Internationalof Computational
Intelligence
Systems
Journal ofJournal
Robotics,
Networking andArtificial Life
Vol.
XX(Z);
Month(2018),(Year), pp.
pp. 194–198
xx–yy
Vol.
5(3);
December
DOI:
10.1080/XXXXXXXXXXXXXX;ISSN XXXX–XXXX
onlineDOI:
10.2991/jrnal.2018.5.3.11;
ISSN 2405-9021
print; 2352-6386
onlinehttps://www.atlantis-press.com/journals/ijndc
https://www.atlantis-press.com/journals/jrnal

Research Article

Design and Fabrication of a Minimally Invasive Surgical Device
with Customized Shape Memory Alloy Spring Actuator
Uditha Roshan1,*, Ranjith Amarasinghe1, Nuwan Dayananda2
Department of Mechanical Engineering, University of Moratuwa, Katubedda 10400, Sri Lanka
Department of Electronic and Telecommunication Engineering, University of Moratuwa, Katubedda 10400, Sri Lanka

12ARTICLE INFOABSTRACT

Article History

A shape memory alloy (SMA)-based spring actuated gripper (SAG) was designed and developed for facilitating minimallyinvasive surgeries (MIS). Specifically the research consists of design and development of a SMA spring which acts as the actuator
for the gripper. A novel mechanism was also developed to transfer the actuator force to the gripper jaws. Further, the actuator
force was characterized by a testing apparatus which uses a full-bridge load cell as the force sensing element. The activation
temperature of the SMA actuator was obtained by performing differential scanning calorimetry (DSC) tests on the samples.

Received 15 July 2018
Accepted 11 November 2018
Keywords
Shape memory alloy
minimally invasive surgeries
gripperdifferential scanning calorimetry© 2018 The Authors. Published by Atlantis Press SARL.
This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

1. INTRODUCTION
Minimally invasive surgery (MIS) is an emerging and highly favor­
able surgery procedure compared to open surgeries. MIS has been
rapidly developed since late 1980s and it was widely accepted due toseveral advantages over conventional surgeries [1]. The equipmentused to perform MIS are ranging from simple to high-end remote-­
controlled devices with the capability to observe the surgical field [2].
The surgical manipulations are performed in a confined space andtherefore miniaturization of actuators in such tools is a necessity.
For this there are several approaches with each one having their own
advantages as well as disadvantages [3]. Among those, using shapememory alloy (SMA) as an actuation material can be considered an
attractive approach [4]. The best known SMA for biomedical applications is called NiTiNOL which has several advantages [5].
Over the years several research groups have developed gripper
prototypes which are actuated by SMA actuators [6] with different
techniques. According to the literature, in most cases Joule heating was used as the activation method of SMA material. However,in this research [7], the SMA material was activated by a heatedfluid flow. Temperature and flow rate of the fluid were controlledcorresponding to the activation requirements. In addition, a compact mechanism was designed to transfer the force generated by
the actuator to the gripping jaws. The kinematics of the mechanism
was analyzed to justify the motions. This is highly important for
the enhanced performance of the SMA-based spring actuated gripper (SMASAG). The SMA actuator was developed as a spring toget suitable stroke and force. Developed spring type actuator wascharacterized by using a specially developed force measuring* Corresponding author. Email: tauroshan@gmail.com

apparatus which uses a full-bridge load cell. Furthermore, testing was conducted to find the transformation temperatures of theNiTiNOL material by differential scanning calorimetry (DSC). The
actuator mechanism was developed by simple manufacturing techniques and 3D printing was used to develop the gripper jaws andthe enclosure of the mechanism.

2. DESIGN OF THE SMASAG AND
WORKING PRINCIPLE
2.1. Working Principle of the SMASAG
Proposed gripper was constructed by integrating several components. The functionality and importance of each will be describedin this section.
The gripper was actuated by a linear actuator structure which wasdesigned as a piston/cylinder assembly (Figure 1). It has an end nut
which facilitates easy disassembly. Both the end nut and the piston
have mounts to attach the SMA spring inside the cylinder. There
are openings in the cylinder body for water to pass for the heating
and cooling of the SMA spring. The stroke length of the actuator
was designed to be 5 mm.
The linear motion of the piston was converted to an angular motion
by the link which is hinged to the base structure. The link consistsof two slots that limit the motion and two pins that connect thepiston and the driver jaw. The slots were designed in a manner inwhich no sudden lockups will occur.
The rotational motion of the link is transferred to the driver jaw by
the reaction force between the pin and slot joint which makes the jaw

U. Roshan et al. / Journal of Robotics, Networking and Artificial Life 5(3) 194–198

195Figure 2 | The SMA shape setting fixture designobject and partial opening of gripper jaws. On the other hand,
having a low spring constant may cause partial gripping. Therefore,
the following Eqs. (1)–(10) were used to design the torsional springwith an appropriate spring constant.
Figure 1 | Proposed gripper assembly [7]rotate around a hinge that is located at the Base. In order to compact
the mechanism, the gripper pair was connected through a sector gearpair. The main two actuation elements are the SMA spring and thesteel torsion spring which support the opening and closing of the gripper jaws respectively. The development and the characterization of theSMA and the torsion spring is elaborated in the following subsections.

