of the shin and thigh connections.

Figure 3.6: Polycentric Knee

This joint has two axes of rotation for the shin and thigh accordingly, the posterior wall of the

knee has been removed to maximize the rotation of the femur and tibia about the knee, while

also providing the joint translational motion to allow for a more natural movement of the leg.

As a safety precaution to prevent hyper extension of a patient’s knee, the front wall of the PC

acts as a stop block, the walls along the medial and lateral sides of the knee act in a similar way

18

to prevent the leg from bowing in either direction. This part was designed with ease of motion

in mind in order to help a patient experiencing osteoarthritis of the knee, similarly to how some

knee braces that have a similar joint structure would help [50,51].

3.2 Force Modeling Simulations

Designs were modeled in SOLIDWORKS and static force simulations were created and

run for each of the joint models and fully assembled leg models, one for each of the knee

designs. This simulation software was utilized over other available software due to its ease of

access when developing SOLIDWORKS files, its prevalence in the wider professional

environment, and to reduce any chance of error or corruption that could occur when

converting the part files into a different format. The use of a static force study allows for the

calculation of both the stress and displacements that the parts will undergo without needing to

account for environmental factors. Each assembly underwent two simulations to account for

the difference in mechanical properties of FDM nylon-12 carbon fiber when printed on either

the XZ or ZX axis, as was described earlier. The forces, listed in Table 3.1, that were to be

simulated acting on the devices included a 35 lbs. force exerted by the LAM and a 200 lbs.

ground reaction force to account for the full weight of the patient when standing or in the

swing phase of their walk cycle. This weight was selected based off of the average reported

weight of elderly men in a 2018 survey conducted by the CDC [52]. Summarized in Table 3.2 are

some of the mechanical properties of the materials used in the study according to material data

sheets provided by Stratasys and Renishaw for the N12CF and Ti6Al4V materials respectively

and are located in Appendix B.

19

Table 3.1: Force Elements Applied to Exoskeleton

Sources of Force Force Strength (lbs.) Application Points Direction of

Force

Weight of Patient 200 Base of ankle Base of thigh shank Base of shin

Always upward

Linear Actuation Motor 35 Thigh motor mount Shin-LAM connection point

Upward Downward 45◦

Table 3.2: Mechanical Properties of Simulated Materials

Material Tensile

Strength (MPa)

Compression Strength

(MPa)

Melting Temperature

(◦C)

Glass Transition Temp (◦C)

Stratasys FDM N12CF XZ Printing Axis 63 67 178 41

Stratasys FDM N12CF ZX Printing Axis 29 92 178 41

Ti6Al4V Alloy 897 1070 1649 1538

3.2.1 Force Modeling of the Thigh

In order to run force simulations on the exoskeleton’s thigh, an assembly file was

created in SOLIDWORKS that comprised of the upper hip connector and lower thigh shank. The

two parts were then mated so as to insert the shank into the bottom of the hip connector. To

create the simulation environment, the ‘Create New Study’ option was selected from the

‘Simulation’ menu with the ‘Static’ option selected as the type of study. The parts were then

given the mechanical properties of the nylon-12 carbon fiber material that had been printed on

the XZ axis.

When applying the simulated forces to the thigh, the ‘Applied Load’ menu was selected

from the ‘Simulation’ menu and the ‘Force’ option was chosen from the list of possible loads.

The 35 lbs. load from the LAM was applied to the rear attachment points of the hip connector

20

piece. The force was set to be applied equally to both points in an upward direction using the

‘Total’ and ‘Specific Direction’ options within the ‘Force’ menu. The 200 lbs. ground force was

applied to the thigh shank in an upward direction at the lower end where the part would

connect to a knee joint.

To ensure that the thigh assembly stayed in place throughout the duration of the

simulation, screw holes where the hip connector would be mounted to the hip motor were

selected as fixed points in space. Without this fixture, the forces applied to the thigh would

cause unrealistic levels of displacement to occur.

