INTELLIGENT EXPANDABLE STRUCTURES BASED ON THE

IMPROVED ACTIVATION OF SHAPE-MEMORY POLYMERS

Andrés Díaz Lantada, Pilar Lafont Morgado, 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: Shape-Memory Polymers (SMPs), Biodevices, Expandable structures, Improved activation.

Abstract: Shape-memory polymers are active materials with thermomechanical coupling and a high capability to

recover from high levels of deformation, which, combined with their low cost and density has favoured the

appearance of numerous applications, particularly those linked to the Medical Industry. In many cases, these

materials are of medical standard, which increases the chances of obtaining biocompatible devices.

In the last decade enormous progress has been made on many areas, regarding these materials, such as

synthesis, characterization, activation, prototyping and others, aimed at improving their applicability.

However, various spheres of action require additional in depth research to promote the production start-up

of various shape-memory polymer-based devices that have had laboratory validation. One of these areas of

improvement is linked to the activation systems of SMPs.

This work sets outs the possibility of obtaining a more homogeneous heating processes for an optimal

activation of the “shape-memory effect”, which promotes the geometric changes of such devices. These

improvements are based on the development of net-shaped SMP structures to which silver thread is knitted

for subsequent activation through Joule heating. First prototypes and trials are explained in detail, as well as

the possible biomedical applications of this concept.

1 INTRODUCTION TO

SHAPE-MEMORY POLYMERS

(SMPS)

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 allow

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

down after manipulation it retains the shape

imposed; the said structure is “frozen” and returns to

a rigid but “non-equilibrium” state. If the material is

again heated above its vitreous 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 ranging from

–30 ºC to 260 ºC, depending on the application

required. Of all the polymers developed that show

shape-memory properties, those most worthy of

mention due to their applicability are epoxy resins,

polyurethane resins, cross-linked polyethilene,

styrene-butadiene copolymers and other

formulations (Lendlein, Kelch, 2002, 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 the

development of sensing devices and actuators,

especially for the aeronautics, automobile and

medical industry.

Figure 1 shows a scheme of the training and

actuation process of SMP structures.

240

Díaz Lantada A., Lafont Morgado P., Muñoz-García J., Luis Muñoz Sanz J., Echavarri Otero J. and Manuel Munoz-Guijosa J. (2010).

INTELLIGENT EXPANDABLE STRUCTURES BASED ON THE IMPROVED ACTIVATION OF SHAPE-MEMORY POLYMERS.

In Proceedings of the Third International Conference on Biomedical Electronics and Devices, pages 240-245

DOI: 10.5220/0002766602400245

 SciTePress

Figure 1: Training process of the shape-memory effect and

recovery of permanent form.

2 POTENTIAL FOR BIODEVICES

2.1 Some Advantages

As polymers, SMPs can be easily conformed into

different complex shapes and their properties

designed or adapted to specific applications and can

also be integrated with other microelectromechanical

sensors (MEMS) to produce “intelligent”

bioactuators and biodevices.

Compared to other shape-memory alloys used in

numerous medical devices, SMPs show a far greater

capability for changing their geometry during

activation. They are also much cheaper to synthesise

and their large scale mass production costs are

reduced by using technologies such as injection

moulding. All this makes them very versatile active

materials with a high potential for industry, provided

they overcome some of the limitations set out in the

following sections.

2.2 Proposed Devices

Bellow are explained some specific proposals for

developing medical devices based on the use of

shape-memory polymers, most of which have

undergone in vitro laboratory testing. After

undergoing in vitro testing and meeting the

requirements for official approval, in some cases

their commercialisation is subject to their attaining

the goals described at the end of this paper.

Self-expanding Stents. Like the stent designed by

Boston Scientific Corporation using the polymer

from CRG Industries known as “Veriflex” under its

trade-name, to treat the problems arising when the

arteries become narrow or obstructed and also for

removing obstructions from other “tube-shaped”

body parts, like the uretheres and the bronchial

tubes. The stent is inserted in its temporary form

(reduced) and the body’s own heat causes it to dilate

and become attached to the artery. They may be

used to replace stents based on shape-memory alloys

such as Nitinol, once the appropriate

biocompatibility studies have been carried out.

Developments of self-expanding stents have also

been carried out by using injected polyurethane

(Wache, 2003).

Intelligent Sutures. Like those developed at the

Forschungszentrum in Karlsruhe by Lendlein’s team

and at the M.I.T. by Langer’s team, which have a

temporary linear shape and a permanent shape in the

form of a knot, with the change in geometry being

activated by the body’s own temperature. They have

numerous applications in minimally invasive surgery

and, as they are biodegradable, they have additional

advantages over the use of textile sutures and metal

clips (Lendlein, Kelch, Langer, 2002, 2005).

