INFRARED THERMOGRAPHY AS A SUPPORT TOOL FOR

DEVELOPING SHAPE-MEMORY POLYMER BIODEVICES

Andrés Díaz Lantada, Pilar Lafont Morgado, Héctor Lorenzo-Yustos, Julio Muñoz-García

José Luis Muñoz Sanz, Javier Echavarri Otero and Juan Manuel Munoz-Guijosa

Grupo de Investigación en Ingeniería de Máquinas – E.T.S.I. Industriales – Universidad Politécnica de Madrid

C/ José Gutiérrez Abascal, nº 2. 28006 – Madrid, Spain

Keywords: Infrared thermography, Shape-memory polymers, Biodevice development, Prototype testing.

Abstract: Infrared Thermography is a technique for carrying out inspections and non-destructive tests that can also be

used as a support tool for developing medical devices based on the use of shape memory polymer materials.

This paper sets out some of the opportunities and advantages provided by this technique for designing the

heating systems associated with these shape-memory polymer based devices. Its application for developing

an active pincer, whose geometry can be changed by heating, is explained in detail. Similar devices can be

used as active catheter ends for minimally invasive surgery tasks.

These thermography tests can also be used as a validation tool for the heating simulations, linked to

optimising this type of devices, and also during “in vitro” tests aimed at obtaining safer active implantable

devices.

1 INTRODUCTION TO

INFRARED THERMOGRAPHY

Infrared (IR) Thermography is a technique for

carrying out inspections and non-destructive tests

which has multiple applications in the development

of machines and products, equipment and facilities

maintenance, and troubleshooting.

Since all bodies emit (according to their

temperature) infrared radiation, which increases in

intensity as the temperature rises, variations in this

intensity can be detected by using infrared sensors.

Thermal cameras can detect radiation in the

infrared range of the electromagnetic spectrum

(usually between a 900 and 14000 nm wavelength,

instead of operating in the visible range of 450 to

750 nm) and can produce images of this radiation.

These cameras are fitted with a sensor matrix

(called microbolometer) that is sensitive to this

radiation. Depending on the intensity of the radiation

more or less current is sent to the camera’s control

electronics, which with the aid of specific software

enables temperature maps to be obtained.

The spheres of application range from

Mechanical Engineering, Electrics, Electronics,

Aeronautics, Architecture and Engineering in

general, to Medicine or Veterinary Science and even

Art and Archaeology. The last decade has seen

enormous progress in the equipment available on the

market as well as more affordable prices that have

led to its expansion as a testing technique in all

kinds of industry.

Some of the fundamental advantages of the

technique are its speed and ease of use, easy to

interpret temperature map-based results and the fact

that it is a non-destructive technique that does not

damage the systems under study (Schindel, 2007,

Maldague, 2001).

Apart from these applications, its use as a

support tool for developing medical devices,

especially those based on the use of shape-memory

polymer (SMP) materials, is detailed in this work.

The main objectives of the study, as exposed in

the following chapters, are:

• To show the importance of validating FEM

thermal simulations with trials.

• To explain how IR thermography can be used in

such validating trials.

• To make clear how both techniques should be

used in a combined way for improving SMP-

based medical devices development.

145

Diaz Lantada A., Lafont Morgado P., Lorenzo-Yustos H., Muñoz-García J., Luis Muñoz Sanz J., Echavarri Otero J. and Manuel Munoz-Guijosa J. (2009).

INFRARED THERMOGRAPHY AS A SUPPORT TOOL FOR DEVELOPING SHAPE-MEMORY POLYMER BIODEVICES.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 145-150

DOI: 10.5220/0001124001450150

 SciTePress

2 SHAPE-MEMORY POLYMERS

AND POTENTIAL BIODEVICES

Shape-memory polymers (SMPs) are materials that

show a mechanical response to external stimuli,

usually to changes of temperature. When these

materials are heated above their “activation”

temperature, there is a radical change from rigid

polymer to an elastic state that will sometimes allow

deformations of up to 400%. If the material is cooled

after manipulation it retains the shape imposed; the

said structure is “frozen” and returns to a rigid but

“unbalanced” state. If the material is again heated

above its glass transition temperature or “activation

temperature” it recovers its initial non-deformed

state.