For an accurate simulation to be created, the two parts of the thigh needed to be

connected together with a simulated fastener and contact constraints for when parts collide

with one another during the simulation. A steel bolt connector was selected to be the fastener

that held the hip connector and shank together. The ‘Rigid’ and ‘Tight Fit’ options were selected

for the bolt so that it did not bend with the plastic thigh when force was applied. To create

realistic contact forces between the parts, a non-penetrative contact set was created between

the outer faces of the shank and the inner faces of the hip connector. The non-penetrative

option was selected so that the computer could calculate how the applied forces would react

with each part individually as opposed to the penetrative/bonded option which would cause

the computer to see the parts as a singular entity when calculating stress and displacement of

the thigh. A semi-fine curvature-based mesh of the models was generated for this study as well.

The same process was done to generate a simulation of the thigh given the mechanical

characteristics of the nylon material when printed on the ZX axis. These parameters are

summarized in Table 3.3.

21

Table 3.3: Thigh Simulation Parameters

Parameter Name Location on Model

Fixture Point

Force 1

Force 2

Bolt Connector 1 (Rigid)

Contact Set 1 (Non-Penetrating)

3.2.2 Force Modeling of the Ankle

To begin the static force studies regarding the exoskeleton’s ankle, an assembly file was

created composed of the ankle motor mount and shin connection shank. The parts were mated

so that the shin connector was inserted into the upper portion of the motor mount. Generation

22

of the static simulation environment was identical to thigh force study and the parts were given

the material characteristics of XZ printed nylon-12 carbon fiber and the parameters are

summarized in Table 3.4. A steel bolt connector was used to secure the shin to the ankle mount

and a non-penetrative contact set was created between the outer walls of the shank and the

inner walls of the insertion point of the ankle to ensure that the forces acted on each part

individually.

Table 3.4Ankle Simulation Parameters


Fixture Point

Force 1

Force 2

Bolt Connector (Rigid)

Contact Set 1 (Non-Penetrative)

23

3.2.3 Force Modeling of the Knee

The 200 lbs. ground reaction force was applied to the base of the ankle mount in an

upward direction to simulate the force that would be applied to the base of the device while a

patient was standing with their leg fully extended. The 35 lbs. force from the LAM was applied

at a downward 45-degree angle to the rear wing protrusions of the shank where the motor

would be mounted if the device was assembled. To prevent the ankle assembly from moving

thru space unrealistically, the shin’s connection point to the knee was chosen as a fixed point in

space. A semi-fine curvature-based mesh was then generated for the assembly before the

simulation was run. These settings were also used to generate the force simulation for the for

the ankle given the characteristics of the material printed on the ZX axis.

Due to the amount of movement that the knees were expected to undergo, and that

their positioning required that they be able to withstand the weight of the full device as well as

any extraneous forces while the device is being used, the knees were designed to be

constructed from a titanium alloy. As such, the force simulations related to the knee joint were

run to see how the stresses applied to the plastic shin and thigh shanks would be affected when

they are attached to components made of a harder material. Simulations were also created

with all of the parts given the characteristics of the nylon-12 carbon fiber material in order to

compare how radically the inclusion of the titanium parts altered the stress values along the

length of the thigh and shin.

3.2.3.1 Forces Simulation of Posterior Offset Knee

To setup the force simulations for the PO knee, an assembly file composed of the two

halves of the knee, the thigh shank, and the shin shank was created. The two halves of the knee

24

were mated using the included ‘Hinge’ mechanical mate so that the rear protrusions could be

concentric and coincident with one another. This allows for the knee to rotate about its rear

axis when modeling the movement of the joint. Both of the shanks were mated using the

concentric mates between the connection holes in the shanks and the bolt holes on the sides of

the knees as well as width mates between the outer faces of the shanks and the inner faces of

the knee sockets. This allows the parts to act as if they have been inserted into the knee sockets

and react as such when testing the mobility of the joint.

The simulation environment was generated as described in previous sections. The knee

parts were given the characteristics of Ti6Al4V alloy and the shanks were given the mechanical

characteristics of the XZ axis printed nylon. As shown in Table 3.5, rigid steel alloy bolt

connectors were used to attach the two halves of the knee together at the rear protrusion

points, as well as between the shanks and their attachment points in the knee sockets. Non-

penetrative contact sets were then created between the outer faces of the shanks and the

inner faces of the knee sockets and a semi-fine curvature-based mesh was generated of each of

the parts.