Thrombectomy Devices. With the recent discovery

that the thermal effect of shape-memory can be

activated by a laser, part of whose energy is

absorbed by the polymer, devices with special

geometries have been proposed for removing clots

(Wilson, 2006). The polymer is shaped in a spiral

mould and then heated and stretched to give it its

temporary shape. When the laser light passes

through the polymer, the shape-memory effect is

activated and the device recovers its spiral shape

trapping the clot which can then be removed.

Active Catheters. By using shape-memory

polymers for the distal point of catheters together

with a subsequent activation of the memory effect

by laser light or body heat, different drugs and

antitumoral agents can be released. The presence of

an active catheter point can also help reach zones

that are difficult to access in minimally invasive

surgery tasks (Yackaki, 2007).

Drug Release Devices. If biodegradable shape-

memory polymers are used for implantable medical

devices, drug supply reservoirs can be incorporated

into the device itself. After implant, the polymer

begins to be absorbed by the organism and the drug

is released. Patents have been taken out in this

respect for self-expanding coronary stents or intra-

urethral stents (Boston Scientific Co. and Surmodics

Inc.). The possibility of obtaining temporary

geometries with micro-reservoirs for drug storage

has also been studied. The drugs would then be

INTELLIGENT EXPANDABLE STRUCTURES BASED ON THE IMPROVED ACTIVATION OF SHAPE-MEMORY

POLYMERS

241

released on activation of the shape-memory effect by

body heat (Gall, 2004).

Active Annuloplasty Rings. Aimed at obtaining a

progressive postoperative treatment of mitral

insufficiency, they are based on the use of a

polymeric ring with heating resistances distributed

around the inside to activate the shape-memory

effect by Joule effect. This activation must allow the

cross section of the mitral ring to be gradually

reduced and, therefore, the mitral insufficiency

improved.

3 CHALLENGES RELATED TO

THE ACTIVATION PROCESS

One aspect where most progress has been made in

the last years is the activation of the memory effect

by various methods, especially:

Joule Effect Activation. Based on distributing

heating resistances at the core of the polymer where

the passing of an electric current generates the

necessary heat.

Light or Laser Activation. Based on projecting a

laser through a shape-memory material with a

similar absorption frequency to that of the laser

used, which produces heating (Lendlein, 2005,

Wilson, 2006)

Magnetic Activation. Based on heating by

induction of magnetic or metallic microparticles,

distributed at the core of the polymer while it is

being conformed to its shape (Buckley, 2006).

However, the biocompatibility of the associated

devices needs to be further optimised.

Support Technologies. Progress in the field of

wireless communications means that devices can

now be remotely activated, which is promoting the

appearance of new active implantable biodevices.

One of the main problems of using Joule effect

activation lies in the fact that distributing punctual

heating resistances among the polymer leads usually

to a decrease of mechanical resistance and to

important temperature differences along the

structure during activation.

On the other hand, “light / laser activation” is

normally limited to tubular / linear structures and

“magnetic activation” usually implies problems due

to the important magnetic fields needed for heating.

We propose and explain in the following

chapters some improvements for SMP activation,

obtained by substituting punctual heating resistances

by silver-cloth thread knitted along the structure and

used as single Joule effect heating element.

4 IMPROVING ACTIVATION:

DISTRIBUTING HEAT

As previously stated our objective is to obtain SMP

structures with an improved distribution of heating

element(s) for a better controlled and more

homogeneous activation of “shape-memory effect”.

Figure 2 shows the prototype of a net-shaped

SMP structure with the heating silver-cloth thread

already knitted to it. Such thread was acquired from

the MUTR (www.mutr.co.uk) special materials

facilities.

For manufacturing the prototypes (directly from

3D CAD files with part geometry) a laser

stereolithography “SLA-3500” machine was used to

polymerise a 3D Systems epoxy resin sold under the

trade name of Accura 60. A total electrical resistance

of around 40 Ω was measured between ends of the

thread, which proves to be an acceptable value for

activating the whole structure through Joule effect

heating.

Figure 2: Prototype of net-shaped SMP structure with

knitted silver-cloth thread for improving activation.

(Resistance around 40 Ω).

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242

Figure 3: Temporary form obtained through heating,

imposing a 90º deformation between extremes of the

structure and finally cooling down.

