PerspectiveChemistry

Synchronized Self-Assembly

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Science  02 May 2008:
Vol. 320, Issue 5876, pp. 620-621
DOI: 10.1126/science.1157225

Self-assembly has long been recognized as a powerful synthetic approach to obtain dynamic structures exhibiting complex functions, such as those found in nature (1). By carefully regulating non-equilibrium self-assembly, two recent studies (2, 3) demonstrate important progress, resulting in new porous membranes whose structure is controlled on several length scales.

There are two main types of self-assembly (4). Static self-assembly deals with equilibrium structures; the shapes and interaction energies of the participating components are altered to achieve various organizations. For molecular components, noncovalent interactions—like hydrogen bonds, electrostatics, and van der Waals forces—are manipulated to encode building blocks with instructions that lead to the spontaneous generation of a desired target (5).

Kinetic control of structure.

Ladet et al. successively interrupt polymer densification by removing a molded hydrogel from neutralization bath to produce multi-membrane hydrogels.

CREDIT: DOROTHY LOUDERMILK/UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN

Dynamic self-assembly, on the other hand, is a non-equilibrium process in which energy is supplied to the system to maintain a steady-state population of ordered structures. Because dynamic self-assembly involves the added complexity of a sustainable driving force, only limited progress has been made in this area (6). Recognizing that nonequilibrium self-assembly may organize matter differently from that which occurs at thermodynamic equilibrium, chemists are challenged to bring kinetic control into their repertoire of methods. This is what the two recent studies achieved.

In the first study, Ladet et al. (2) formed chitosan gels by slowly removing the water from aqueous alcohol solutions of a chitosan polyelectrolyte. This alcohol gel can be molded into shapes such as tubes and spheres of various sizes. When the gel object was bathed in a solution of an aqueous base, hydrophobic interactions within the network dominated, causing the polymer molecules to contract and form a membrane-like skin around the original object. The rate of membrane formation via densification typically occurred on the time scale of minutes. Simply removing the object from the neutralization bath interrupts membrane thickening. Insertion of the object back into the bath initiates formation of a second membrane layer between the gel-core and first membrane. Interestingly, this process results in an intermembrane space that can accommodate payloads such as chondrocyte cells. Repetition of this sequence produced concentric shells of hydrogel membranes extending inward toward the core (see the first figure).

The process used to create these layered hydrogels exploits a simple type of kinetic control. Another way to drive new modes of nonequilibrium structure would be to time the release of the potential energy stored in the self-assembling components. A recent report by Capito et al. elegantly demonstrates such a regulated process, in which ordered structures rapidly self-assemble at the interface of two chemically distinct electrolyte solutions. This process produced functional porous membranes with complex hierarchical structures (3).

Synchronized self-assembly.

In the study by Capito et al., macroscopic sacs and membranes are prepared when a solution of macromolecules contacts a solution of self-assembling molecules (bottom). Upon contact (A), the components are attracted by electrostatics (B) and instantly form a diffusion barrier (C). Sustained membrane growth proceeds by release of counterions (D), enhancing the osmostic pressure imbalance between the two liquids (E) that drives the polymeric electrolyte to uncoil and extrude through the barrier (F). New polyelectrolyte is then exposed (G) onto which new nanofibers assemble (H) causing further release of counterions (D).

CREDIT: DOROTHY LOUDERMILK/UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN

The authors assemble membranes instantly when two liquids come in contact, establishing a physical barrier that hinders dissipation of the ion imbalance between the small molecule electrolyte and the polyelectrolyte solutions. High osmotic pressure within the polyelectrolyte solution and the requirement for electroneutrality impels the megamolecules to extrude through pores in the structured barrier and enter the electrolyte solution. This leads to the growth of perpendicular nanofibrils in a dynamic process that is sustained by osmotic pressure (see the second figure).

The study shows that membrane-enclosed sacs of macroscopic sizes can be rapidly formed upon contact between the two liquids; moreover, the presence of polymeric electrolyte in the sac interior imparts unique self-healing function to these compartments. The resulting membranes are permeable to proteins and could be used to entrap cells by simple liquid-liquid contact to create mini-cell biology laboratories. These could then be used as controlled environments for cell expansion, stem cell differentiation, or studies of bio-signaling from neighboring sacs entrapping colonies of other cells. By tailoring the small molecule electrolytes, the structures might be customized for a diverse array of applications in biomedicine, catalysis, and energy generation.

These recent findings point to new synthetic concepts whereby the final supramolecular structure depends on the mechanistic pathway of the assembly, rather than the thermodynamic endpoint. Ladet et al. show that even a relatively simple kinetic scheme can produce intriguing structures from simple components. Capito et al. identify the possibility of a self-sustaining pathway in which static self-assembly and a kinetically regulated mechanism combine to generate diverse architecture and functions. This finding opens the way to exciting opportunities for novel materials that may stem from incorporating pathway-directing information into the constituents and processes of self-assembly.

References

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