The forces applied to the knee include the 200 lbs. ground reaction force applied in an

upward direction to the shin shank at its attachment point to the ankle, and the 35 lbs. force

from the linear motor applied to the shin shank as described in 3.2.2. The attachment point

between the thigh shank and the thigh motor mount was selected to be the fixed point for this

model to prevent unrealistic movement through space. The simulation was then run to assess

the stresses applied to the leg. These steps were then repeated to create a simulation for the

25

shanks given the characteristics of the ZX printed nylon-12 material as well as for the

simulations where all of the components were set to be made out of the nylon-12 material.

Table 3.5: PO Knee Simulation Parameters


Fixture Point

Force 1

Force 2



(table continues)

26





3.2.3.2 Force Simulation of Polycentric Knee:

An assembly file was created composed of the PC knee, shin shank, and thigh shank

parts. A concentric mate between the shanks’ knee connection point and the PC knee’s bolt

insertion sockets and a width mate was used between the outer faces of the shanks and the

inner faces of the knee sockets. This way the shanks act as if they had been inserted into the

knee.

Four simulation environments were created, two where all of the components were

27

given the characteristics of the nylon-12 material printed on either the XZ or ZX plan and two

where the thigh and shin were given the nylon material characteristics and the knee was given

the properties of the Ti6Al4V alloy. Rigid steel alloy bolt connectors were selected to attach the

shanks to the knee and non-penetrating contact sets were created between the outer faces of

the shanks and the inner faces of the PC knee sockets as shown in Table 3.6. Forces and fixtures

were applied as described in 3.2.3.1. A semi-fine curvature-based mesh was generated for each

of the models and the simulations were run.

Table 3.6: PC Knee Simulation Parameters


Fixture Point

Force 1

Force 2


(table continues)

28





3.2.4 Force Modeling of Leg

With the simulations completed for the individual sections of the leg simulation sets had

to be conducted to see how all of the parts interacted with one another. As such two models of

the leg were constructed using both the PO and PC knees in order to find out which leg had the

best reaction to the applied forces. As with the knee models four simulation environments

were set up, two where all of the components of the leg were given the properties of the nylon-

12 material and two where the parts were a hybrid of nylon and titanium. This was done to

verify if the legs could be constructed wholly from nylon as well as to compare how the

integration of titanium affected how the stress was distributed along the length of the leg.

3.2.4.1 Force Simulation of Leg with Posterior Offset Knee

For this simulation, an assembly file was generated consisting of the two halves of the

29

PO knee along with the thigh and ankle assemblies. The PO knee parts were mated using the

mechanical hinge mate and the shin and thigh shanks were mated to the knee as was described

in 3.2.3.1. Non-penetrating contact sets were made between the thigh motor mount and thigh

shank, thigh shank and PO knee socket, shin shank and PO knee socket, and the shin shank and

ankle motor mount. Rigid steel alloy bolt connectors were selected to fasten the components

together at the respective attachment points along the length of the leg. The 35 lbs. force from

the LAM was applied to the thigh motor mount and the shin shank as described in previous

section and the 200 lbs. ground reaction force was applied to the base of the ankle motor

mount. The screw holes at the top of the thigh motor mount were selected as the fixture for

this simulation to prevent unrealistic movement once the forces are applied during the

simulation as shown in Table 3.7. A semi-fine curvature-based mesh was applied to the models

and the simulation was conducted. These same parameters were also used to set up the other

three simulations.

Table 3.7: PO Leg Simulation Parameters


Fixture Point

Force 1

(table continues)

30


Force 2

Force 3





(table continues)

31







(table continues)

32



3.2.4.2 Force Simulation of Leg with Polycentric Knee:

For the final simulation set, an assembly file was created with the PC knee, thigh, and

ankle assemblies. The shin and thigh shanks were mated to the PC knee as described in 3.2.3.2,

and four simulation environments were created for the plastic parts printed on the XZ and ZX

axes in which two of them the knee was given the characteristics of Ti6Al4V alloy as in the

previous PC knee simulation.