Once the prototypes had been fabricated, and the

heating thread knitted to the structure, the “in vitro”

heating was carried out to verify the designed

heating system and show that it is possible to exceed

the glass transition temperature that leads to the

“shape-memory effect” being activated.

This was controlled using a thermographic IR

camera marketed under the name of “Flyr Systems

Thermacam E300” with its accompanying

“Thermacam Reporter 8.0” analytical software

which enables the thermographs taken to be more

exactly and thoroughly analysed.

A remarkable fact for the trials is to verify that

no short-circuit appears during the training process,

which limits the attainable size reduction. When

bending and reducing structure size during training,

the heating thread can separate slightly from the

polymeric material and promote such problems.

Therefore, fixing points for the heating element

have to be taken into account from the design stage,

so as to improve final performance. Another option

consists of totally embedding the heating element

within the polymeric structure during manufacture,

but that leads to additional problems when casting or

injecting into the moulds.

Figure 4: Recovery process by distributed heating. (View

using IR thermography). Total recovery time Æ 20

seconds. Images taken every 5 seconds (image

corresponding to t=0 not included).

INTELLIGENT EXPANDABLE STRUCTURES BASED ON THE IMPROVED ACTIVATION OF SHAPE-MEMORY

POLYMERS

243

Figure 5: Recovery process by distributed heating.

Recovery of SMP net-shaped structure with improved activation

100

0 5 10 15 20 25 30

Time (s)

Bending angle (º)

Temperature (ºC)

Recover

rate of protot

pe Temperature

Figure 6: Recovery process (from 90º to 0º) as a function

of time and temperature.

The results show a remarkable homogeneous

heating of the net-shaped structure and the

possibility of going beyond the glass transition

temperature of the polymer, so as to activate the

“shape-memory effect” and the subsequent

geometrical changes.

Additionally, as Figure 5 shows, it is important

to mention that the presence of the silver-cloth

thread for Joule heating does not limit the “shape-

memory effect training” capability or the ability to

recover its original shape through subsequent

heating above glass transition temperature. The

mismatch between original shape “A

” and final

shape “A

” after activation fulfills the relation:

- A

] / A

< 4%.

5 IMPROVING RESULTS AND

CONCLUSIONS

While the new capabilities brought by these

materials give rise to expectations that many medical

devices will become more effective, considerably

great effort still needs to be put into research and

development, so as to obtain robust and effective

actuators based on these materials.

Some advances linked to obtaining a more

homogeneous heating for improving activation have

been explained. First prototypes of net-shaped SMP

structures with silver-cloth thread knitted to them for

Joule activation have helped to validate the proposal.

It should be mentioned that although the

activation temperature of the materials used of

around 50-55 ºC could not give rise to safe

intracorporeal devices, there are shape-memory

polymers whose activation temperatures are closer

to human body temperature which could be

subjected to a process similar to the one described

here.

The choice of this material was influenced by its

availability in the UPM’s Product Development

Laboratory and by its good processing properties, as

well as its suitability for prototype manufacturing

directly from 3D CAD models, through “laser

stereolithography”. Future work will be focused to

the search and application of SMP formulations with

improved biocompatibility and more suitable

activation temperatures.

Additionally, once having obtained the

prototypes in end materials, different surface

deposits can be used by means of physical or

chemical steam deposition technologies (particularly

the new DLC “diamond like carbon”) coatings, as

well as textile coatings to ensure the

biocompatibility of the device and facilitate its

implantation.

At the same time other medical grade metals and

alloys will be studied as heating coils, so as to obtain

final more appropriate prototypes for “in vitro”

trials. The use of such “in vitro” trials, together with

results from simulations, will help to prepare “in

vivo” studies in a more confident way.

Non-medical applications of this kind of “SMP-

based devices” or “SMP-based smart structures”,

with similar or more complex geometries, can also

be promoted by the possibilities supplied by

distributed threads for Joule activation. Infrared (IR)

thermography proves to be a useful tool for

validating the capabilities of new concept prototypes

and as a support for design tasks.

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REFERENCES

Lendlein, A., Kelch, S., 2002. Shape-Memory Polymers.

Angewandte Chemie International.

Lendlein, A., Kelch, S., 2005. Shape-Memory Polymers.

Encyclopedia of Materials: Science and Technology.

Liu, C., Mather, P., 2007. Review of progress in Shape-

Memory Polymers. Journal of Materials Chemistry.

Wache, H., 2003. Development of a polymer stent with

shape-memory effect as a drug delivery system.

Journal of Materials Science – Materials in Medicine.

