Stimuli-responsive self-assembly

This chapter aims to highlight the recent accomplishments made in the development of smart, dynamic, biological surfaces and their relevance for biomedical applications. The smart dynamic surfaces are mostly based on stimuli-responsive self-assembled monolayers (SAMs) [66,67] and polymer films [68-73] or on utilizing the SAMs and the polymer films as platforms for linking the stimuli-responsive material [74]. The chapter is organized according to the external stimuli used to manipulate the properties of the dynamic surface chemrcal/biochemical, thermal, electrical, and optical stimuli. A brief look at the current status and the future outlook of the field will conclude this chapter. 

Stimuli-Responsive Self-Assembling System into Nanofibers. 41... 

In 2000, the first example of ELP diblock copolymers for reversible stimulus-responsive self-assembly of nanoparticles was reported and their potential use in controlled delivery and release was suggested [87]. Later, these type of diblock copolypeptides were also covalently crossUnked through disulfide bond formation after self-assembly into micellar nanoparticles. In addition, the encapsulation of l-anilinonaphthalene-8-sulfonic acid, a hydrophobic fluorescent dye that fluoresces in hydrophobic enviromnent, was used to investigate the capacity of the micelle for hydrophobic drugs [88]. Fujita et al. replaced the hydrophilic ELP block by a polyaspartic acid chain (D ). They created a set of block copolymers with varying... 

Eloi J-C, Rider DA, Cambridge G, Whittell GR, Winnik MA, Manners I (2011) Stimulus-responsive self-assembly reversible, redox-controlled micellization of polyferrocenylsilane diblock copolymers. J Am Chem Soc 133 8903-8913... 

Yan Q, Yuan JY, Zhang FB, Sui XF, Xie XM, Yin YW, Wang SF, Wei Y (2009) Cellulose-based dual graft molecular brushes as potential drug nanocarriers stimulus-responsive micelles, self-assembled phase transition behavior, and tunable crystalline morphologies. Biomacromolecules 10 2033-2042... 

In reeent years, polypeptides have been intensively studied as stimulus-responsive polymers owing to their obvious attractive features such as their inherent ionizable groups Le., NH2 and COOH), hierarchical self-assembly, and multiple secondary or tertiary structures. The biocompatibility and bio-degradability of hybrid pol)q)eptides or proteins have attracted great attention for possible clinical use some have entered clinical trials. 

Self-assembly via an induced stimulus was reported by Chilkoti etal. utilizing an elastin-like polypeptide that forms micelles at the critical micelle temperature (Figure 14). These induced multivalent architectures presenting RGD epitopes, designed to bind integrins, were shown to exhibit dynamic modulation of a receptorbinding affinity in response to an external trigger. ... 

Polyurethanes (PUs) are found everywhere the chair we sit on, the comfortable bed we sleep on, every nook and comer of our house, the car we drive, the refrigerator and air conditioner that comfort us, and many other day-to-day objects. PUs can also be used for dmg delivery and may be fabricated as bioinert, biodegradable, stimulus-responsive, shape-memory, conjugated, self-assembled, rigid, flexible, and porous systems. 

Another type of stimulus-responsive supramolecular polymer was produced using the complementary affinity of a bisporphyrin (Figure 7.20). The bispor-phyrin units were connected with 1,3-butadiyne units to produce homoditopic tet-rakisporphyrin 23. The iterative self-assanbly of 23 gave rise to supramolecular nanometric polymeric assemblies. An electron-deficient aromatic guest can aggressively bind within the bisporphyrin cleft, causing the supramolecular polymeric assemblies to dissociate. 

An early example of a self-assembled calix[4]pyrrole system was reported in 1996 by Sessler and coworkers [79]. Solid state analysis of the calix[4]pyrrole monoacid 38 revealed that it forms a homoditopic complex both in the solid state (cf. Fig. 12.24) and in organic media upon deprotonation. This dimer could be broken (i.e., disassembled) by exposure to a source of fluoride anions. However, it is only in recent years that calix[4]pyrroles have been explored for the creation of capsule-like systems that can be responsive to a specific stimulus [79]. 

Overall, this initial report by Sessler and collaborators served to demonstrate that TTF-C4Ps could be used to create heterocomplementary supramolecular oligomers, both in solution and in the solid state, and that stimulus-induced responsive behavior was an inherent feature of the resulting self-assembled systems. 

A number of studies of the self-association features of TTF-C4P-based systems are ongoing presently in the Sessler and Jeppesen laboratories. The interested reader is advised to consult the current literature for updates that may arise as the result of these efforts. Based on what has been reported to date, it is clear that functionalized calixpyrroles have a role to play in the design and synthesis of stimulus-responsive supramolecular constructs, including capsules and self-assembled polymers. 

Combining peptide sequences and synthetic polymers is useful not only for enhanced control over nanoscale structure formation, but also for production of biologically interactive materials. Biomimetic hybrid polymers may also produce sophisticated superstructures with new material properties. Smart materials based on polypeptides may reversibly change conformation and associated properties in response to an environmental stimulus, such as a shift in pH or temperature (Rodriguez-Hernandez et ah, 2005). Polypeptide block copolymers may also be used as model systems to study generic self-assembly processes in natural proteins. Obviously, such materials could be of significant interest for a variety of biomedical and bioanalytical applications. 

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