Programmable Materials

The ability to synthesize materials by design presents one of the greatest scientific challenges. Programmable materials can be designed to be highly dynamic either in form or function. Advances made toward the fabrication of programmable materials can lead to new, unexpected discoveries and novel functionalities in emerging fields, such as enhanced plasmonics, selective catalysis, efficient energy harvesting, precision medicine, and autonomous actuators. Vast research interest have been raised in material design for programmable materials to pursue these cutting-edge features, such as programmable molecules and nanomaterials. These can be self-assembled and processing with dynamic forms under various environments and emerging applications such as flexible electronics, showing high adaptability coupled with multidimensional transformations. The thriving research into programmable materials has revolutionized the materials science and engineering community from the perspective of synthetic biology, chemistry, and computational designs, etc. Programmable materials exhibiting unique adaptability upon various applications have generated new opportunities related to processing technology, especially regarding uniformity and scalability. Coupled with recent breakthroughs in programmable materials and related manufacturing, this creates potential impact in research and industrial applications. Moreover, the bioinspired dynamic transformation of programmable materials opens more opportunities for autonomous and soft robotics with enhanced intelligence upon their application. The prospects are significant for future applications of smart robotics and autonomous cyber–machine interfaces. The flourishing programmable materials may facilitate a vision for responsive materials on multiple scales showing a thriving future with progressed synthetic biology and chemistry, adaptable printing processes, autonomous robotics, and intelligent cyber–machine interfaces. To feature recent materials-oriented advances in this rapidly growing field, this special issue of Advanced Materials on the topic of “Programmable Materials” is presented, gathering contributions from leading experts to showcase four thriving pillars of programmable materials: Programmable materials are produced from small molecules with programmable self-assembly of nanomaterials with controllable organization and morphology. These responsive materials are appealing and create stringent demands on their assembly and resulting structures. For example, nanoparticles can be programmed precisely by assembling “atoms” functionalized with a shell of oligonucleotides into crystalline superlattices with tuneable compositions, symmetries, and lattice parameters, through DNA base pairing. Jin and co-workers (article number 2006591) summarize atomically precise Au nanoclusters (NCs) with programmable kernel structures and their electronic/optical absorption properties by controlling the size/shape and surface ligand of Au NCs. Sub-wavelength-scale metal nanoparticles (NPs) show localized surface plasmon resonance (LSPR) when interacting with electromagnetic waves, which is highly dependent on the size, composition, and assembly structure of these plasmonic NPs. Park, Nam, and co-workers (article number 2002700) outline recent bottom-up chemical approaches toward the precise assembly of uni- and multi-component plasmonic NPs. Including those with static assemblies through fixed structural parameters, as well as those with dynamic, reconfigurable spatial arrangement upon diverse stimuli. Meanwhile, strongly enhanced optical responses in the nanogap have also led to extensive investigations in plasmonic gap nanostructures (PGNs) and their potential sensing applications. Given the challenges in reproducible and controlled fabrication of highly uniform plasmonic nanogap structures in the nanometer and atomic-level range, Nam and co-workers (article number 2006966) elaborate recent advances in the synthesis, assembly and fabrication strategies, the prediction and control of optical properties, as well as the sensing applications of PGN with diverse gap sizes, morphologies, and compositions. Recent advances in organic electronics can be partly ascribed to the development of conjugated polymers with advantageous optoelectronic and mechanical properties, which are greatly dependent on crystallinity, orientation, morphology, domain size, and π–π interactions. Seferos and co-workers (article number 2006287) examine recent progress in assembly techniques for the three stages of π-conjugated programming (i.e., covalent programming, solution programming, solid-state programming) that can control the ordering, structure, and function of π-conjugated polymers from nano- to microscale. Layered 2D materials (e.g., graphene, black phosphorus, MXenes) have equally drawn intensive attention due to their unique electrical, mechanical and optical properties. Furthermore, recent developments of nanodots derived from layered materials (with lateral dimensions below 10 nm) have introduced added intriguing properties, including high water solubility, facile doping and functionalization, and tunable photoluminescence properties. Zhang, Tan, and co-workers (article number 2006661) present a summary of the materials categories, advantages, synthesis, and the potential applications of such emerging nanodots derived from layered materials. The catalyst can provide highways for chemical reactions, usually selectively and desirably. Due to the exceptional component tunability, high surface area, adjustable pore size, and uniform active sites, these catalysts based on metal–organic frameworks (MOFs) have been intensively investigated in the last decades. Huo, Zhang, and co-workers (article number 2007442) present a programmable modular design strategy on MOF-based catalysts’ synthesis with on-demand catalytic performance that includes five crucial modules: metal ions/clusters, organic ligands, modifiers, functional materials, and post-treatment. When it comes to solar photocatalysis, plasmonic NPs and plasmon-generated hot carriers (electron–hole pairs) are widely utilized to drive photochemical reactions and solar photocatalysis, whereas the underlying mechanism of hot-carrier transfer can be unveiled by photoelectrochemistry (PEC). PEC can prolong the lifetimes of hot carriers and localize hot holes on photoanodes and hot electrons on photocathodes. Wei and co-workers (article number 2006654) discuss PEC approaches and working principles for the mechanistic understanding of hot-hole and hot-electron transfer in plasmonic photocatalysis. Besides isolated plasmonic NPs, the plasmonic coupling architectures can allow further facilitate solar photocatalysis through enhanced LSPR. Xue and co-workers (article number 2005738) present the principles of the plasmonic coupling effect and discuss recent progress on the construction of plasmonic coupling architectures (i.e., metal nanostructures’ morphology, size, dispersion, composition, and the dielectric environment) and their integration with semiconductors for enhanced photocatalytic performance. Meanwhile, CO2 utilization is another