Intelligent materials

Progress in material science stimulates the economy andgreatly in¯uences the living standards of a society.Intelligent materials are based on a new concept wherethe information science is introduced to the material.Today it is a challanging task to manufacture newmultifunctional materials which posses intelligence at thematerial level.We refer tomaterial intelligence in terms ofthree main functions: sensing changes in environmentalconditions, processing the sensed information and ®nallymaking judgement (actuating) by moving away from orto the stimulus [1]. The innovative functions of these newmaterials are in many respects similar to those of livingorganisms. Biological systems continuously adapt toa dynamically evolving environment, while developinginnate properties of homeostasis. The intelligentmaterials should also display these characteristics whilemaintaining harmony with the environment.

The current generation of intelligent materials can bedivided into two main parts.

1. Hard and dry materials, such as metals, ceramics andplastics. Intenstive research is currently being devoted

to compound semiconductors, dielectric ceramics,thin ferroelectric ®lms and photovoltaic energy con-verters.

2. Soft and wet materials, such as electrorheological¯uids, magnetic ¯uids and polymer gels.

In technical applications we generally exploit theactuation characteristics of these materials.

In the last few years we have observed a migration ofinterest and activity from the traditional hard materialstowards soft materials, such as biomaterials, self-assem-bly materials, complex ¯uids and polymer gels. We areaware that soft materials will probably never replacehard ones, but their study has led to signi®cant advancesin areas such as arti®cial tissues, controlled drugdelivery, molecular recognition, chemical valves, arti®-cial muscles and chemomechanical energy conversion.

For any technical application it is of great impor-tantance to have a quick and reliable control system.Electric or magnetic ®elds are the most practical stimulifrom the point of view of signal control. In this paper we®rst brie¯y summarise the possible application of electricand magnetic ®elds as new driving mechanisms for

Colloid Polym Sci 278:98±103 (2000)Ó Springer-Verlag 2000 ORIGINAL CONTRIBUTION

Received: 26 July 1999Accepted: 27 August 1999

Presented at the 39th General Meeting ofthe Colloid-Gesellschaft, WuÈ rzburg 27±30September 1999

M. ZrõÂ nyiDepartment of Physical ChemistryTechnical University of BudapestH-1521 Budapest, Hungarye-mail: zrinyi.fkt@chem.bme.huTel.:36-1-4633229Fax: 36-1-4633939

Abstract In this paper we summar-ise the e�ects induced by electric andmagnetic ®elds on the mobility andshape of polymer gels containing acomplex ¯uid as a swelling agent.Magnetic-®eld-sensitive gel beadsand monolith gels have been pre-pared by introducing magnetic par-ticles of colloidal size into chemicallycross-linked poly(N-isopropylacryl-amide) and poly(vinyl alcohol)hydrogels. The in¯uence of uniformand nonuniform ®elds has beenstudied. In uniform magnetic ®elds

the gel beads form straight chainlikestructures, whereas in nonhomoge-neous ®elds the beads aggregates dueto the magnetophoretic force direct-ed to the highest ®eld intensity. Theability of magnetic-®eld-sensitivegels to undergo quick, controllablechanges in shape can be used tomimic muscular contraction.

Key words Intelligent material áResponsive gels á Magnetic-®eld-sensitive polymer gel á Arti®cialmuscle

M. ZrõÂ nyi Intelligent polymer gels controlledby magnetic ®elds

polymer gels, and this is followed by a summary ofmagnetically controlled polymer gels.

Responsive gels

Polymer gels are unique materials in the sense that noother class of materials can be made to respond to somany di�erent stimuli as polymer gels. The stimuli thathave been demonstrated to induce abrupt changes inphysical properties are diverse and include temperature,pH, solvent or ionic composition, electric ®eld, lightintensity as well as the introduction of speci®c ions [2±5].In the last few years, these gels have become of majorinterest as novel intelligent materials. Many kinds ofsuch gels have been developed and studied with regardto their application to several biomedical and industrial®elds, for example, controlled drug delivery systems,musclelike soft linear actuators, biomimetic energy-transducing devices and separation techniques.

