Smart Materials: The eighth age of materials

The seven ages of materials, from the Stone Age to the plastic age, have shaped modern civilization. Recent advances in the development of ‘smart materials’ mark the beginning of a new material age with the potential to revolutionize diverse fields of research, ranging from robotics to medicine. Imagine a sentient flexible flying goo like the one in the movie Flubber, capable of changing shapes and attaining tremendous speeds. Although presently fictional, such a life-like material is a strong possibility in the future. Researchers are developing plastic- based compliant materials that closely resemble biological systems capable of undergoing changes based on environmental conditions. Robots made with soft materials could be better suited to work closely with humans as they would become capable of making decisions such as ‘what to do next’, based on their interactions with the surroundings.

Plastics are primarily made of synthetic polymers that have the same repeating unit… On the other hand, organic life with enormous complexity and intelligence is built upon biopolymer architecture that is capable of self-regulated autonomy

Nature-inspired responsive materials

In the past century plastics have shaped the modern world by penetrating almost every aspect of our daily lives. Plastics are primarily made of synthetic polymers that have the same repeating unit. These materials are static and have a single purpose. On the other hand, organic life with enormous complexity and intelligence is built upon biopolymer architecture that is capable of self-regulated autonomy and responds to external stimuli. For example, Venus flytrap, a carnivorous plant, closes its leaves to entrap insects that touch its trigger hairs. Inspired by such externally controlled natural movements of biological systems, researchers have developed soft flexible materials that are programmed to undergo reversible geometrical changes, movements, and accomplish tasks by responding to external stimuli such as light, humidity, pH, electrochemical changes, magnetic field, or temperature. Such materials with life-like functional properties are generally made of special polymers called liquid-crystalline elastomers, which are embedded with tiny entities that can change their geometry by external stimuli. One such molecule is a dye called azobenzene.

Azobenzene can reversibly change its chemical structure upon excitation with light of different wavelengths. This light-induced reversible isomerizationis associated with large geometrical changes between its thermally stable ‘trans‘ structure with a length of 0.9 nm and a metastable ‘cis’ structure with a length of only 0.55 nm (Figure 1). Interestingly, these small molecular events can also affect the macroscopic structures of the materials they are intertwined into, i.e. it can enable visible reversible deformations of different geometries in the whole material. Researchers have exploited these properties to develop light controlled artificial flytrap, actuators, inchworm, self-propeller, etc. However, these materials have their drawbacks including low strength, which hinders the development of high-performance robotics. Moreover, the cost of manufacturing liquid crystalline based materials for robotics and smart devices is still prohibitively high for mass production.

Commodity polymers, on the other hand, are inexpensive, making them attractive for smart applications. However, their usage is limited by difficulties in fine-tuning the reversibility of its material properties. Recently, researchers from the Eindhoven University, Netherlands, in collaboration with leading European industrial partners, have developed a high-performance material that responds to external stimuli such as light.

Materials responding to light

This composite material consists of a photo-sensitive azobenzene dye and a high-performance polyethylene. The dye units were uniaxially oriented, which ensures that the photo-induced response is uniform and gives the desired directional deformation. Polyethylene is lightweight and possesses extraordinary mechanical properties and it is even stronger than steel! The combined light-induced molecular level responses of the highly aligned azobenzene molecules, as well as the polyethylene matrix, result in a large stress in the whole material. To illustrate the application of such externally controlled changes in stress, light-induced motion of a composite film glued to an aluminium strip was demonstrated. Shining light on the polymer-aluminium surface results in reversible hinge-like bending of the metal strip (Top panel of Fig. 1). Since these materials are made of inexpensive bulk commodity polymer polyethylene, they can be used in large-scale applications. Although the new material do not yet have any practical applications, preliminary results from proof-of-principle experiments look promising for the development of responsive commodity polymers. From an industry application standpoint, they have potential in fields such as soft-robotics and microdevices with greater adaptability, sensors, responsive textile, smart windows, self-cleaning surfaces, motors, and artificial muscles.

Figure 1: Light-induced reversible isomerization of azobenzene dyes resulting in macroscopic changes in the whole material.

Research on such responsive materials requires the large-scale collaboration of experts in different fields of synthetic, computational, and analytical chemistry, material science, and physics. Soft-material research, which aims to bring life-like functional features to materials suitable for the development of personal soft robots, could be as revolutionary as the personal computer. Let’s wait and watch.

Author:

Shaji Varghese, a molecular materials scientist with extensive experience in the academic and health care industry. He holds a Ph.D. degree in Chemistry, for his work on the topic of chemical transformations in the cavity of porous materials. He has authored/co-authored research articles that have been published in leading peer-reviewed journals and books. He is passionate about enhancing patient health and enriching people’s lives through scientific innovations. He firmly believes that scientific innovations should be made to have a positive impact on the environment and society.

Editors:

Saurja Dasgupta is originally from Kolkata, India. He obtained his Ph.D. at the University of Chicago, where he studied the structure, function, and evolution of catalytic RNA. He is currently doing his postdoctoral research at Massachusetts General Hospital, Boston, where he is trying to understand the biochemical milieu that could have given birth to life on earth (and elsewhere) and reconstruct primitive cells. One of his scientific dreams is to observe the spontaneous emergence of Darwinian evolution in a chemical system.  When not thinking about science, Saurja pursues his love for the written word through poetry and song-writing (and meditating on Leonard Cohen’s music). His other passions are trying to make science easier to understand, and fighting unreason and pseudoscientific thinking with a mixture of calm compassion and swashbuckling spirit.

Sumbul Jawed Khan received her Ph. D. in Biological Sciences and Bioengineering from the Indian Institute of Technology Kanpur, where she studied the role of microenvironment in cancer progression and tumor formation. During her post-doctoral research at the University of Illinois at Urbana-Champaign, she investigated the gene regulatory networks that are important for tissue regeneration after damage or wounding. She is committed to science outreach activities and believes it is essential to inspire young people to apply scientific methods to tackle the challenges faced by humanity. As an editor, her aim is to simplify, translate, and excite people about the current advances in science.