The cycle can be repeated numerous times

without degrading the polymer and most suppliers

can formulate different materials with activation

temperatures of between –30 ºC y 260 ºC, depending

on the application required. Of all the polymers

developed that show shape memory properties, those

most worthy of mention are epoxy resins,

polyurethane resins, cross-linked polyethilene,

styrene-butadiene copolymers, polynorbornene and

other formulations (Lendlein, 2002, 2005, Liu,

2007).

They are therefore active materials that present

thermomechanical coupling and a high capability for

recovery from deformation, (much greater than that

shown by shape memory metal alloys), which

combined with their lower density and cost has

favoured the appearance of numerous applications.

Their properties permit applications for

manufacturing sensing devices or actuators,

especially for the aeronautics, automobile and

medical industry.

They have been proposed to develop numerous

medical devices such as self-expanding stents

(Wache, 2003), intelligent sutures (Lendlein, Kelch,

Langer, 2002, 2005), thrombectomy devices

(Wilson, 2006), active catheters (Yackaki, 2007),

drug delivery devices (Gall, 2004) or annuloplasty

systems (Díaz Lantada, 2008).

As an example of the capability of these

materials to recover their geometry Figure 1 shows

the closure of a pincer manufactured in epoxy resin

(whose trade name is Accura

60) when its memory

is activated by heating (under forced convection

using a hot-air gun with air at 80 ºC).

Similar devices, introducing appropriate changes

to their geometry according to the application, could

be used as the active end of a catheter to remove

harmful particles, clots and other elements.

Figure 1: Shape-memory effect in an epoxy resin pincer.

The following section explains how infrared

thermography can help optimise the development of

shape memory polymer-based medical devices.

3 THERMOGRAPHY AND SMP

BIODEVICE DEVELOPMENT

3.1 Operational Considerations

Generally speaking, the total power emitted per unit

of area is given by Stefan-Boltzmann’s Law, which

for a black body is expressed using the constant σ:

· T

(1)

It is important to point out that the concept of a

“black body” is an ideal concept as the real objects

are “grey bodies”, which means the concept of

emissivity “ε” has to be taken into account, giving

the equation:

ε ·

· T

(2)

Where emissivity shows values in the 0 < ε < 1

range and relates the radiation that would be emitted

by an ideal black body at the same temperature. This

constant is highly dependent on the material’s

surface and its finish, and on the wavelength and

surface temperature, and can have a marked

influence on the results of the tests performed with

infrared thermography equipments.

So, when conducting thermography tests, the

value of the said emissivity needs to be selected

depending on the material of the object or device

under study. The camera itself incorporates typical

values for different metals, polymers, ceramics and

other materials.

For materials not included in the camera

software, the emissivity of the material can be

determined by painting a part of the object or device

with black optical paint, for example Nextel Black

Velvet, which gives an emissivity very close to 0.94.

5 mm

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146

Then the value of the emissivity in the camera can

be changed for the original material surface until its

reading is adjusted to the pre-measured value for the

reference painted zone.

Figure 2 shows an example of the influence of

surface colour on the emissivity of the pincer

prototypes manufactured in shape-memory epoxy

resin. It shows how the different emissivity leads to

erroneous measurements of temperature, with

differences of around 3 ºC, even though all the

prototypes are the same temperature.

Figure 2: Importance of calibrating the emissivity of the

material.

Therefore, for the above prototypes, according to

the data in Figure 2, the emissivity values would be:

Black pincer.- 0.94

White pincer.- 0.92

Translucent pincer.- 0.86

This calibration of material emissivity is

particularly important when the device has to fulfil

its mission inside the human body, where

differences of 2 to 3 ºC can be decisive for avoiding

damage to tissue surrounding the end device.

Other environmental factors such as wind, rain

or snow, whose influence is usually listed in the

correction tables supplied by the thermography

equipment manufacturers, may be omitted if the

characterisation tests are conducted in a laboratory.