Table 3.8 shows, non-penetrating contact sets were created between the upper thigh

and thigh shank, thigh shank and knee socket, shin shank and knee socket, and shin shank and

ankle motor mount as described in previous sections. Two tension spring connections were

used to connect the shin and thigh shanks together with a tension of 35 lbs. The springs were

added in place of the force from the LAM to account for stability of the leg because with all of

the normal forces being applied, the computers were unable run an accurate simulation due to

the shape of the knee. The 200 lbs. ground force was applied to the base of the ankle mount.

The screw holes at the top of the thigh were again selected as the fixtures for the simulation. A

semi-fine curvature-based mesh was generated for the models and the study was run.

33

Table 3.8: PC Leg Simulation Parameters


Fixture Point

Force 1

Spring Connector 1 (Compression/Extension)

Sprint Connector 2 (Compression/Extension)



(table continues)

34







(table continues)

35



36

CHAPTER 4

RESULTS AND DISCUSSION

4.1 Simulation Results

Three dimensional plots were generated to map out the stress, strain, and displacement

of the models when the forces were applied during the simulations. The following sections

describe the results of the force study applied to the parts for both the XZ and ZX printed nylon-

12 carbon fiber components.

4.1.1 Thigh Simulation Results

From the stress plots that were calculated during the simulation of the thigh portion of

the device, it was determined that parts manufactured from the either the XZ or ZX printing

axes would not undergo catastrophic failure when undergoing the expected forces. For the

plots in this section the blue regions show areas of low stress build up while the red regions are

areas where the highest amount of stress is calculated to occur. The green and yellow areas are

calculated to be the median stress values. The maximum stress applied to the parts printed

from the XZ and ZX axis was calculated to be 26.06 and 7.420 MPa respectively, these max

stresses were found to be applied to the fixed screw holes of the upper thigh motor mount. As

shown in Figures 4.1 and 4.2, even with the difference between the compression strengths of

the two plastics, overall stresses were dispersed similarly across the thigh as a whole. Both

versions of the thigh were calculated to have a maximum displacement of 2.098 mm and max

strains of 2.169x10-3 and 2.214x10-3 respectively. These values are summarized in Table 4.1.

37

Figure 4.1: Stress Plots for N12CF Thigh Printed on XZ Axis (A)Front view, (B) Right view, (C) Rear view,

(D) Left view

Figure 4.2: Stress Plots for N12CF Thigh Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear

view, (D) Left side view

38

Table 4.1: Cumulative Results of Thigh Static Force Simulations

Material Max Stress (MPa) Max Displacement (mm) Max Strain

FDM N12CF Printed on XZ Axis 26.06 2.098 2.169x10-3

FDM N12CF Printed on ZX Axis 7.420 2.098 2.214x10-3

4.1.2 Ankle Simulation Results

The plots constructed from the simulations, as shown in Figures 4.3 and 4.4, of the ankle

portion of the exoskeleton, calculated the maximum stresses applied to the joint to be 18.19

and 21.56 MPa for the XZ and ZX printing axes respectively. The max stresses were mapped to

occur at the bolt connection point between the shin shank and the ankle mount on both

versions. The maximum displacement was calculated to be 4.696 and 1.278x101 mm for the XZ

and ZX axis respectively at the base of the ankle mount. The maximum strain applied to the

ankle joint was also found to be 1.729x10-3 and 5.747x10-3 respectively. These values are also


Table 4.2: Cumulative Results of Ankle Static Force Simulations


FDM N12CF Printed on XZ Axis 18.19 4.696 1.729x10-3

FDM N12CF Printed on ZX Axis 21.56 12.78 5.747x10-3

39

Figure 4.3: Stress Plots for N12CF Ankle Printed on XZ Axis (A) Front view, (B) Right view, (C) Rear

view, (D) Left view

Figure 4.4: Ankle Stress Results for ZX Printing Axis (A) Front view, (B) Right view, (C) Rear view, (D)