environmentally friendly approach toward sustainable energy, and the electroreduction of CO2 into multi-carbon (C2+) products is of particular interest. Zheng and co-workers (article number 2005798) summarize the latest advances in Cu-catalyzed CO2 electroreduction and offer an investigation of relevant theoretical and mechanistic studies. Strategies for programming Cu catalysts implemented for improving the C2+ product selectivity and electrocatalytic activity are also presented, including tandem catalyst design, electronic structure tuning, selective facet exposure, and molecule/additive strategy. A broad portfolio of programmable materials developed for biological applications and customized according to features of interfacing biological entities. Nanomedicines and materials receptive to specific disease-associated and microenvironmental stimuli, including pH, redox-state, small molecules and upregulated proteins, have empowered novel diagnostics and therapies. Gianneschi and co-workers (article number 2007504) summarize programmable soft materials receptive to disease-associated stimuli through dynamic processes such as morphological and size transitions, changes in mobility and retention, as well as assembly and disassembly. They highlight the programmable soft materials deployed for chemo- and immunotherapies for cancer and other diseases like myocardial infarction and diabetes mellitus. Cell-based living materials have aroused intensive interest due to their extensive applications in cancer therapy, regenerative medicine, drug development, etc. Microfluidic platforms have been serving as high-throughput, scalable, and versatile tools in this field. Qin and co-workers (article number 2005944) profile recent advances in microfluidic platforms for programming cell-based living materials, including individual cells, cell-laden fibers, cell sheets, organoids, and organs in a high-throughput and efficient manner for diverse biomedical applications, such as cell therapy, tissue engineering, disease modeling, and drug screening. In addition to soft materials, semiconducting materials possess unique properties that are optimal for unconventional bioinformatics, biosensing, and healthcare. Ivanisevic and co-workers (article number 2004655) discuss concepts of inorganic substrate/microorganism biointerfaces serving as “smart” materials with desired responses to external conditions. These are deployable for the developing novel approaches of biosensing, bioelectronics, and biocompatible materials. They also identify optimal properties of inorganic materials along with target microorganisms. Beyond materials to be interfaced with them, biological systems have equally inspired us of functional programming and abiotic devices that emulate the functions of their biological counterparts, of which brain-inspired neuromorphic computing is a rising topic. Neuromorphic computing offers high-speed operation and low power consumption. Liu, Xie, and co-workers (article number 2006469) summarize the principles and characteristics of biological neurons and synapses, followed by recent developments of memristive materials and devices that emulate synaptic functions based on electric, magnetic, and optical stimuli. They emphasize the roles and performance of the devices, such as switching rate, switching ratio, power consumption, and endurance. Adaptive materials offer unique features in intelligent transformative systems, since the materials can realize adaptation to intervene in their environment, and are deployable in many aspects, such as wearable electronics, infrastructures, actuators, and the automotive industry. For flexible electronics, programmable materials demonstrate varying stiffness and flexibility under various conditions, which may be well-adapted to designed scenarios. The ongoing paradigm shift of flexible and wearable products creates a demand for intrinsically flexible batteries that can adapt to mechanical deformations without damages. Lithium-ion batteries are the dominant choice for such a power supply. However, the simultaneous achievement of high energy density and high mechanical flexibility remains a challenge. Zheng and co-workers (article number 2004419) discuss the criteria of flexible lithium batteries (FLBs), strategies to achieve flexibility, materials, and cell-design principles for FLBs. A flexible-battery plot is equivalent to the benchmarked performance of FLBs. Modern printing technologies have differentiated the advancements of flexible and wearable electronics, whereas ink materials with intrinsic programmability can be a promising enabler. Wang and co-workers (article number 2005890) investigate recent developments in nanocarbon ink materials (e.g., fullerenes, carbon nanotubes, graphene, and their derivatives). They emphasize that the programmability and multifunctionality can be applicable for flexible electronics, sensors, and actuators. In addition to these mechanically adaptive materials, mechanically active materials (MAMs) can sense and transduce external stimuli into mechanical outputs and vice versa. Owing to their excellent mechanical compliance, programmability, and biocompatibility, hydrogel-based MAMs can offer promising potential in next-generation robotics and smart systems. Salaita and co-workers (article number 2006600) discuss the compositions and fabrication of hydrogel-based MAMs and elaborate specific top-down or bottom-up approaches for programming hydrogels with multiple responsivity mechanisms, including optical, thermal, magnetic, electrical, chemical, and mechanical stimuli. The past decades also witnessed a paradigm shift toward proactive programming of materials’ functionality by leveraging the force–geometry–property relationships from conventional mechanics of materials that offer passive access to mechanical properties of materials in existing forms. Chen, Gao, and co-workers (article number 2007977) coined this rising field as “mechanomaterials” and profile the concept. By outlining the design principles on four scales: the atomic scale, the molecular scale, the manipulation of nanoscale materials, and the microscale design of structural materials, and further elaborating specific techniques involved to satisfy the multiscale programming of functional mechanomaterials, they also impactfully highlight the deployment of mechanomaterials related to their intriguing mechanical, optical, electrical and biological properties. More importantly, a closed-loop workflow is developed for the reverse design of programmable mechanomaterials to offer for the future development of the emerging field. We highly appreciate the valuable support from the editorial team of Advanced Materials, particularly Dr. Ulrike Kauscher Pinto and Dr. Jos Lenders. We are equally grateful to the colleagues who have shared their insights and perspectives. Finally, we would like to take this chance to congratulate Professor Chad Mirkin from Northwestern University in the USA for his 30 years of influential research with some topics covered by this issue. We sincerely hope that the readers enjoy this informative issue on programmable materials. The authors declare no conflict of interest.