Design and Simulation of a Shape Memory Alloy Based Microgripper for Minimally Inversive Surgery K

Design and Simulation of a Shape Memory Alloy Based Microgripper for Minimally Inversive Surgery
K. Pradeep Rathnayake, Eng. H. S. Lakmal PereraAbstract:Shape memory Alloy is a special kind of material which can memorize one (one-way SMA) or two (two-way SMA) shapes and able to recover when corresponding stimulation applied. This effect is called as shape Memory Effect (SME). This effect able to generate usable force when recovering hence able to do a mechanical work. Minimally inverse surgeries (MIS) are very popular and effective method for internal surgeries, because it only leaves few marks in the body after the surgery, which will disappear within few years. Existing microgrippers which used in Minimally Invasive Robotic Surgery are actuated by a motor pack that is located outside the patient’s body and the power to this end effector is transmitted by means of sliding link or tendon driven mechanisms. This method of power transmission results in less number of degrees of freedom to the gripper. This project was focused to design a microgripper which is flexible and ease in controlling, using SMA.

Most existing SMA microgrippers were took advantage of two techniques that is net shaped SMA and spring shaped SMA. For these project, two main factors were used with the high power to weight ratio of SMA. These are shape memory effect and Superelastic effect by using shape memory wires. To accomplish the selected design of microgripper, FEM (Finite Element Method) was used. This microgripper was very small (micro level) when compared to existing micro surgery tools, which allows to use tiny incisions in the human body to insert the microgripper, and it is possible to insert multiple microgrippers using one incision due to the size of the gripper. This gripper also can be developed for other types of surgeries by placing the required SME and SE wire size in order to generate the corresponding gripping forces and jaw width.

Keywords: Shape Memory Alloy, Minimally Inversive Surgery, Microgripper
1.Introduction
This type of material is significant class of materials called stimulus-responsive materials (SRMs), which are able to respond to a certain stimulus, such as, chemical, light or heat. This class of materials can be divided into two groups, shape change material (SCM) and shape memory materials (SMMs), Also call as Smart Materials.

Among these types of SMMs that have been developed so far are shape memory polymers (SMPs), shape memory hybrid (SMH), and shape memory alloy (SMA) 1. This project is focused on the Shape Memory Alloy to design and simulate of a Microgripper which can use in a biomedical Applications.

The alloys, which behave according to above effect called Shape Memory Alloy (SMA). First SMA is founded as AuCd in 19322. But the attraction of this phenomenon was not so apparent until significant recoverable strain was founded in NiTi at Naval Ordnance Laboratory. Therefore, it got the name of nitinol. NiTi (nitinol) is a most common and best SMA from all the SMAs. Now there are wide range of SMAs were developed in solid, thin film and even shapes. SMA has many usable characteristics such as High biocompatibility, High mechanical performances, High power to weight ratio, Large deformation recovery, Large actuation force.

Using this effective it is very effective to develop a microgripper even further small scale. A microgrippers is a microscopic servomechanism that supplies and transmits a measured amount of energy for to generate a usable work. It uses to grip micro objects with high accuracy, applications such as assembly in microsystems, micromanipulators for cells and endoscope. There are few different materials that use to manufacture MEMS (Micro Electro Mechanical Systems). Few of them can be state as follows,
Piezoelectric microgrippersElectromagnetic microgrippers23495349250Mr. K. Pradeep Rathnayake,
BTech(Eng)(Hons)(OUSL), Department of Mechanical Engineering, The Open University of Sri Lanka.

Eng. H. S. Lakmal. Perera
BTech (Eng)(Hons)(OUSL), MEng (Moratuwa),
AMIE (SL), AMIAE (SL), Probational Lecturer, Department of Mechanincal Engineering, The Open University of Sri Lanka.

00Mr. K. Pradeep Rathnayake,
BTech(Eng)(Hons)(OUSL), Department of Mechanical Engineering, The Open University of Sri Lanka.

Eng. H. S. Lakmal. Perera
BTech (Eng)(Hons)(OUSL), MEng (Moratuwa),
AMIE (SL), AMIAE (SL), Probational Lecturer, Department of Mechanincal Engineering, The Open University of Sri Lanka.

Shape Memory microgrippers2.Literature review
2.1 Types of Shape Memory Alloys
There are wide range of different types of SMAs around the world. Among all these founded SMAs only three SMAs are commercially important. They are, NiTi-based (NiTi, NiTiCu, NiTiPd, NiTiFe), Cu-based (CuZn, CuZnAI, CuAINiMn), Fe-based (FePt, FeMnSi, FeNiC).
2.2 NiTi-Based shape memory alloys
Among all these types NiTi is the best SMA for the biomedical field since it has high bio compatibility. It has different transformation temperatures according to its composition. Following table 1 shows the transformation temperature of NiTi with the composition.

Table 1: Transformation temperature with composition of each Ni, Ti element 3.