3.2 Characterization of Materials and

Applications

As already explained, in order to produce geometric

changes in active implantable devices based on

shape-memory polymers the temperature of the

material needs to be raised above its glass transition

temperature. In the development process of these

devices infrared thermography can be used as a

support tool for other characterization and design

technologies, particularly regarding:

• Determining the activation temperature in

the different candidate materials.

• Designing and optimising the heating

system to activate a change in the device

geometry.

• Validating thermal simulations so that

design decisions can be made more quickly.

• Carrying out the shape memory training

process at the most suitable temperatures.

• Carrying out “in vitro” tests, as a prior step

to tackling “in vivo” tests.

The last four points put forward are set out in the

following sections, since determining the activation

temperatures of the different candidate materials is

usually performed with DSC or DMTA tests

(Mather, 2002, Volk, 2005).

3.3 Designing the Activation System

To make the preliminary design for the heating

system the number and value of the heating resistors

can be pre-selected (in case of Joule effect based

activation) in line with the following calculation

procedure.

If the resistors in the active device are required

to reach a steady state temperature above shape-

memory activation temperature, the power generated

must be equal to the leakage at that temperature. It

must be thus verified that:

q = n · R · I

= C · S · (T

iodevice

– T

) (3)

With the following notation:

q.- Heat generated [W].

n.- Number of resistors connected in series.

R.- Resistance [Ω].

I.- Intensity through the resistors [A].

C.- Heat transfer coefficient [W/(m

·K)].

S.- External surface of the device [m

Having selected the heating resistors the

transient state can also be evaluated and the time

INFRARED THERMOGRAPHY AS A SUPPORT TOOL FOR DEVELOPING SHAPE-MEMORY POLYMER

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147

estimated that the device will take to reach

activation temperature, as will now be explained:

q = n · p = m · c

· dT/dt + C · S · (T – T

) (4)

With the following notation:

m.- Device mass [kg].

.- Specific heat of the SMP [J/(kg·K)].

T.- Device temperature [ºC ó K].

p.- Power generated by each resistor [W].

The differential equation is integrated between

the initial temperature, usually 37 ºC, and the final

temperature required for activation, which gives the

necessary activation time.

However, in order to optimise the design of the

heating system required to activate the geometric

change in a shape-memory material based device,

the combined use of infrared thermography tests and

simulations made applying the finite element

method is highly valuable.

This procedure will be explained below for the

shape memory pincers already mentioned, which

could be used as an active catheter end in minimally

invasive surgery tasks.

The properties of the epoxy resin used for

manufacturing the prototypes are listed in Table 1

and have been used when designing the device as

well as for the simulations performed.

Table 1: Properties of the material used.

Accura

60 Epoxy Resin

Density 1.21 g/cm

Tensile Strength 58 – 68 MPa

Tensile Modulus 2690 – 3100 MPa

Glass Transition (T

) 58 ºC

Hardness, Shore D 86

It is important to emphasise that although its

activation temperature of around 60 ºC could not

result in a safe intracorporeal device, there are

several shape-memory polymers whose activation

temperatures are closer to human body temperature

and which could be subjected to a procedure similar

to that set out in this work.

A heating resistor of 4.7 Ω was chosen for the

tests and it was fitted into the device in an

specifically designed housing, which was finally

filled with additional epoxy resin. The prototype is

shown in Figure 3.

Figure 3: Epoxy resin pincer with built-in heating resistor

for activation.

This prototype was heated above the glass

transition temperature of the epoxy resin by using a

hot air gun with air at 80 ºC for forced convection.

The temporary shape was obtained by inducing

opening using traction perpendicular to the middle

plane of the pincer, just 2 mm above the resistor. In

this way, most of the temporary deformation is

produced in the zone near to the heating resistor,

which means that the initial shape can be recovered

by heating a small area of the device.

The low conductivity of the polymers means that

the use of several heating resistors is usually

required for devices with geometric changes in

different zones. The way of giving our device its

temporary shape means that the tests can be carried

out with a single resistor.