Left view

40

4.1.3 Knee Simulation Results

4.1.3.1 PO Knee Results

For both the XZ and ZX simulations, the maximum stress applied to this portion of the

device was calculated to occur at a point on the knee itself. A stress of 141.20 MPa was found

to occur at the rear side of the knee on the XZ simulation, and a stress of 17.85 MPa was

calculated to occur at the bolt connection point between the knee and shin on the ZX

simulation as shown in Figures 4.5 and 4.6. As for the nylon thigh and shin, the stresses ranged

from 11.77 to 94.15 MPa for the XZ printed parts and 1.488 to 11.90 MPa for the ZX printed

parts. The higher stresses found on the plastic parts were found to occur at the force contact

points as well as the bolt attachment points in both of the simulations. For both simulations the

maximum displacement of the device was calculated to occur at the shin with a measurement

of 12.62 and 4.210 mm for the XZ and ZX simulations respectively. The maximum strain applied

to this section of the device was also calculated to be 1.028x10-2 and 3.558x10-3 respectively.

Figures 4.7 and 4.8 show the stress applied to the posterior offset knee assembly when

constructed entirely from nylon-12 carbon fiber. The maximum stress, displacement and strain

values were calculated to be 9.230 and 16.03 MPa, 2.575 and 7.099 mm, 8.547x10-04 and

4.853x10-03 for the XZ and ZX printed nylon components respectively. These values are


41

Figure 4.5: Stress Plot for Ti6Al4V PO Knee with N12CF Thigh and Shin Printed on XZ Axis

Figure 4.6: Stress Plot for Ti6Al4V PO Knee with N12CF Thigh and Shin Printed on ZX Axis

42

Figure 4.7: Stress Plots for N12CF PO Knee Printed on XZ Axis (A) Front view, (B) Right view, (C) Rear

view, (D) Left view

Figure 4.8: Stress Plot for N12CF PO Knee Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear

view, (D) Left view

43

Table 4.3: Cumulative Results of PO Knee Static Force Simulations


FDM N12CF Printed on XZ Axis (Ti64) 94.15 (141.20) 12.62 1.028x10-02

FDM N12CF Printed on ZX Axis (Ti64) 11.90 (17.85) 4.210 3.558x10-03

FDM N12CF Printed on XZ Axis, all parts 9.230 2.575 8.547x10-04

FDM N12CF Printed on ZX Axis, all parts 16.03 7.099 4.853x10-03

4.1.3.2 PC Knee Results

As with the PO knee plots, the maximum stresses applied to the PC knee were also

found to occur on the titanium knee itself as shown in Figures 4.9 and 4.10. These stresses were

calculated to be 251.70 and 2452000 MPa for the XZ and ZX environments respectively. The

higher stress in the ZX plot far exceeds the yield strength of the knee. Therefore, plastic

deformation and mechanical failure can be expected to occur. The stresses applied to the

plastic components were found to be in the ranges of 20.99 to 167.8 and 0.1319 to 1022000

MPa for the XZ and ZX environments respectively. These high stresses were also found to occur

at the bolt attachment points for the thigh and shin and result in plastic deformation to occur

for these parts. The maximum displacement and strain were calculated to be 4.422x107 mm

and 1.605x10-2 for the XZ parts and 3.890x1011 mm and 3.563x101 for the ZX parts. Figures 4.11

and 4.12 show the stress plots for the PC knee assembly constructed wholly from nylon-12

carbon fiber printed on the XZ and ZX axes respectively. The maximum stress, displacement and

strain values were calculated to be: 2.664x1009 and 2.056x1013 MPa, 511.6 and 3.448x1012 mm,

44

and 5.406x10-01 and 1.479x1004 for the XZ and ZX printed parts respectively. These values have

been summarized in Table 4.4.