Atomic Composition Mf( 0C ) Ms( 0C ) As( 0C ) Af( 0C )
Ni51%Ti49%-9 49 4 71
Ni49.4%Ti50.6%57 63 86 100
Ni49.5%Ti50.5%-17 -3 22 35
Ni47.4%Ti52.6%30 50 71 86
Ni47.5%Ti52.5%45 66 81 98
Where:
Mf: Martensite Start
Ms: Martensite finish
As: Austenite starts
Af: Austenite finish
2.3 Biocompatibility of NiTi Shape Memory Alloy
NiTi has high biocompatibility since the mechanical behaviour of NiTi is more similar to biological tissue response if compared with that of other metallic materials commonly used for biomedical devices, as stainless steel 316L and chromium-cobalt (Cr-Co) alloys. Figure 1 shows the surface of NiTi clamp before implantation (left) and after 17 months exposure in the human body (right). There is no trace of corrosion visible. Porosity can be neglected on this scale is approximately 10?m 4.

Figure 1: surface of NiTi clamp before implantation (left) and after 17 months exposure in the human body (right).

2.4 Biomedical applications with microgrippers
Bio-MEMS devices are designed to provide viable neutral interfaces for long term, high density and two-way communication with selected areas in the body. Bio-MEMS devices can be categorized into three main categories.

Diagnostic microsystems: It includes rapid point-of-care, systems on a chip, cell and molecule sorting and also DNA diagnostics.

Surgical microsystems: Minimally Invasive Surgery (MIS), CAD assisted surgery (micro robotics).

Therapeutic microsystems + prostheses: Drug and gene delivery, tissue augmentation and repair, bio-capsules, micro invasive surgical systems
2.5 Dimensions of Biomedical microgrippers
Minimally inverse surgical procedures are carried out using small tubes, these tubes insert to small incision, it is between 5 to 10mm in diameter 5. Following table 3.1 shows the some existing microgripper jaw dimensions in Minimally Inverse surgery. Following Table 2 shows the dimensions of exiting grippers Jaws in MIS.

Table 2: Jaw dimensions of exiting grippers in MIS
MIS Microgripper Dimensions of the gripper
G1. For Measuring the mechanical properties of cells and tiny biological tissues 6
Length 17 mm, overall width 7.5 mm, thickness 0.4 mm
G2. Design of SMA Micro-Gripper for Minimally Invasive Surgery 7
Major diameter 3mm, Length 6mm
G3. Surgery grippers for minimally invasive heart surgery 8
Diameter 5mm, Length 10mm
G4. Structure forceps 9
Diameter 2.8mm, Length 8mm
G5. Rongeurs forceps 9
Diameter 3mm, Length 11mm
G6. Needle holder 9
Diameter 0.35mm, Length 2mm
G7. RESANO toothing forceps9
Diameter 3mm, Length 16mm
2.6 Human body temperature
Normal human body temperature, also known as normothermia or euthermia, is the typical temperature range found in humans. The normal human body temperature range is typically stated as 36.5–37.5 °C 10. But human body temperature is changing according to each stage as shown in the following Table 3.

Table 3: Human body temperature for each stage.

Temperature stage Temperature (°C)
Hypothermia: Hypothermia is reduced body temperature that happens when a body dissipates more heat than it absorbs. ;35.0 °C
Normal 36.5–37.5 °C
Fever: having a temperature above the normal range ;37.5 or 38.3 °C
Hyperthermia: Hyperthermia is elevated body temperature due to failed thermoregulation. ;37.5 or 38.3 °C
Hyperpyrexia: Hyperpyrexia is an extreme elevation of body temperature ;40.0 or 41.0 °C
3.Design
This concept has two distinct Shape memory alloys And Bio-compatible polymer as shown in the Figure 2.

Figure 2: Proposed design.

Gripper jaws by Bio Compatible Polymer.

Gripper fixing area.

Clamp for fixing SME Alloy and SE Alloy
This the NiTi wire which has Shape Memory Effect.

This is the NiTi wire which has Superelastic behavior in human body temperature.

3.1 Simulation of the design
Iterative simulation was done to get the correct dimensions by applying required force of 0.15N which create by the part E as shown in the figure 2. to B and C end as shown in the figure 3.

Figure 3: Boundary conditions of the simulation
A – Fixed end
B, C – Applied force of 0.15N
D – Applied thermal load (over 100 0C)
Following figure 4 shows the Final simulation the gives the required behaviour.

Figure 4: Strain recovery over time
Middle part (E as shown in the figure 2) also designed by the iterative simulations which can withstand the strain recovery force which occurs when thermal load was not applied. And not able to withstand the strain recovery force which occurs when thermal load was applied.
3.2 Selection of dimensions and material of gripper
When selecting dimensions and the materials for the microgripper, information was used which gathered from the literature review.
Materials: –
NiTi (Superelastic): This material is the part which provide the grip opening force (E as shown in the figure 2). Which is now using widely in the dental applications. For this design also, same material which used in the dental application was used because it behaves as SE material in body temperature. (SE nitinol wire provides a low constant force at human body temperature used to straighten teeth while reducing the need for wire retightening. The transition temperature of these wires is made so that they generate force at the temperature of the human mouth 11) according to the table 3 the temperature is between 35°C and 41°C. Therefore, according to table 1 this material has Ni49.5%Ti50.5% in composition and which has As 20°C.

