Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Synthetic molecular motors and mechanical machines.

23 Stereocontrol of Lactide ROP 6164 231 Isotactic Polylactides 6164 232 Syndiotactic Polylactides 6166 233 Heterotactic Polylactides 6166 3 Anionic Polymerization 6166 4 Nucleophilic Polymerization 6168 41 Mechanistic Considerations 6168 42 Catalysts 6169 421 Enzymes 6169 422 Organocatalysts 6169 43 Stereocontrol of Lactide ROP 6170 44 Depolymerization 6170 5 Cationic Polymerization 6170 6 Conclusion and Perspectives 6171 7 Acknowledgments 6173 8 References and Notes 6173

Controlled ring-opening polymerization of lactide and glycolide.

BACKGROUND Hydrogels are formed through the cross-linking of hydrophilic polymer chains within an aqueous microenvironment. The gelation can be achieved through a variety of mechanisms, spanning physical entanglement of polymer chains, electrostatic interactions, and covalent chemical cross-linking. The water-rich nature of hydrogels makes them broadly applicable to many areas, including tissue engineering, drug delivery, soft electronics, and actuators. Conventional hydrogels usually possess limited mechanical strength and are prone to permanent breakage. The lack of desired dynamic cues and structural complexity within the hydrogels has further limited their functions. Broadened applications of hydrogels, however, require advanced engineering of parameters such as mechanics and spatiotemporal presentation of active or bioactive moieties, as well as manipulation of multiscale shape, structure, and architecture. ADVANCES Hydrogels with substantially improved physicochemical properties have been enabled by rational design at the molecular level and control over multiscale architecture. For example, formulations that combine permanent polymer networks with reversibly bonding chains for energy dissipation show strong toughness and stretchability. Similar strategies may also substantially enhance the bonding affinity of hydrogels at interfaces with solids by covalently anchoring the polymer networks of tough hydrogels onto solid surfaces. Shear-thinning hydrogels that feature reversible bonds impart a fluidic nature upon application of shear forces and return back to their gel states once the forces are released. Self-healing hydrogels based on nanomaterial hybridization, electrostatic interactions, and slide-ring configurations exhibit excellent abilities in spontaneously healing themselves after damages. Additionally, harnessing techniques that can dynamically and precisely configure hydrogels have resulted in flexibility to regulate their architecture, activity, and functionality. Dynamic modulations of polymer chain physics and chemistry can lead to temporal alteration of hydrogel structures in a programmed manner. Three-dimensional printing enables architectural control of hydrogels at high precision, with a potential to further integrate elements that enable change of hydrogel configurations along prescribed paths. OUTLOOK We envision the continuation of innovation in new bioorthogonal chemistries for making hydrogels, enabling their fabrication in the presence of biological species without impairing cellular or biomolecule functions. We also foresee opportunities in the further development of more sophisticated fabrication methods that allow better-controlled hydrogel architecture across multiple length scales. In addition, technologies that precisely regulate the physicochemical properties of hydrogels in spatiotemporally controlled manners are crucial in controlling their dynamics, such as degradation and dynamic presentation of biomolecules. We believe that the fabrication of hydrogels should be coupled with end applications in a feedback loop in order to achieve optimal designs through iterations. In the end, it is the combination of multiscale constituents and complementary strategies that will enable new applications of this important class of materials.

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Advances in engineering hydrogels

Hydrogels attract great attention as biomaterials as a result of their soft and wet nature, similar to that of biological tissues. Recent inventions of several tough hydrogels show their potential as structural biomaterials, such as cartilage. Any given application, however, requires a combination of mechanical properties including stiffness, strength, toughness, damping, fatigue resistance and self-healing, along with biocompatibility. This combination is rarely realized. Here, we report that polyampholytes, polymers bearing randomly dispersed cationic and anionic repeat groups, form tough and viscoelastic hydrogels with multiple mechanical properties. The randomness makes ionic bonds of a wide distribution of strength. The strong bonds serve as permanent crosslinks, imparting elasticity, whereas the weak bonds reversibly break and re-form, dissipating energy. These physical hydrogels of supramolecular structure can be tuned to change multiple mechanical properties over wide ranges by using diverse ionic combinations. This polyampholyte approach is synthetically simple and dramatically increases the choice of tough hydrogels for applications.

https://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/56662/1/Gong-Manuscript-1.pdf

Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity

Polyisoprenes were found to form inclusion complexes with cyclodextrins with high selectivity to give crystalline compounds. β-Cyclodextrin formed complexes only with polyisoprene of low molecular weigth, whereas γ-cyclodextrin formed complexes with polyisoprenes of high molecular weight. α-Cyclodextrin did not form complexes with polyisoprene of any molecular weight. The yields of γ-cyclodextrin complexes increased with increasing molecular weights of the polyisoprenes and reached a maximum of around several thousands, and then decreased.

Complex Formation between Polyisoprene and Cyclodextrins

Branched supramolecular polymers formed by bifunctional cyclodextrin derivatives

Formation of supramolecular isomers; poly[2]rotaxane and supramolecular assembly.

Even smallerartificialmolecularmotorsareextremelydifficulttosee,andthus, additional visualization techniques are necessary toobserve rotational behavior. Rotaxanes have a suitablestructure for the construction of molecular motors. We havepreviously investigated the rotational motion of a cyclo-dextrinrelativetoanaxismoleculeusingNMRtechniques.

Single‐Molecule Imaging of Rotaxanes Immobilized on Glass Substrates: Observation of Rotary Movement

Supramolecular polymeric materials have received much attention in recent years due to the fabrication of supramolecular materials with mechanical properties and stimuli-responsive properties in response to polymeric designs based on noncovalent cross-linkers. Host–guest interactions are useful noncovalent interactions that can be employed to realize supramolecular materials. To understand the relationship between the mechanical properties of supramolecular polymeric materials based on host–guest interactions and the molecular structure of guest molecules on the polymer side chain, we prepared supramolecular hydrogels cross-linked by inclusion complexes between cyclodextrin (CD) and cationic alkyl guests on the polymer side chain. The mechanical properties were influenced by an electric barrier due to the cationic group, CD cavity size, length of the guest unit, charge number of the electric barrier, and reduction responsiveness. Although the introduction of an electric barrier to the end of the alkyl gue...

Mechanical Properties of Supramolecular Polymeric Materials Formed by Cyclodextrins as Host Molecules and Cationic Alkyl Guest Molecules on the Polymer Side Chain

Yoshinori Takashima

Papers

Complex Formation between Polyisoprene and Cyclodextrins

Branched supramolecular polymers formed by bifunctional cyclodextrin derivatives

Formation of supramolecular isomers; poly[2]rotaxane and supramolecular assembly.

Single‐Molecule Imaging of Rotaxanes Immobilized on Glass Substrates: Observation of Rotary Movement

Mechanical Properties of Supramolecular Polymeric Materials Formed by Cyclodextrins as Host Molecules and Cationic Alkyl Guest Molecules on the Polymer Side Chain