Stimuli-Responsive Materials

From Molecules to Nature Mimicking Materials Design

Marek W Urban

• Publisher: Royal Society of Chemistry
• Year: 2019
• ISBN:
  9781788018098
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

The ability for a material to change properties in response to external stimuli is an attractive feature for numerous applications and as such stimuli responsive materials are gaining attention across many different fields. This book introduces the concepts of stimuli-responsiveness, including the fundamental materials properties required for design. It provides readers with comprehensive scientific principles and developments of stimuli responsive materials, as well as the recent technological advances. Written by a renowned expert in the field, this book is suitable for anyone interested in stimuli responsive materials working in polymers, biochemistry, biotechnology and materials science.

1What is Stimuli Responsiveness?

1.1 Introduction

It is well established that materials, and particularly polymeric materials, may serve many functions. These functions range from life-saving medical devices to paints that protect cars from rusting, or plastic utensils that rapidly fill up our landfill sites, to name just a few. Regardless of their function, there is a global need for functional materials, devices or complex systems to be either fully sustainable or entirely degradable. Making an object sustainable implies that all initial functions will be retained during its lifetime, but at some point it will lose its functions. While one term does not exclude the other, making materials degradable requires that at the end of its useful life, a given object will be turned into a new product of equal if not greater value. This concept, known as the cradle-to-cradle approach, seeks to create functional materials, devices or systems in such a way that they are not only efficient, but also waste-free. While it is environmentally advantageous to design all materials using the cradle-to-cradle approach, it is even more desirable to incorporate into materials design active functions that will be manifested by their property changes. For example, on a sunny hot day, it would be desirable to have a house with a white roof in order to minimize absorption of the sun's rays so the house would stay cool. But in winter we want the same white roof to change color to black in order to absorb sunlight to keep the house warmer. Thus, there is a stimulus, sun radiation, and there is a response, the change of color from white to black. If this process can be repeated many times, we have a roof that is stimuli-responsive to the external source of energy. Can internal sources lead to stimuli? If there are internally built mechanical stresses or concentration gradients within a given material, molecular relaxations may lead to responses manifested by crack formation, which typically results from the non-equilibrium state of matter.

Although Mother Nature provides multiple inspirations for the design and development of new materials, creating synthetic systems capable of responding to stimuli in a controllable and predictable fashion presents significant but fascinating challenges. Particular challenges lie in mimicking biological systems where structural and compositional gradients at various length scales are necessary for orchestrated and orderly responsive behaviors. To tackle these challenges several stimuli-responsive systems have been developed, with the majority of studies dealing with polymeric solutions, gels, surfaces and interfaces, and to some extent, polymeric solids. These states of matter impose different degrees of restriction on the mobility of polymeric segments or chains, thus making dimensional responsiveness easily attainable for systems with a higher solvent content and minimal energy inputs. Significantly greater challenges exist when designing chemically or physically crosslinked gels and solid polymeric networks that require maintenance of their mechanical integrity. Restricted mobility within the network results from significant spatial limitations, thus imposing limits on obtaining stimuli responsiveness. The challenge in designing these stimuli-responsive polymeric systems is to create networks capable of inducing minute molecular yet orchestrated changes that lead to significant physicochemical responses upon external or internal stimuli.

To illustrate spatial restrictions on mobility in the x-, y- and z-directions in solutions, at surfaces and interfaces, in gels, and in solids, Figure 1.1 is a schematic diagram of the four states and relative dimensional restrictions within each state.¹ When going from a solution phase to surfaces and interfaces, or gels to solid state phases, segmental mobilities of polymer chains decrease due to significant spatial restrictions manifested by smaller displacement vectors in the x-, y- and z-directions. Consequently, the energetic requirements for responses to temperature, mechanical stimuli, electromagnetic irradiation, or electrochemical stimuli, pH, ionic strength, or bioactive species will be different for each physical state. Examples of responses are depicted in Figure 1.1 and are classified into chemical and physical categories, where multiple stimuli may result in one or more responses, or one stimulus may result in more than one response. Because spatial restrictions also dictate energetic requirements, the next section will discuss these relationships.

Figure 1.1 Physical state and dimensional changes in chemical and physical stimuli-responsive processes. Reprinted from ref. ¹, Copyright (2010), with permission from Elsevier.

