Shape-changing hydrogels that can bend, twist, or actuate in response to external stimuli are critical to soft robots, programmable matter, and smart medicine. Shape change in hydrogels has been induced by global cues, including temperature, light, or pH. Here we demonstrate that specific DNA molecules can induce 100-fold volumetric hydrogel expansion by successive extension of cross-links. We photopattern up to centimeter-sized gels containing multiple domains that undergo different shape changes in response to different DNA sequences. Experiments and simulations suggest a simple design rule for controlled shape change. Because DNA molecules can be coupled to molecular sensors, amplifiers, and logic circuits, this strategy introduces the possibility of building soft devices that respond to diverse biochemical inputs and autonomously implement chemical control programs.

Self-assembly creates natural mineral, chemical, and biological structures of great complexity. Often, the same starting materials have the potential to form an infinite variety of distinct structures; information in a seed molecule can determine which form is grown as well as where and when. These phenomena can be exploited to program the growth of complex supramolecular structures, as demonstrated by the algorithmic self-assembly of DNA tiles. However, the lack of effective seeds has limited the reliability and yield of algorithmic crystals. Here, we present a programmable DNA origami seed that can display up to 32 distinct binding sites and demonstrate the use of seeds to nucleate three types of algorithmic crystals. In the simplest case, the starting materials are a set of tiles that can form crystalline ribbons of any width; the seed directs assembly of a chosen width with >90% yield. Increased structural diversity is obtained by using tiles that copy a binary string from layer to layer; the seed specifies the initial string and triggers growth under near-optimal conditions where the bit copying error rate is <0.2%. Increased structural complexity is achieved by using tiles that generate a binary counting pattern; the seed specifies the initial value for the counter. Self-assembly proceeds in a one-pot annealing reaction involving up to 300 DNA strands containing >17 kb of sequence information. In sum, this work demonstrates how DNA origami seeds enable the easy, high-yield, low-error-rate growth of algorithmic crystals as a route toward programmable bottom-up fabrication.DNA nanotechnology ͉ nucleation ͉ crystal growth G rowth from seeds confers both flexibility and control to a synthetic method: A single process can generate a wide range of products, with the specific choice of product determined by information contained in the seed; side products can be reduced dramatically because of the presence of a nucleation barrier, resulting in high-yield synthesis; and growth can proceed under near-ideal chemical conditions, resulting in products with few defects. Mineral and chemical compounds exhibit the simplest form of seeded growth, wherein a supersaturated solution of a polymorphic material can be inoculated with seed crystals to produce large, pure crystals of the desired form (1). In nanostructure synthesis, seeds may be used to control the diameter and crystal type of carbon nanotubes and metal nanowires (2-4). Biological organisms use information-bearing seeds with amazing control over the type, place, and timing of the structures grown from the same starting materials: Minimal media consisting of glucose, nitrogen, sulfates, and salts can be seeded with a single bacterium that will convert the starting material to biomass whose structure and composition is dictated by genomic information (5). Multicellular development, where genomic information in the zygote directs the algorithmic construction of the entire organism (6, 7), demonstrates the ultimate potential of seeded growth. Accessing this potential...

A central goal of chemistry is to fabricate supramolecular structures of defined function and composition. In biology, control of supramolecular synthesis is often achieved through precise control over nucleation and growth processes: A seed molecule initiates growth of a structure, but this growth is kinetically inhibited in the seed's absence. Here we show how such control can be systematically designed into self-assembling structures made of DNA tiles. These structures, ''zig-zag ribbons,'' are designed to have a fixed width but can grow arbitrarily long. Under slightly supersaturated conditions, theory predicts that elongation is always favorable but that nucleation rates decrease exponentially with increasing width. We confirm experimentally that although ribbons of different widths have similar thermodynamics, nucleation rates decrease for wider ribbons. It is therefore possible to program the nucleation rate by choosing a ribbon width. The presence of a seed molecule, a stabilized version of the presumed critical nucleus, removes the kinetic barrier to nucleation of a ribbon. Thus, we demonstrate the ability to grow supramolecular structures from rationally designed seeds, while suppressing spurious nucleation. Control over DNA tile nucleation allows for proper initiation of algorithmic crystal growth, which could lead to the high-yield synthesis of micrometer-scale structures with complex programmed features. More generally, this work shows how a selfassembly subroutine can be initiated.algorithmic self-assembly ͉ DNA nanotechnology ͉ self-assembly ͉ supramolecular chemistry B iology demonstrates how self-assembly can create sophisticated organization on the molecular scale. The fundamental challenge in engineering a self-assembled object is that its molecular components must themselves contain the information needed to guide self-assembly.Controlling the nucleation of a self-assembled object is the first step in controlling the self-assembly process, as exemplified by the formation of actin networks (1, 2), the growth of microtubules on the centrosome (3), and the assembly of bacterial flagella (4). These systems avoid the difficulty of controlling homogeneous (unseeded) nucleation by relying on heterogeneous (seeded) nucleation: Growth is rare except in the presence of a seed molecule, from which it proceeds with little or no kinetic barrier. Here we demonstrate a general strategy for designing self-assembled molecular structures whose nucleation is controlled by a seed. We use programmable DNA tiles to create a series of seeded ''zig-zag ribbons'' that have increasing kinetic barriers to homogeneous nucleation.DNA tiles (5, 6) are a general-purpose nanoscale construction material. A DNA tile consists of multiple, interwoven strands that form double helices connected by ''crossover points'' (Fig. 1a). Tiles bind to each other when their single-stranded (''sticky'') ends hybridize (Fig. 1b) and can assemble into extended structures, including one-and two-dimensional crystals (7-10). Tile interactions a...

Control over when and where nanostructures arise is essential for the self-assembly of dynamic or multicomponent devices. We design and construct a DNA origami seed for the control of DAE-E tile DNA nanotube assembly. Seeds greatly accelerate nanotube nucleation and growth because they serve as nanotube nucleation templates. Seeds also control nanotube circumference. Simulations predict nanotube growth rates and suggest a small nucleation barrier remains when nanotubes grow from seeds.

Peptides or peptide conjugates capable of assembling into one-dimensional (1D) nanostructures have been extensively investigated over the past two decades due to their implications in human diseases and also their interesting applications as biomaterials. While many of these filamentous assemblies contain a β-sheet-forming sequence as the key design element, their eventual morphology could assume a variety of shapes, such as fibrils, ribbons, belts, or cylinders. Deciphering the key factors that govern the stacking fashion of individual β-sheets will help understand the polymorphism of peptide assemblies and greatly benefit the development of functional materials from customized molecular design. Herein, we report the decisive role of electrostatic interactions in the lamination and untwisting of 1D assemblies of short peptides. We designed and synthesized three short peptides containing only six amino acids (EFFFFE, KFFFFK, and EFFFFK) to elucidate the effective control of β-sheet stacking. Our results clearly suggest that electrostatic repulsions between terminal charges reduce the pitch of the twisting β-sheet tapes, thus leading to highly twisted, intertwined fibrils or twisted ribbons, whereas reducing this repulsion, either through molecular design of peptide with opposite terminal charges or through coassembly of two peptides carrying opposite charges, results in formation of infinite assemblies such as belt-like morphologies. We believe these observations provide important insight into the generic design of β-sheet assemblies.