2.2. Design of the SMASAG
After the designing of the SMASAG, the actuator supporting structure or the outer packaging of the actuator, the SMA based spring
and the torsional spring had to be designed.

r F cos b		 FLJ = A SMA
(1)rB		 L 1x = L 10 −The outer packaging or the structure of the actuator consists of a cylinder which has an outer diameter of 20 mm, a bore of 10 mm and alength of 50 mm. Also, the piston has a diameter which matches wellwith the cylinder bore to provide smooth movement and sealing toprevent fluid leakages. Here, the piston is 33.7 mm in length. Theend nut facilitates the final assembly of the actuator. It has 15 mm
across flats and threaded to the cylinder by an M12 standard thread.
There are two mounts for the SMA spring attachment and both arethreaded to the end nut and piston by M6 standard thread.

2.2.2. Design of the SMA spring
The design of the SMA spring is related to the SMA shape setting
operation within which a fixture was designed to constrain thespring shape.
According to the design dimensions (in mm) in Figure 2, the SMA
spring has four turns with 2.24 mm pitch and the spring diameteris 8 mm.

2.2.3. Design of the torsion spring
The torsion spring provides the closing action of the gripper jaws.
Having a high spring constant may cause excessive force on the2

(2)

22
C = L 10+ L21x - 2 L10 L1x cosq (3)		 		 L Cx = L C 0 + C (4)F LJ L H cos(a + b ) + FG L G		 F JS =(5)L Cx		 N E =L 10 + L 20
3p D T(6)		 N A = N B + N E (7)kper turn =dT4
(8)10.8D T N A		 FSJ =dT 4q
(9)10.8DT N A 2p		

2.2.1. Design of the outer packagingof the actuator

DT q		 FSJ = FJS (10)Here NA, NB and NE are the actual, body and effective turns of thetorsional spring, respectively.
Figure 3 illustrates the parameters used in the above equations.
Equations (1)–(5) were used to calculate the force exerted on thetorsional spring due to the actuation force by the SMA spring.
Equations (6)–(10) are related to finding the number of turns ofthe torsion spring.

3. FABRICATION OF THE SMASAG ANDFORCE CHARACTERIZATION OF
THE SMA ACTUATOR
3.1. Fabrication of the SMA Actuator
A commercially available SMA material called NiTiNOL was used
to fabricate the spring and its composition is Ni - 55%; H - 0.001%;
O - 0.05%; N - 0.001%; C - 0.05%; Ti - Balance. The acquired
NiTiNOL material was in the form of straight wires with different
diameters and transformation temperatures. Hence, a fixture wasrequired in order to get them in the correct spring shape while conducting shape setting at high temperature.

U. Roshan et al. / Journal of Robotics, Networking and Artificial Life 5(3) 194–198

196

As the NiTiNOL wire tends to regain its trained shape whenincreasing the temperature beyond Austenitic final temperature,
it is important to use fixtures (Figure 4) when inducing newly
defined shape into the material. Hence, the fixture was placed ina Muffle furnace according to the temperatures and aging times
indicated in Table 1. Finally, the sample was quenched in water toobtain the final spring shape (Figure 5).
After shape setting it is important to investigate the SMA characteristic temperatures which are commonly known as transformation
temperatures, through a DSC test.

3.2. Investigating the Transformation
Temperatures of Shape Set Springs
Differential scanning calorimetric test is primarily based on monitoring the energy changes that are happening in the samples during
heating and cooling cycles [8]. These energy changes would either
be exothermic or endothermic. By obtaining a heat flow graph withrespect to temperature, characteristic temperatures of a SMA sample
can be calculated. For this procedure, tests were conducted by DSC
Q200 instrument [9]. Tzero aluminum pans were used as the reference and the sample pans. The samples were subjected to a thermal
program where they were ramped from 0 to 120°C at 2°C/min temperature increments and then cooled from 120 to 0°C at 2°C/min
temperature decrements. Then the data was stored and plotted as isdepicted in the following Figures 6 and 7 and Table 2 summerizes

Figure 5 | The Shape set NiTiNOL spring

Figure 3 | Free body diagram and calculation parameters

Figure 6 | DSC thermogram for sample #1

Figure 4 | Fabricated fixture
Table 1 | NiTiNOL wire shape setting details
Sample no. #12

Wire diameter(mm)Shape settingtemperature (°C)

Aging time(min)

1.0
1.0

500
500

2530

Figure 7 | DSC thermogram for sample #2

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197

the results observed from the DSC test. Ms, Mf, As and Af indicate
the Martensite start and finish temperatures and Austenite start andfinish temperatures respectively (Table 2).

and the other end spring was attached to measure the force as inFigure 9c. Finally, the container was filled with water and its temperature was gradually increased by a heater. The full-bridge loadcell was connected to a 24-bit load cell amplifier (HX711).