Figure 4 shows heat activation of the shape-

memory polymer based pincer. For the infrared

thermography tests, shown in Figure 5, a variable

voltage transformer was used. The first image shows

the situation under a steady state with a 0.6 W

heating power (geometric change activation is not

reached) while the second shows the situation under

a power of 1 W (which allows T

to be exceeded and

activates the device).

Figure 4: Activation by heating the pincer and recovery of

its geometry.

The ANSYS 9.0 finite element calculation

program was used to carry out the heating

simulations. Heating powers of 0.6 and 1 W were

used, similar to those used in the thermography tests,

with the purpose of comparing the results.

Additional data on the material and boundary

conditions used in these tests are set out in Table 2.

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Figure 5: Infrared thermograph of the pincer during the

heating process, below (54.7ºC) and above (92.5ºC)

activation temperature.

Table 2: Additional data for finite element simulations.

Thermal conductivity 0.2 W/(m·K)

Specific heat 1.6 J/(g·K)

Emissivity 0.86

Convection coefficient 15 W/(m

·K)

Ambient temperature 26 ºC

Isotropic material hypothesis

The results of the simulations carried out are

shown in Figure 6 as steady state temperature maps

and its relation with the thermography tests

performed is also explained.

It is important to point out the similarity of the

results given by simulations and tests, which makes

us specially confident in the usefulness of our

simulations for future heating system optimisations.

For a 0.6 W heating, the steady state temperatures of

the hottest zone given by testing and simulation are

54.7 ºC and 52.8 ºC respectively. For 1 W of

heating, these temperatures reach 92.5 ºC in the test

and 92 ºC in the simulation. The temperature

distribution is also similar, with errors of less than

5% overall.

So, infrared thermography can be used as a tool

in non-destructive tests for validating the results of

simulations performed as part of the design process

for shape memory polymer-based devices.

Figure 6: Heating simulations. Solution of steady state

temperatures for heating powers of 0.6 and 1 W.

3.4 The “Memory” Effect Training

Process

To enhance the process of obtaining the temporary

shape of the shape memory polymer device, instead

of using oven convection heating or the help of a hot

air gun, the resistors in the activation system of the

device itself can be used.

In this way, with the help of associated control

electronics, the glass transition temperature can be

exceeded in specific zones of the device where the

geometry could be mechanically changed.

The use of thermography devices can be useful

for controlling and limiting the heating of the device

so that it will only exceed the T

in the zone whose

geometry is wished to change temporarily, the rest

of the structure remaining unaltered.

3.5 Carrying Out “in vitro” Tests

Infrared thermography is also useful when it comes

to carrying out “in vitro” tests for validating the

device before going on to the “in vivo” tests.

In a similar way to the examples shown, the

temperature reached by the surface of a prototype of

a SMP-based implantable device during activation

can be checked. It can be checked whether this

INFRARED THERMOGRAPHY AS A SUPPORT TOOL FOR DEVELOPING SHAPE-MEMORY POLYMER

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149

temperature is harmful for the tissue that will be in

contact with the device. In addition the effectiveness

of using protective coatings can also be studied.

Indeed, by implanting the device “in vitro” and

taking a thermograph during the activation process,

the temperature of the tissue surrounding the device

can be measured at any time to ensure that no

harmful heating is being produced.

4 CONCLUSIONS

The work presented shows the use of infrared

thermography as a support tool for the development

of medical, surgical or implantable devices based on

the use of thermally activated SMPs.

This technology is extremely helpful for

designing the activation system of these devices and

for validating heating simulations aimed at

optimising their development. As an application,

tests and simulations carried out with active pincers

obtained by rapid prototyping are shown. With slight

changes to their geometry and size, such devices

could be used as active catheter ends for minimally

invasive surgery.

Thermography is also extremely useful for

carrying out “in vitro” tests and obtaining safer end

implantable devices, which would not potentially

harm the surrounding tissues by heating, as a result

of optimising the heating and isolating system using

combined FEM simulations and thermography trials.

As future challenges it would also be interesting

to apply such a procedure to medical devices based

on other active materials that can be thermally

activated. It could therefore be applied to improve

the response of actuators manufactured with shape

memory alloys or sensors based on pyroelectric

materials.

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