Poly(N-isopropylacrylamide) hydrogel, abbreviatedas NIPA gel, is one of the most frequently studiedtemperature-responsive hydrogels. This gel has negativethermosensitivity, i.e. it shrinks with increasing temper-ature. A noncontinuous collapse transition takes placearound 34 °C. There are several other gels which showreversible noncontinuous swelling and shrinking transi-tions at di�erent temperatures [5, 6].

Attempts at developing stimuli-responsive gels areoften complicated by the fact that structural changes,such as volume changes, are kinetically restricted byrelatively slow swelling or deswelling process [3]. Thisdisadvantage often hinders the e�orts to design opti-mum gels for di�erent purposes. Other applicationsrequire noncontact modes of deformation and movabil-ity. In order to accelerate the response rate and toachieve noncontact agitation a new driving mechanismhad to be found. Electric and magnetic ®elds seemed tobe promising stimuli.

Polymer gels in electric and magnetic ®elds

All materials experience forces or torques when subject-ed to electric or magnetic ®elds. These interactions arestrong for certain solid materials, but are rather weakfor ¯uid systems. In order to enhance the in¯uence ofexternal ®elds on solution and/or gel properties, it isnecessary to combine solidlike and ¯uidlike behaviour.New colloidal solutions, termed complex ¯uids, havebeen introduced. Electrorheological ¯uids, magneto-rheological ¯uids and ferro¯uids contain dispersed smallparticles in the size range from nanometres to micro-metres [7]. These ¯uids respond to an applied ®eldsby rapidly changing their apparent viscosity and yieldstress. Since polymer gels contain a substantial amountof liquid as a swelling agent, it is possible to make ®eld-

sensitive gels by using a polymer network swollen by acomplex ¯uid. The colloidal particles incorporated,characterized by strong adsorptive interactions betweensolid particles and polymer chains, couple the shapeand physical properties of the gel to the external ®eld.These ®eld-sensitive gels can be exploited to constructnew types of soft actuators, sensors, micromachines,biomimetic energy-transducing devices and controlleddelivery systems.

If a ®eld-sensitive gel is exposed to an external ®eld,two distinct types of interactions can be identi®ed: ®eld±particle interaction and particle±particle interaction [7,8]. If the ®eld is nonuniform, the ®eld±particle interac-tions are dominant. The particles experience a dielec-trophoretic (DEP) or a magnetophoretic (MAP) force.As a result the particles are attracted to regions ofstronger ®eld intensities. Because of the cross-linkingbridges in the network, changes in molecular conforma-tion due to either DEP or MAP forces can accumulateand lead to macroscopic shape changes and/or motion.The main features of the DEP and MAP forces aresummarised in Table 1.

In uniform ®elds the situation is completely di�erent.Due to the lack of a ®eld gradient, there are no attractiveor repulsive ®eld±particle interactions. The particle±particle interactions become dominant. The imposed®eld induces electric or magnetic dipoles. As a result,mutual particle interactions occur if the particles are soclosely spaced that the local ®eld can in¯uence theirneighbours. This mutual interaction can be very strong,leading to a signi®cant change in the structure of particleensembles. The particles attract each other when alignedend to end, and repel each other in the side-by-sidesituation. Due to the attractive forces pearl-chainstructure develops. The ®eld-induced chain formationhas major implications for a number of technologies[20].

Table 1 Particle±®eld interactions in nonuniform ®elds. R meansthe radius of solid particles, e and l stands for the permittivity andpermeability, respectively. The index 2 refers to the colloidal par-ticle and the index 1 denotes the swelling agent

Electric ®eld Magnetic ®eldDielectrophoretic (DEP) force Magnetophoretic (MAP) force

fDEP � 2pe1R3KrE20 fMAP � 2pl1R3KrH 2

0

Permittivities Permeabilities

K � e2ÿe1e2�2e1 K � l2ÿl1

l2�2l1

Field gradient Field gradient

rE0 rH0

Low energy consumption Signi®cant energy consumption

may be dangerous safe

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Magnetic-®eld-sensitive NIPA gels

We have prepared a novel polymer gel (MNIPA),characterised by temperature and magnetic ®eld sensi-tivity. In the abbreviation of MNIPA, M Means NIPAwith magnetic particles incorporated. By incorporationof magnetite nanoparticles into NIPA hydrogels we areable to target magnetic gel beads to a certain place andto separate the gel beads by a nonuniform magnetic®eld. Our main intention was to study both temperatureand magnetic ®eld sensitivity of MNIPA gels. Gel beadswith an average diameter of 2,0 mm and a narrow sizedistribution were prepared and investigated [21].