Figure 4.9: Stress Plot for Ti6Al4V PC Knee with N12CF Thigh and Shin Printed on XZ Axis

Figure 4.10: Stress Plot for Ti6Al4V PC Knee with N12CF Thigh and Shin Printed on ZX Axis

45

Figure 4.11: PC Knee all parts N12CF printed on XZ axis (A) Front view, (B) Right view, (C) Rear view,

(D) Left view

Figure 4.12: PC Knee all parts N12CF printed on ZX axis (A) Front view, (B) Right view, (C) Rear view,

(D) Left view

46

Table 4.4: Cumulative Results of PC Knee Static Force Simulations


FDM N12CF Printed on XZ Axis (Ti64) 167.8 (251.70) 4.422x107 1.605x10-02

FDM N12CF Printed on ZX Axis (Ti64) 1022000 (2452000) 3.890x1011 3.563x1001

FDM N12CF Printed on XZ Axis, all parts 2.664x109 511.6 5.406x10-01

FDM N12CF Printed on ZX Axis, all parts 2.056x1013 3.448x1012 1.479x1004

4.1.4 Leg Simulation Results

4.1.4.1 PO Leg Results

The stress plot generated from the force study shown in Figures 4.13 and 4.14

calculated the maximum stresses to be applied to the titanium alloy knee on both the XZ and ZX

printed legs with a value of 30.29 and 63.79 MPa respectively. The stresses applied to the nylon

components ranged from 3.029 to 21.20 MPa and 6.379 to 25.52 MPa respectively, with the

higher stresses being applied to the connection bolt holes as was found with the ankle

simulation. Maximum displacement of the leg was found to be 8.577 and 2.471x101mm for the

maximum strain values were found to be 1.908x10-3 and 6.073x10-3 for the XZ and ZX printed

legs respectively. Figures 4.15 and 4.16 show the stress plots for the posterior offset leg with all

parts constructed from the nylon-12 filament for printed on the XZ and ZX axes respectively.

The maximum stress, displacement, and stress were calculated to be: 41.91 and 22.66 MPa,

11.06 and 29.29 mm, and 2.004x10-04 and 6.121x10-03 for the XZ and ZX printed parts

respectively. All these values have been summarized in Table 4.5.

47

Figure 4.13: Stress Plot for N12CF PO Leg printed on XZ Axis with Ti6Al4V Knee (A) Front view, (B),

Right view, (C) Rear view, (D) Left view

Figure 4.14: Stress Plot for N12CF PO Leg printed on ZX Axis with Ti6Al4V Knee (A) Front view, (B),

Right view, (C) Rear view, (D) Left view

48

Figure 4.15: Stress Plot for N12CF PO Leg printed on XZ axis (A) Front view, (B) Right view, (C) Rear

view, (D) Left view

Figure 4.16: Stress Plot for N12CF PO Leg Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear

view, (D) Left view

49

Table 4.5: Cumulative Results of PO Leg Static Force Simulations



FDM N12CF Printed on ZX Axis (Ti64) 25.52 (63.79) 2.471x101 6.073x10-3

FDM N12CF Printed on XZ Axis, all parts 41.91 11.06 2.004x10 -03

FDM N12CF Printed on ZX Axis, all parts 22.66 29.29 6.121x10-03

4.1.4.2 PC Leg Results

The stress plots of the PC leg, shown in Figures 4.17 and 4.18, calculated the maximum

stresses to be 17.77 and 21.94 MPa for the XZ and ZX printed nylon parts respectively. For both

legs the maximum stresses were calculated to occur along the length of the thigh shank. The

maximum displacements and strain values were also calculated to be 1.426x101 and

4.186x101mm and 1.636x10-3 and 5.587x10-3 respectively. Figures 4.19 and 4.20 show the stress

plots for the polycentric leg with all parts constructed from the nylon-12 carbon fiber material

when printed on the XZ and ZX axes. The maximum stress, displacement, and strain have been

calculated to be: 17.38 and 44.89 MPa, 13.61 and 23.58 mm and 1.584x10-03 and 1.041x10-02

for the XZ and ZX printed parts respectively (see Table 4.6).