3.3. Fabrication of the Outer Packaging andthe Gripper

The HX711 was connected to a microcontroller (ATmega 2560)
based development board in order to acquire data. After obtaining raw data values from the amplifier, the development board
was made to transmit the raw data to a computer through serial
communication and all the data processing tasks were conductedwithin a LabVIEW generated program. In addition, a graphicaluser interface (GUI) was developed to plot and display the data.

The fabricated SMASAG is depicted in Figure 8. The base structure
and the gripper jaws were made from ABS Plus material with theuse of 3D printing technology.
Actuator piston and cylinder was made from aluminum and brassmaterials respectively. SMA element mounts were made out of brass
and threaded to the piston and End nut which is made out of steel.
All the commonly available materials were used because of achieving easiness of prototyping. When miniaturizing for the actual
application more reliable bio-compatible materials need to be used.
Miniaturization of the actuator for real application would makemore challenges. Commonly used sensing methods in the macroscale cannot be used in such scenario for temperature measurement. So MEMS based temperature sensing [10] would be more
ideal in such situations.

Simultaneously the temperature of the water was measured througha four and half true RMS Digital Multimeter which is compatible
with thermocouple probes. The acquired values were plotted andFigure 10 shows the variation of force with the temperature forabc

3.4. Force Characterization of theSMA Spring
After shape setting the NiTiNOL wire into a spring, it is important to have an understanding about the amount of force that canbe exerted through it on the mechanism. Therefore, an apparatus
was developed to measure the force that can be achieved when thesprings are constrained to recover their initial shape.
The apparatus is shown in Figure 9a. It consists of a full-bridge loadcell and a container which was used to immerse the spring in water
while one end was rigidly connected to the base of the container
Table 2 | Transformation temperatures estimation from DSC test
Sample no. #12

Sample weight (mg)17.62518.462Ms (°C) Mf (°C) As (°C) Af (°C)
37
23.2

24.6
4.8

Figure 8 | The fabricated SMASAG

46.2
55.5

59.5
72.4

Figure 9 | (a) Force measuring apparatus. (b) Calibrating by known
weights. (c) NiTiNOL spring attachment

Figure 10 | Force characterization results

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198

the NiTiNOL springs which had the same coil and wire diameterwith different transformation temperatures. These forces can beachieved in a controlled manner [11,12].

4. CONCLUSION
In this study, a SMA spring actuator-based gripper was designedand fabricated. For this purpose, the tasks carried out included
the force characterization of a NiTiNOL spring and development of a novel mechanism to transfer the actuator force to thegripper jaws. The results showed that the required force could
be achieved quickly for the springs having lower transformation temperatures if they were subjected to the same conditions.
Furthermore, to investigate the transformation temperature of
the actuator material, a DSC test was conducted. The test result
showed that the Austenite finish (Af ) temperature had increased
with the annealing time.

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Authors Introduction
Dr. Ranjith Amarasinghe

Mr. Uditha Roshan
Uditha Roshan received his B.Sc. (Hons.)
degree in Mechanical Engineering fromUniversity of Moratuwa, Sri Lanka,
in 2016, and is currently pursuing hisM.Phil. degree in the Department of
Mechanical Engineering, University ofMoratuwa.

Dr. Nuwan Dayananda
Dr. Nuwan Dayananda is a Senior Lecturer
at the Department of Electronic & Tele­
communi­cation Engineering, Univer­sity of
Moratuwa, Sri Lanka. He received his Ph.D.
in Biomedical Engineering from Western
University, London, Ontario, Canada in2008. His research is focused on using technology to solve problems in medicine andbiology. He is specifically interested in medical imaging and medical image processing related research such as image based diagnosis, image guided surgery and therapy, and surgical simulators.

Dr. Ranjith Amarasinghe received his B.Sc.
(Hons.) degree in Mechanical Engineeringfrom University of Moratuwa, Sri Lanka,in 2000, his Masters degree in Information
Science and Systems Engineering in 2003
and Dr. Eng. degree in Micro Electro
Mechanical Systems (MEMS) in 2007
from Ritsumeikan University, Japan.
He served as a postdoctoral fellow under Japan Society for thePromotion of Science (JSPS) from 2008 to 2010, and as a Chair
Professor from 2010 to 2011 in the Department of Micro Systems
Technology, Ritsumeikan University, Japan. From 2011 he joined
the University of Moratuwa as a Senior Lecturer.