Viewpoint: Pavlovian Materials—Functional Biomimetics Inspired by Classical Conditioning

Abstract

Herein, it is discussed whether the complex biological concepts of (associative) learning can inspire responsive artificial materials. It is argued that classical conditioning, being one of the most elementary forms of learning, inspires algorithmic realizations in synthetic materials, to allow stimuli‐responsive materials that learn to respond to a new stimulus, to which they are originally insensitive. Two synthetic model systems coined as “Pavlovian materials” are described, whose stimuli‐responsiveness algorithmically mimics programmable associative learning, inspired by classical conditioning. The concepts minimally need a stimulus‐triggerable memory, in addition to two stimuli, i.e., the unconditioned and the originally neutral stimuli. Importantly, the concept differs conceptually from the classic stimuli‐responsive and shape‐memory materials, as, upon association, Pavlovian materials obtain a given response using a new stimulus (the originally neutral one); i.e., the system evolves to a new state. This also enables the functionality to be described by a logic diagram. Ample room for generalization to different stimuli and memory combinations is foreseen, and opportunities to develop future adaptive materials with ever‐more intelligent functions are expected.

Stimuli‐Responsive Nanocomposite Hydrogels for Biomedical Applications

“…In a forward‐looking perspective, emerging trends are beginning to unveil the development of nanocomposite hydrogels inspired by the logic of classical conditioning. [ 48 ] Traditional stimuli responsive systems are engineered to instinctively react to certain stimuli, but their programming often does not allow for independent decision‐making processes (i.e., intelligent behavior), for instance by learning to react to new stimuli inputs and adapting their behavior based on past events. Recent works have begun to explore such systems, namely through the incorporation of nanoparticles as i) logic gate memory modules that can record information from previous stimuli states (i.e., temperature) and reset under a different type of input (i.e., magnetic); [ 49 ] ii) memory elements/actuators in hydrogel platforms that learn to melt under originally neutral stimuli and also allow for cognitive forgetting; [ 50 ] and iii) DNA‐oligonucleotide modified nanoparticle neural networks that are able to process DNA instructions under Boolean logic operations.…”

Stimuli-responsive engineered living materials

“…Stimuli-responsive materials that respond with high sensitivity has attracted the attention of material scientists to create materials that mimic the functions and behaviors of living organisms. Although significant efforts have been developed, 22,23 it is still challenging to create materials that perform complex biological functions such as chemotaxis, adaptation, growth, and metabolic functions in response to highly specific and weak stimuli. Living organisms respond to weak stimuli in ways that are encoded by the information in the genome.…”

Liquid Crystal Polymer‐Based Soft Robots

“…Recently, the associative learning LCNs, also termed as Pavlovian materials, that "learn" to respond to an initially neutral stimulus only after being trained by an independent stimulus were designed by Priimagi and co-workers, providing unforeseen routes toward self-studying soft robots. [130,131] The principle of a Pavlovian material is shown in Figure 10A. Initially, the material is insensitive to stimulus 2 but responds to stimulus 1.…”