Table 4.6: Cumulative Results of PC Leg Static Force Simulations



FDM N12CF Printed on ZX Axis (Ti64) 21.94 (10.97) 41.86 5.587x10-3

FDM N12CF Printed on XZ Axis, all parts 17.38 13.61 1.584x10-03

FDM N12CF Printed on ZX axis, all parts 44.89 23.58 1.041x10-02

50

Figure 4.17: Stress Plot for N12CF PC Leg Printed on XZ Axis with Ti6Al4V Knee (A) Front view, (B),

Right view, (C) Left view

Figure 4.18: Stress Plot for N12CF PC Leg printed on ZX Printing Axis with Ti6Al4V Knee

(A) Front view, (B), Right view, (C) Left side view

51

Figure 4.19: Stress Plot for N12CF PC Leg printed on XZ Axis (A) Front view, (B) Right view, (C) Rear

view, (D) Left view

Figure 4.20: Stress Plot for N12CF PC Leg Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear

view, (D) Left view

52

4.2 Discussion

The purpose of this thesis was to determine the viability of nylon-12 carbon fiber

filaments as a material to be utilized in the construction of lower body exoskeletons using

additive manufacturing processes. As mentioned previously, all of the simulations were

conducted in SOLIDWORKS utilizing the available software tools. As such, the results found in

this paper may not be able to be replicated in other simulation software such as COMSOL

Multiphysics or ANSYS. A major limitation that is important to note is that SOLIDWORKS is only

able to construct solid parts and does not allow for the customization of parameters such as the

filament diameter or the printing direction of the filament when constructing a simulation.

Therefore, the overall results may not reflect what a printed part may experience when

undergoing stress testing.

Even with the exact same parameters set up between the XZ and ZX simulation

environments, the calculated stresses were found to be different. This is attributed to the

difference in compression moduli between the XZ and ZX printed materials, with the ZX

modulus being the higher value. This difference in compression strength is due to the direction

in which the nylon is distributed during extrusion as it affects the overall shape of the printing

pattern and how the extruded layers interact with any applied forces as discussed earlier. In

this case, the ZX printed layers would be normal to the applied compression forces, allowing

them to sandwich together and form a more rigid material.

Each of the simulation plots show that the stresses applied to the device disperse along

the length of the thigh and shin and condense at the various bolt attachment points. This is

because of the difference in the material properties between the nylon-12 carbon fiber and the

53

steel bolt connectors. These connectors were also selected to be rigid which causes it to remain

relatively stationary when forces are applied to the parts around it. The same can be said for

why the larger stresses would build up around the fixture points on each of the simulation

models. It was important to test each section of the leg independent of one another in order to

make sure that each of the main components could withstand the applied forces so that if the

leg failed, the failure points could be located and redesigned more easily. The separate

simulations were also conducted to make sense of how the stresses would be arranged along

the individual section and to determine how they changed once all the parts were connected

together. The unusually high amount of stress applied to the PC simulation is most likely due to

the fact that it was less stable because there was no rear wall preventing the thigh and shin

from moving around, causing the computer to have a more difficult time generating a solution.

This theory is aided by the fact that introducing the spring connectors to act as the 35 lbs. force

being applied to the thigh and shin stabilized the PC leg model overall and reduced the

calculated amount of stress, while also mapping it out almost identical to that of the PO leg that

underwent the normal applied forces.

Comparing the simulations containing the titanium knee to those that contain the nylon

knee it can be determined that the titanium part can be replaced with the nylon one without

causing the device to fail. In all of the simulations, save for the ZX printed polycentric knee,

none of the stress on the nylon parts was calculated to exceed the material’s yield strength and

under go plastic deformation. From the posterior offset leg simulations, it can be seen that the

maximum stress calculated on the knee happens on the same spot in all of the simulations and

the values on the XZ printed knee are extremely close together. The stresses are mapped out in

54

identical patterns along the length of the leg for each group of simulations, as in all of the PO

leg plots look identical and all of the PC leg plots look identical.