Several MNIPA gel beads distributed randomly ina Petri dish are shown in Fig. 1a. Without an externalmagnetic ®eld, the beads do not attract each other andas a result they do not form aggregates. This visualobservation tells us that the gel beads have no perma-nent magnetisation. This result is also supported bymagnetisation measurements.

Magnetisable particles, such as MNIPA gel beads,experience a force in nonuniform magnetic ®elds. Thisphenomenon, called magnetophoresis, is the result of aMAP force directed along the gradient of the magnetic®eld (Table 1): this e�ect is shown in Fig. 1b. Thenonuniform magnetic ®eld is induced by a permanentmagnet (not seen in the ®gure) placed under the Petridish. The beads are attracted to point where themagnetic ®eld has its maximum intensity and arerepelled from the point where the magnetic ®eld is aminimum. As a consequence all the beads gather wherethe ®eld intensity has the highest value. This positioncorresponds to the surface of the permanent magnet,which cannot be seen in the ®gure. It can also be seenfrom the ®gure that the time required to collect all thebeads is rather short: this process took place within 24 s.When the ®eld is turned o�, the induced magnetic

moment of the beads vanishes and the aggregation canbe destroyed by slight mechanical agitation.

The situation is somewhat di�erent if we put themagnetic gel beads in a homogeneous magnetic ®eld. Inthis case no force is exerted on the gel beads, butthe magnetic ®eld polarises the beads and induces amagnetic moment in them. The induced magneticmoments are aligned parallel to the imposed ®eld. Theattractive interactions between the induced magneticdipoles lead to chain formation (Fig. 1c). The interact-ing particles are aligned parallel with respect to theapplied ®eld.

We have studied the temperature dependence of themagnetic gel beads. The temperature sensitivity is animportant property for several applications, for exam-ple, controlled drug release, soft actuator. It was foundthat as for the nonmagnetic NIPA gels, an abruptvolume change in response to the temperature changeobserved. This is demonstrated in Figs. 2 and 3.

Figure 2 shows that due to an increase in tempera-ture, the volume of the gel bead decreases signi®cantly.The degree of swelling has changed form 1 to 0.15.

In order to study the e�ect of incorporated magnetitenanoparticles on the temperature sensitivity of the gels,we determined the temperature dependence of the beadsize. These results are shown in Fig. 3. It is seen that thetemperature dependence of the volume (or size) chargesof MNIPA gels is very similar to that of unloaded NIPAgels. This ®nding means that incorporation of magnetitenanoparticles into NIPA gels does not a�ect thetemperature sensitivity signi®cantly.

Muscular contraction mimicked by magnetic gels

Since muscle tissue consists of 80% multicomponentaqueous solution and 20% proteins that comprise the

Fig. 1a±c In¯uence of uniform and nonuniform magnetic ®elds on j (MNIPA) gel beads. a No magnetic ®eld is applied, b nonuniformmagnetic ®eld is created by a permanent magnet, c homogeneous magnetic ®eld. The arrow indicates the direction of the ®eld

100

elastic material, it is not surprising that e�orts to developarti®cial muscle have been devoted to polymer gels [9].

From an engineering point of view, muscles are softand wet mechanical transducers, capable of performingtheir functions by quick and reversible shortening in aprocess called unidirectional contraction. Forces inter-nal to the muscle are derived from a special mechanism

which is designed to transform chemically bound energyinto mechanical work or locomotion. During muscularcontraction one end of the muscle remains ®xed, whilethe other end moves towards the origin. Since thevolume of the muscle remains essentially unaltered, italso experiences an increase in diameter.

