Intelligent Stimuli-Responsive Materials

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NATURE-INSPIRED STIMULI-RESPONSIVE SELF-FOLDING MATERIALS

LEONID IONOV

1.1 INTRODUCTION

Engineering of complex 3D constructs is a highly challenging task for the development of materials with novel optical properties, tissue engineering scaffolds, and elements of micro and nanoelectronic devices. Three-dimensional materials can be fabricated using a variety of methods including two-photon photolithography, interference lithography, molding [1]. The applicability of these methods is, however, substantially limited. For example, interference photolithography allows fabrication of 3D structures with limited thickness. Two-photon photolithography, which allows nanoscale resolution, is very slow and highly expensive. Assembling of 3D structures by stacking of 2D ones is time-consuming and does not allow fabrication of fine hollow structures.

Fabrication of 3D microobjects using controlled folding/bending of thin films-self-folding films-is novel and a very attractive research field [1,2]. Self-folding films are the examples of biomimetic materials [1]. Such films, on the one hand, mimic movement mechanisms of plants [3] and, on the other hand, are able to self-organize and form complex 3D structures. The self-folding films consist of two materials with different properties. At least one of these materials, active one, can change its volume. Because of the non-equal expansion of the materials, the self-folding films are able to form a tubes-, capsules- or more complex-structure. Similar to origami, the self-folding films provide unique possibilities for the straightforward fabrication of highly complex 3D micro-structures with patterned inner and outer walls that cannot be achieved using other currently available technologies. The self-folded micro-objects can be assembled into sophisticated, hierarchically organized 3D super-constructs with structural anisotropy and highly complex surface patterns. Until now most of the efforts were focused on the design of inorganic self-folding films [4]. On the other hand, due to their rigidity, limited biocompatibility, and non-biodegradability, application of inorganic self-folding materials for biomedical purposes is limited. Polymers are more suitable for these purposes. First, there are many polymers changing their properties in physiological ranges of pH and temperature as well as polymers sensitive to biochemical process [5]. Second, polymers undergo considerable and reversible changes of volume that allows the design of a variety of actively moving microconstructs [6]. Third, there are a variety of biocompatible and biodegradable polymers [7]. This chapter overviews recent progress in development of the polymer films which are able to fold and form 3D microstructures.

1.2 DESIGN OF SELF-FOLDING FILMS

Bending is essentially required for design of self-folding films and allows conversion of semi one-dimensional and two-dimensional objects into 2D and 3D ones, respectively. Typically, bending is the result of either expansion or contraction of a material caused by change of environmental conditions. In most cases, change of conditions, however, results in homogenous expansion or contraction in all directions and does not lead to increase of dimensionality. Bending is produced as a result of inhomogeneous expansion/shrinking, which occurs with different magnitudes in different directions. Bending could be achieved either (i) by applying gradients of field to homogenous materials or (ii) by applying non-gradient stimuli to inhomogeneous materials. The example of the first case is the bending of polyelectrolyte hydrogel during electrolysis [8]. The examples of the second case are the bending of liquid crystalline films [9], hydrogel with the lateral gradient monomer concentration [10], cantilever sensors [11], and shape-memory polymers [12].

In fact, design of self-folding objects using homogenous materials is technically very complicated because a very complex spatial force gradient must be formed and kept for a considerable period of time. For example, this can be achieved using surface tension by depositing a water droplet on a thin film [13]. The film folds immediately after the droplet is deposited. The formed 3D object changes its shape during drying of the droplet and unfolds when water is completely evaporated. In physiological buffer environment, surface tension effects are, however, weak. Fabrication of self-folding objects using inhomogeneous films is more straightforward. The inhomogeneous films fold due to difference in the properties on constituting materials in pre-programmed manner, which is defined by the film structure/pattern.

To date, three general approaches for design of self-folding polymer films using inhomogeneous materials are reported (Fig. 1.1). First approach is based on shape-memory polymers, which are partially liquid crystalline with directional anisotropy of properties (Fig. 1.1a). At low temperature, the shape-memory materials are in their temporary shape. The films recover their permanent shape by heating. In second and third approaches, two polymers are used. One of the polymers is passive and its properties remain unchanged. Another polymer is active and its volume or shape is changed when stimulus is applied. The second approach is based on the use of polymer bilayers (Fig. 1.1b). Active polymer swells or shrinks in response to signal. The swelling in one direction is restricted by the passive polymer. As a result, the bilayer does not uniformly expand/shrink but it does fold and unfold. Third approach is based on the use of patterned film of passive polymer with insertion of the active one (Fig. 1.1c). Active polymer undergoes shape transition, which might be caused by surface forces, that results in folding of the film.

FIGURE 1.1 Approaches for design of self-folding polymer objects (a) relaxation of shape-memory polymers (b) folding of polymer bilayer due to expansion of one of the polymers (c) folding of patterned polymer film caused by shape change of one of polymers. Reproduced from Reference 14, with kind permission from Royal Society of Chemistry. Copyright 2011.

1.3 MECHANISM OF FOLDING

Timoshenko [15] was the first who investigated bending of bilayer, which consist of two materials with different expansion coefficients. He assumed that the bilayer can bend in only one direction and results in a bilayer with uniform curvature

(1.1)

(1.2)

(1.3)

where E are the elasticity modulus, a are the thickness of the layers, h is the total thickness (h = a1 + a2), is the stress of the films, ? is the radius of curvature. As it comes from the Equations (1.3), radius of curvature is inversely proportional to film stress. Moreover, radius of curvature first decrease and then increase with the increase of m. The resultant curvature is not very sensitive to the difference in stiffness between the two layers, and is mainly controlled by the actuation strain and the layer thickness. The Timoshenko equation applies to a beam bending in only one direction and does not predict the folding direction. Moreover, Timoshenko equation considers elastic deformations, the polymers and hydrogels often demonstrate viscoelastic properties.

More recent models have considered complex bending of bilayer in two dimensions. Mansfield found analytical solutions for large deflections of circular [16] and elliptical [17] plates having lenticular cross sections with a temperature gradient through the thickness. For small gradients, the plates formed spherical caps, curved equally in all directions. At a critical gradient, a configuration with greater curvature in one direction became more favorable. Because of the lens-shaped thickness profile, even though the elliptical plate had a major axis it showed no preferred direction for bending even for large deflections. Freund determined the strain at which the spherical cap, formed by circular bilayer of uniform thickness, becomes unstable using low order polynomial solutions and finite element simulations [18].

Later, Smela et al. showed that short-side rolling was preferred in the case of free homogeneous actuation and that this preference increased with aspect ratio (ratio of length to width of rectangular pattern) [19]. Li et al. [20] and Schmidt [21] experimentally demonstrated the opposite scenario, namely a preference for long-side rolling, in the case where bilayers are progressively etched from a substrate. They observed that when the tube circumference was much larger than the width or the aspect ratio of the rectangle was high, rolling always occurred from the long side. When the tube circumference was much smaller than the width and the aspect ratio of the membrane pattern was not very high, the rolling resulted in a mixed yield of long- and short-side rolling, as well as a "dead-locked turnover" shape. Short-side rolling occurred at small aspect ratios when the deformed circumference is close to the width. In these self-rolling systems, the active component undergoes relatively small volume changes or actuation strains, which are nearly homogenous over the whole sample. Control of rolling/folding direction is very important for programmed folding. For example, Schmidt demonstrated that introduction of wrinkles allows switching to short-side rolling [21].

In inorganic self-rolling systems, the active component undergoes relatively small volume changes or actuation strains, which are nearly homogenous over the whole sample. Hydrogels, however, demonstrate considerably different properties. First, hydrogels...