Stratasys also manufactures a nylon-12 filament without the carbon fiber filler and a

Nylon-6 filament to be used in FDM manufacturing processes. The yield strengths for these

materials along the XZ and ZX printing axes are 32 and 28 MPa and 49.3 and 28.9 MPa for the

bylon-12 and nylon-6 materials respectively. Comparing the simulation data to these values

shows the maximum calculated stresses for some sections of the exoskeleton are very close to

the yield strengths of the of the ZX printed versions of both filaments. The stresses calculated to

occur on the XZ printed parts do not exceed the expected yield strengths of the XZ nylon-12 or

nylon-6 materials. It can be concluded that the XZ printed nylon-6 material would be the better

choice of the two as the factor of safety is calculated to be <2 for the ankle and thigh sections

of the device when comparing the stress values to the yield strength of the nylon-12 material. It

is important to note that this comparison is only being made with Stratasys nylon filaments and

does not eliminate the possibility of using nylon-12 and nylon-6 from other companies as their

different formulation methods can result in different mechanical characteristics of the

materials.

55

CHAPTER 5

CONCLUSIONS

5.1 Experimental Conclusion

This thesis aimed to verify if nylon-12 carbon fiber filaments could be used in the

construction of a powered lower body exoskeleton using FDM processes by using static force

simulations to model the perceived forces that a prototype exoskeleton would be expected to

encounter. The results of the static force simulations conclude that nylon-12 carbon fiber FDM

filaments can be utilized in the construction of lower body powered orthotic devices for use by

the average elderly patient. From the maximum applied stress values, it was determined that

manufacturing the parts via the XZ printing axis would provide the most stable version of the

device as the calculated stresses for the ZX simulations were deemed too close to the yield of

the material to safely construct the device. Based on the plots calculated from the knee

simulations as well as both of the full leg simulations, not only does the use of titanium affect

the overall stress applied but also the shape of the knee affects results. From these studies, it

was determined that the usage of a polycentric knee joint would be the optimal choice for the

device as long as it has the right support structures as the maximum stress applied to the ankle

and thigh were found to be less than when using the posterior offset knee. Due to the way that

the forces are applied to the device, the mounting points between all of the parts of the leg

were found to be points of high stress. As such, reinforcement of these junctions may be

required for prolonged use of the device. Further simulations and testing are required to

determine the lifespan of the device.

56

5.2 Future Avenues of Research

This thesis focused on the usage of nylon-12 carbon fiber as a material, a comparison

between orthotic devices constructed from other common polymers used in FDM, such as

acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA), should be conducted. The

various parts of this orthotic were also designed to be hollow in order to lessen the weight of

the overall device. As such, a comparison between how the forces act on solid parts could also

be made to determine whether a hollow or solid design of the device is more suitable for usage.

The introduction of lattice structures to the various plastic parts could also be used as a basis

for a stress comparison. Additionally, this thesis only focused on computer simulations of forces

applied to the device. It is important to run tests on the exoskeleton prototype in practice to

compare the theoretical calculations to practical findings. This device was designed to be

manufactured using a planar printing pattern, because of this it would be a good idea to test

the mechanical differences between these parts and part printed using a non-planar pattern.

The primary focus of this thesis was testing to see if the exoskeleton could handle the

maximum forces that are expected to be applied to it when a patient is shifting their weight

during the swing phase of their walk cycle. As such, dynamic force study should also be

conducted on this device to see how the various parts react to the varying forces that would be

applied over the course of an average patient’s walk cycle. This can be done in SolidWorks by

setting up a non-linear study with the modal time history option applied. This option will allow

the ability to apply the expected loads of the LAM and weight of the patient over a set period of

time which can be altered based on the walking speed of an average patient as well as taking

into account the amount of time needed to extend the LAM.

57

To determine the theoretical lifespan of the device a fatigue simulation with constant

amplitude events should be run to see how many times the exoskeleton leg can undergo the

stresses calculated in the simulations before failing. This can be done in SolidWorks as long as

the static force simulations have already been conducted, they can be applied as the stress

parameters to the fatigue study, a SN-curve will need to be calculated for each of the materials

to account for expected material fatigue over time. This plot can be created by testing on a

sample piece of material with constant cyclic loading until it fails, the data may also be provided

by material vendors if possible.

58

APPENDIX A

ENGINEERING DRAWINGS OF EXOSKELETON

69

APPENDIX B

MATERIAL DATA SHEETS

75

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