Lendlein, A., Kelch, S., 2005. Shape-Memory Polymers as

Stimuli-sensitive Implant Materials. Clinical

Hemorheology and Microcirculation.

Lendlein, A., Langer, R., 2002. Biodegradable, Elastic

Shape-Memory Polymers for Potential Biomedical

Applications. Science.

Wilson,T., et al., 2006. Shape-memory Polymer

Therapeutic Devices for Stroke. Lawrence Livermore

National Laboratory.

Small, W., et al., 2005. Laser-activated Shape-Memory

Polymer Intravascular Thrombectomy Device. Optics

Express.

Yakacki, C.M. et al., 2007. Unconstrained Recovery of

Shape-Memory Polymers Networks for

Cardiovascular Applications. Biomaterials.

Yakacki, C.M. et al., 2008. Deformation Limits in Shape-

Memory Polymers. Advance Engineering Materials.

Gall, K.; Kreiner, P. et al., 2004. Shape-memory Polymers

for MEMS Systems. Journal of

Microelechtromechanical Systems.

Díaz Lantada, A., Lafont, P. et al., 2008. Treatment of

Mitral Valve Insufficiency by Shape-Memory Polymer

Based Active Annuloplasty. Biodevices 2008 –

International Conference onBiomedical Electronics

and Devices. INSTICC Press.

Lafont, P., Díaz Lantada et al., 2006. Patent Document

P200603149: Sistema activo de anuloplastia para

tratamiento de la insuficiencia mitral y otras patologías

cardiovasculares. Oficina Española de Patentes y

Marcas.

Bellin, I., et al., 2006. Polymeric Triple-Shape Materials.

Proceedings of the National Academy of Sciences.

Volk, B. et al., 2005. Characterization of Shape-memory

Polymers. NASA Langley Research Centre. Texas

A&M University.

Tobushi, H. et al., 2008. Shape Recovery and

Irrecoverable Strain Control in Polyurethane Shape-

Memory Polymer. Science and Technology of

Advanced Materials.

Liu, C. and Mather, P., 2002. Thermomechanical

Characterization of a Tailored Series of Shape-

memory Polymers. Journal of Applied Medical

Polymers.

Liu, Y. et al., 2003. Thermomechanical Recovery

Couplings of Shape-memory Polymers in Flexure.

Smart Materials and Structures.

Huang, W. and Lee, C., 2006. Thermomechanical

Behaviour of a Polyurethane Shape-memory Polymer

Foam. Journal of Intelligent Material Systems and

Structures.

Liu, Y. et al., 2006. Thermomechanics of shape-memory

polymers: Uniaxial experiments and constitutive

modelling. Int. Journal of Plasticity.

Yakacki, C.M. et al., 2007. Unconstrained recovery of

shape-memory polymer networks for cardiovascular

applications. Biomaterials.

Harrysson, O., et al., 2007, Custom-designed orthopaedic

implants evaluated using FEM analysis of patient

computed tomography data. BMC Musculoskeletal

Disorders.

Paumier, G., et al., 2008. Thermoresponsive Polymer-

Based Microdevice for Nano-Liquid Chromatography.

Biodevices 2008 – Int. Conference on Biomedical

Electronics and Devices. INSTICC Press.

Lendlein, A., et al., 2005. Light-induced shape-memory

polymers. Nature.

Buckley, P., et al., 2006. Inductively Heated Shape-

memory Polymer for the Magnetic Actuation of

Medical Devices. IEEE Transactions on Biomedical

Engineering.

Conti, S., et al., 2007. Modelling and Simulation of

Magnetic Shape-Memory Polymer Composites.

Journal of Mechanics and Physics of Solids.

Yang, B., et al., 2004. On the effects of moisture in a

polyurethane shape-memory polymer. Smart Materials

and Structures.

Yakacki, C.M. et al., 2008. Cytoxicity and

Thermomechanical Behaviour of Biomedical Shape-

Memory Polymer Networks Post-sterilization.

Biomedical Materials.

Cabanlit, M., et al., 2007, Polyurethane Shape-Memory

Polymers Demonstrate Functional Biocompatibility In

Vitro. Macromolecular Bioscience.

Sokolowsky, W., et al., 2007, Medical Applications of

Shape-memory Polymers. Biomedical Materials.

Bar-Cohen, Y., 2006. Artificial Muscles using

Electroactive Polymers (EAP): Capabilities,

Challenges and Potential. SPIE Press.

INTELLIGENT EXPANDABLE STRUCTURES BASED ON THE IMPROVED ACTIVATION OF SHAPE-MEMORY

POLYMERS

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