In the analyses of the mechanical behaviour of anisolated muscle it is customary to introduce two types ofstrain [10]. Under load conditions the muscle is ®rststretched passively before being stimulated to contract.This process is then followed by active contraction dueto shortening of ®bres. Therefore under load conditionsmuscular thickening may be considered as a net e�ect ofboth passive and active strains taking place simulta-neously.

When contraction takes place against a resistance, aforce is generated and mechanical work is done. Whencontraction occurs against the maximum resistance, themaximum muscular force develops, but no measurablechange in muscle length, as well as in mechanical work,can be observed. This isometric contraction representsthe largest force that a muscle can actively generate.Under no load conditions the stimulation of musclewill cause it to contract to its smallest length withoutdevelopment of any active tension.

The passive±active nature of muscular contractioncannot be realised by a traditional elastic material, thedeformation of which depends only on the surfacetraction. In a material designed for arti®cial muscleapplication there must be a physical or chemical processthat can generate mechanical stress inside the materialindependently of the acting surface traction. Thiscontrollable ``body force'' is able to determine the rateand measure of contraction even if the material ispreloaded and it is needed to make the material capableof mimicking the passive and active mechanisms ofmuscles.

An arti®cial muscle must reproduce at least two maincharacteristics of real muscle ®bres, namely the high andfast contractility. It is also important to have a reliablecontrol system.

The ability of magnetic-®eld-sensitive gels to undergoa quick controllable change of shape can be used tomimic muscular contraction. The peculiar magnetoelas-tic properties of these gels may be used to create a widerange of motion and to control the shape change andmovement, that are smooth and gentle, similar to what isobserved in muscles [11±17].

In order to be able to mimic muscular contraction westudied the unidirectional shortening of magnetic gelsamples excited by a nonhomogeneous magnetic ®eld.A cylindrical gel sample of poly(vinyl alcohol) (PVA)containing magnetite nanoparticles was suspended ver-tically in water between the plane-parallel poles of anelectromagnet. The position of the top surface of the gelwas ®xed and the highest ®eld strength was located at

Fig. 2a, b Collapse transition of a MNIPA gel bead. a Temperaturebelow the lower critical solubility temperature (LCST), T = 25 °Cand d = 1.87 mm; b temperature above the LCST, T = 40 °C andd = 1.00 mm

Fig. 3 The dependence of the size of a MNIPA gel bead on thetemperature. d is the diameter of the gel bead and d0 represents thisvalue at 25 °C

101

this point. The steady current intensity in the solenoid-based electromagnet was varied in order to producedi�erent magnetic ®eld distributions. The maximummagnetic ®eld strength (magnetic ¯ux density) of300 mT developed at the upper part of the gel anddisappeared within 120 mm along the axis of thecylindrical gel sample. It is worth mentioning that300 mT is a ®eld strength which is less than the ®eldstrength measured at the surface of common permanentmagnets. Due to the ®eld gradient directed from bottomto top along the gel axis, contraction occurs. Figure 4shows that under load conditions stimulation of amagnetic gel will cause it to contract.

Since the highest magnetic ®eld strength can becontrolled by the intensity of the steady current ¯owingthrough the electromagnet, it is possible to realisedi�erent degrees of contraction as shown in Fig. 4.The maximum ®eld strength can be seen to increase fromleft to right in Fig. 4. One can establish the fact thatsigni®cant contraction can be induced by moderatemagnetic ®eld gradients. In this case the strongest ®eldintensity is 300 mT, and this disappears within 120 mmalong the gel axis.

The contractile activity of magnetic gels can be usedto lift a load that is to produce work. A nonmagneticload (lead) of variable mass was connected to the lowerend of the gel. A charge-coupled-device camera provid-ing the displacement of the lower part of the gel with anaccuracy of 0.01 mm monitored the contraction of thegel.

In the presence of a load the gel ®rst elongates(passive strain). When a magnetic ®eld is created, as aconsequence, a contraction (active strain) takes place. Ifthe mass of the load is not too high, the net e�ect is asigni®cant contraction. It can be seen that the magnetic

stimulus results in a large decrease in the length of thegel. When the ®eld is turned o�, the gel stretches again.