1.2 An Interplay of Chemical and Physical Responsiveness

How do our eyes work, and why we are able to see? Why do flowers bloom every spring? Why does self-healing of wounds occur in mammals and plants? How do we age? Why do living organisms metabolize and materials do not? Why are grizzly bears able to hibernate in the winter and humans cannot? Some of these questions might be obvious, but for the most part they are not. Why? The majority of these processes are only partially understood and the complexity arises from the lack of correlations between chemical reactions and physical processes responsible for the overall outcome. Another way of looking at it is that using chemically the same material to make different objects will require different physical designs. Making a chair for a child and an adult from the same material will require different designs. On the contrary, using the same design for a child and an adult may require different materials. To take a closer look at stimuli responsiveness let us consider the processes involved in vision. To see we need an object, visible light and a detector. This detector is a human eye, which can only sense reflected rays when an object is illuminated by light – in darkness we cannot see. Some may consider the eye as a camera; but is it really that simple? As depicted in Figure 1.2 the human eye has many components, which act in a synchronized manner. This rather remarkable device is capable of adjusting to various distances and illumination conditions, converting light signals to impulses and transmitting them to the brain where an image is created.

Figure 1.2 The complexity of a human eye structure. (Eyeball cross-section image © 1989–2001 by Lippincott Williams & Wilkins, Baltimore, MD.)

To realize the interplay of chemical and physical processes let us now consider selected chemical processes responsible for the basic physical eye function, vision.

Figure 1.2 illustrates the complex structure of a human eye. It is covered by a white, tough wall called the episclera, a fibrous layer between the conjunctiva and sclera. The eye muscles are connected to the conjunctiva. The cavity in the front part of the eye, between the lens and cornea, is the anterior chamber, which is filled with aqueous fluid that is recirculated every 100 minutes; this is produced by the ciliary body and drains back into the blood circulation through channels in the chamber angle. The structure behind the iris (mostly invisible), which produces the fluids filling the front part of the eye and maintains the eye pressure, is the ciliary body. Another important function of this part of the eye is to facilitate focusing. The iris acts like the diaphragm of a camera (and is responsible for the color of the eyes), allowing only a certain amount of light to enter the eye by dilating and constricting the pupil. The pupil is the dark opening in the center of the colored iris controlling how much light enters the eye. Immediately behind the iris is the lens, which is responsible for focusing light rays onto the retina. The white part of the eye, which is a thin lining over the sclera and inside the eyelids, is the conjunctiva. The cells of the conjunctiva produce mucous, which lubricates the eye. The primary focusing element of the eye is the cornea, also known as the epithelium. It is made of transparent cells capable of rapid regeneration. Its inner layer is transparent, allowing light to pass through. The narrow channel that runs from the optic disc to the back surface of the lens is called the hyaloid canal, and the body of the eye is filled with a jelly-like clear substance called vitreous humor. The retina is a layer of membrane lining the back of the eye. It contains photoreceptor cells that react to light intensity by sending impulses to the brain via the optic nerve. The light passes through the cornea and lens, creating an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. The macula is the most sensitive part of the retina and is responsible for the central (or reading) vision. Being near the optic nerve, directly at the back of the eye, it is also responsible for color vision. The optic disc is located in the back of the eye where the nerve, along with arteries and veins, enters the eye. The optic nerve consists of a bundle of a million nerves and is responsible for transferring information from the retina as electrical signals and delivering it to the brain, where this information is processed as a visual image.

There are two types of photoreceptor cell within the retina layer: rods and cones. As the light hits the photoreceptor cells, the first step is for chromophore 11-cis-retinal to isomerize to all-trans-retinal. The protein that is covalently bonded to 11-cis-retinal is opsin, and the complex 11-cis-retinal–opsin is known as rhodopsin. Opsin has seven hydrophobic α-helical regions that pass through the lipid membrane of the pigment-containing discs (Figure 1.3), forming a hydrophobic pocket where 11-cis-retinal resides. In the dark, 11-cis-retinal binds the opsin as an inverse agonist and holds it in an inactive conformation. When light strikes the visual pigment, the isomerization of 11-cis-retinal to all-trans-retinal in the binding pocket pushes the opsin into an active conformation and initiates phototransduction, which ultimately leads to the generation of nerv...