NiTi (Shape Memory Effect): This material is the part which provide the grip closing force (D as shown in the figure 2). That is this material should able to provide the SME above human body temperature. According to the table 3, it should be over 41°C. Therefore, according to the table 1 this material has Ni47.4%Ti52.6% in composition and which has As at 71°C.

Bio-compatible polymer: There are series of silicon-based polymers has developed which suitable for additive manufacturing . One of the suitable material is silica nanoparticles. The biocompatible polymers and their composites with silica nanoparticles were successfully utilized to deposit both simple cubic structures, as well as a more complex twisted pyramidal feature. These polymers were found to be not toxic to a chondrogenic cell line, according to ISO 10993-5 and 10993-12 standard tests 12. A, B and C (as shown in the figure 2) parts of the selected gripper can be manufacture with this material using additive manufacturing
Jaw width: –
Gripping jaw width is depending on the cardiac tissue thickness. This is different from person to person. As an average the thickness is minimum 500µm 13. Therefore, the jaw width is taken as 0.5mm to 2mm.

Figure 4: Jaw width of the microgripper
Gripping force: –
Gripping force is depending on the surgery type. This project is focusing on the cardiac surgeries. For these kind of surgeries, the for which is safe is 0.1N to 0.3N 14. Therefore, this range is taken to be as the input of the gripper jaws in simulation.

Dimensions and design parameters: –
Following Table 4 and Figure 4 shows the selected design parameters.

Table 4: Final dimensions of the design
Materials SE NiTi (Ni49.5%Ti50.5%) SME NiTi (Ni47.4%Ti52.6%)
As(°C) Af(°C) 22 35 71 86
Initial Dimensions Width – 1mm, Length 3.2mm, Thickness 0.5mm
Gripping force 0.15N
Jaw width Minimum 0.99782mm, Maximum 1.82112mm
Electrical potential(T) 1v;T;2v

Figure 5: Dimensions of the microgripper.

5.Conclusions
As discussed Shape memory alloy is a material that has unusual behaviour compared to other materials. This type of materials also calls as memory material or smart material. Because it can remember one or more specific shapes which can memorize at corresponding stimulate. In this case it is temperature. These materials are used in various files such as bioengineering automotive field. Because this material has ability to recover up to 10% of its strain by overcoming the force or act which cause to generate the strain. Generated strain is proportional to generated force. That is when strain get large (not more than 10%), the force which occurs when recovering the strain get larger. This phenomenon was used to design a microgripper which able to use Minimally Inverse Surgery. These kind of surgeries uses up to 8 small holes (5 to 10mm) on the body to insert micro surgical tools. Most of these tools are mechanical linkages and need special training to operate. Also, it has only one end effector, therefore each tool needs separate hole in the body to insert. Another main disadvantage is, they are not flexible. Therefore, bending is not allowed. This project was focused to design micro gripper which is more miniature and flexible than existing micro grippers. According to the simulation results a gripper has designed to fulfil the goal.

For this project selected material was NiTi (Nitinol) as the shape memory alloy. According to the literature review, the reason was the biocompatibility of the NiTi. NiTi has wide range of types according to their composition which decide their transformation or activation temperature. For this project two compositions were used. One composition act SME over 71°C (?Ni?_(47.4%) ?Ti?_(52.6%)) other one act as SE in human body temperature (?Ni?_(49.5%) ?Ti?_(50.5%)). These two phenomena were every effective when designing the micro gripper. Four conceptual designs were proposed to the microgripper and design no:4 was selected by considering few facts for the simulation. Shape memory alloys are nonlinear material which was very complex to create mathematical model. Therefore, FEA (Finite Element Analysis) software ANSYS was used to simulate named the microgripper. Microgripper dimensions were decided according to the information of literature review. These selected dimensions were justified by using iterative simulation.

This SMA microgripper was for a MIS cardiac surgery with less number of holes on the human body. Using this microgripper it is possible to insert multiple microgrippers due to the size of the gripper. This gripper also can develop for other surgeries by placing the required SME and SE wire size in order to generate the corresponding gripping forces and jaw width.

Acknowledgements
I would like to offer my special thanks to my project Supervisor Eng. H.S.L. Perera, His patient guidance, enthusiastic encouragement and useful critiques guided me to accomplish the goal of this project successfully.

I would also like to extend my thanks to all other academic staff teachers of the Department of Mechanical Engineering at Open University of Sri Lanka for their help in offering me the necessary resources for successful completion of the project.

Finally, I wish to thank my parents for their support and encouragement to completion of this project.

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