We have determined the work done by the ferrogel asa function of the load. These results can be seen in Fig.5.It is worth mentioning that we used a cylindrical gel,which had an overall volume of 12 cm3. The gelcontained 0.72 g polymer (PVA) and 0.96 g magnetitenanoparticles.

Control of pseudomuscular contraction

Besides theoretical interest, for future applications it isnecessary to understand the basic relationships betweenthe work done, the properties of gels as well as themagnetic ®eld distribution. We consider here a verticallysuspended cylindrical magnetic gel by means of whicha nonmagnetic load can be lifted up. First the gel ispreloaded with a mass, M. As a consequence a passivestrain, kM, develops. The magnitude of this extensioncan be calculated on the basis of rubber elasticity theory[18]:

k3M ÿ ak2M ÿ 1 � 0 : �1�In this equation it is assumed that Gaussian behaviourcould be used as an approximation. The dimensionlessquantity a includes the ratio of nominal stress, rn andthe modulus, G:

a � rn

G� Mg

a0G: �2�

Here g represents the gravitational constant and a0

denotes the undeformed cross-sectional area of the gel atrest. When an external magnetic ®eld is applied thestrain will be changed to kM;H, which is the overallstrain. Considering the homogeneous deformationand the linear relationship between magnetisation andmagnetic ®eld strength, on the basis of the additivity ofmechanical and magnetic stress it is possible to deriveEq. (3) [17].

k3M;H ÿ ak2M;H ÿ b H2h ÿ H2

m

ÿ �kM;H ÿ 1 � 0 ; �3�

Fig. 4 Active deformation of a magnetic-®eld-sensitive gel under aload

Fig. 5 Mechanical work released by a ferrogel as a function of theload

102

where Hh and Hm denote the magnetic ®eld strength atthe bottom and the top of a ferrogel cylinder, respec-tively. b can be considered as the stimulation coe�cientde®ned as

b � l0v2G

: �4�where l0 means the permeability of the vacuum and vdenotes the susceptibility of the gel.

The mechanical work produced by the active straincan be expressed as follows

W � MgDh � Mgh0�1ÿ kM;H� ; �5�where h0 denotes the undeformed length of the magneticgel and Dh represents the displacement of the load.

As an example, we have calculated the mechanicalwork as a function of the load [17]; the results are shownin Fig. 6. It is obvious that the mechanical work stronglydepends on the applied load. One can see that for smallloads the work increases with the load. M = 18 grepresents the highest mechanical work. Above a certainvalue it is seen that if the load is comparatively heavy,the work decreases with increasing mass. Similar resultshave been found for mechanical work produced byswelling [19]. Any other experimental situation can bestudied with the aid of Eqs. (3)±(5).

Conclusion

Magnetic-®eld-sensitive polymer gels have been pre-pared by introducing magnetic particles of colloidal sizeinto chemically cross-linked NIPA and PVA hydrogels.These gels represent a new class of stimuli-responsivegels. The in¯uences of uniform and nonuniform ®elds aswell as the temperature have been studied. In uniform®elds the magnetic gel beads form straight chainlike

structures, whereas in nonhomogeneous ®elds the beadsform aggregates due to the MAP force directed to thehighest ®eld intensity. It is possible to target thetemperature- and magnetic-®eld-sensitive gels to certainpositions by using MAP interactions.

An understanding of magnetoelastic coupling in gelswill hasten the gel engineering of switches, sensors,micromachines, biomimetic energy-transducing devicesand controlled delivery systems. Signi®cant contractioncan be realised by magnetic gels. Their unique magne-toelastic properties make it possible for them to mimicmuscular contraction. If the magnetic ®eld is createdinside the gel by incorporating small powerful electro-magnets and the ®eld is co-ordinated and controlled by acomputer, then the magnetic-®eld-sensitive gel may beused as an arti®cial muscle.

Acknowledgements This work was supported by the HungarianAcademy of Sciences under contract OTKA T 030654 and by theEuropean Commission, Directorate-General XII Science, Researchand Development, grant no. IC 15 CT 96-0756.

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Fig. 6 The mechanical work as a function of the load calculated onthe basis of Eqs. (3)±(5)

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