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= Supramolecular polymers = Supramolecular polymer s are a kind of polymers whose monomeric units hold together via highly directional and reversible non-covalent interactions. Unlike covalently bonded polymers utilizing covalent bonds to construct the polymers skeleton, supramolecular polymers form themselves by non-covalent bonds (e.g. hydrogen bonding,  π-π interaction, metal coordination  and host-guest interaction    ). Due to their polymersic nature as well as reversible noncovalent interactions, supramolecular polymers show not only some polymers-like properties, but also dynamic properties such as self-healing.

= Properties of supramolecular polymers =

Self-healing property
Due to the nature of supramolecular polymers, the noncovalent interactions make supramolecular polymers more dynamic and reversible. Such properties enable supramolecular polymers to construct a dynamic and reversible network, which are able to develop self-healing materials based on noncovalent bonds. Compared with self-healing materials based on covalent bonds, these supramolecular polymers-based self-healing materials can restore the initial structure and function of polymers before being exposed to damages, and can also undergo repeating damage-heal process.

Optoelectronic property
To achieve the light-to-charge conversion is the prerequisite step in artificial photosynthesis systems. By incorporating electron donor and electron acceptor into the supramolecular polymers, a number of artificial systems, including photosynthesis system, can be constructed. Due to the existence of more than one interactions (π-π interaction, hydrogen bonding interaction and the like), electron donor and electron acceptor can be held together in a proper proximity to afford long-lived charge separated states. Then a light-to-charge conversion system with faster photoinduced electron transfer and higher electron-transfer efficiency can be achieved in these artificial polymers.

Biocompatible property
It is quite common that biomolecule s, such as DNA, protein and the like, come into being through various noncovalent interactions in biological system. Likewise, supramolecular polymers assembles itself by means of combination of noncovalent interactions. Such formation manner endows supramolecular polymers with features, being more sensitive to external stimuli and able to render reversibly dynamic changes in structures and functions. By modifying monomeric units of supramolecular polymers with water-soluble pendants, bioactive moieties as well as biomarkers, supramolecular polymers can realize various kinds of functions and applications in biomedical field. At the same time, their reversible and dynamic nature make supramolecular polymers bio-degradable, which surmounts hard-to-degrade issue of covalent polymers and makes supramolecular polymers a promising platform for biomedical applications. Being able to degrade in biological environment lowers potential toxicity of polymers to a great extent and therefore, enhances biocompatibility of supramolecular polymers.
 * [[File:Fig 2. A titin-mimicking supramolecular polymers.png|thumb|450x450px|Fig 2. A titin-mimicking supramolecular polymers, with quadruple hydrogen bonding between 2-ureido-4[1H]-pyrimidone (UPy) moiety. ]]

= Applications of supramolecular polymers =

Self-healing material
Based on the formation manners of supramolecular polymers, there are mainly three types self-healing materials, namely hydrogen bonding-based, metal coordination-based and π-π interaction-based self-healing supramolecular polymers.

Hydrogen bonding-based self-healing supramolecular polymers
Binder et al. developed a bivalent poly(isobutylene)s (PIBs) with barbituric acid functionalized at head and tail. Multiple hydrogen bonding existed between the carbonyl group and amide group of barbituric acid enable it to form a supramolecular network. In this case, the snipped small PIBs-based disks can recover itself from mechanical damage after several-hour contact at room temperature.

Metal coordination-based self-healing supramolecular polymers
Taking advantage of coordination interactions between catechol and ferric ions, Niels et al. developed pH-control self-healing supramolecular polymers. The formation of Mono-, bis- and triscatehchol-Fe3+ complexes can be controlled under different pH, of which the bis- and triscatehchol-Fe3+ complexes show elastic moduli as well as self-healing capacity. For example, the triscatehchol-Fe3+ can restore its cohesiveness and shape after being teared.

π-π interaction-based self-healing supramolecular polymers
Burattini et al. firstly reported a π-π interaction-based self-healing supramolecular polymers, utilizing chain-folding polyimide and pyrenyl-end-capped chains to construct supramolecular network. This polymers can realize self-healing process by compressing two separated part together at 80℃ and simultaneously this healing process can be repeated several times.

Optoelectronic application
One of the successful attempt of establishing photosynthesis system with supramolecular polymers is the combination of perylene derived bisdiimide (PDI) and diketopyrrolopyrrole (DPP) by Braunschweig’s group. Through triple hydrogen bonding between a diamidopyridine group of DPP and diimide groups of PDI as well as π-π stacking of PDI-DPP complex, the helical supramolecular skeleton was formed. Both LUMO-to-LUMO electron transfer and HOMO-to-HOMO hole transfer can occur upon excitation with 435nm light, along with fast charge separation (<0.2ps).

Biomedical application
With the excellent nature in biodegradation and biocompatibility, supramolecular polymers show great potential on development of drug delivery, gene transfection and etc.

Drug delivery
There are multiple stimuli (e.g. pH, oxidants, reductant, enzymes and temperature) in milieu interne. Drug carriers based on supramolecular polymers show rapid stimuli-responsive capacity. Take hydrogen bonding-based supramolecular polymerss as an example, they are quite sensitive to the pH changes in external environments. Therefore, the dynamic molecular skeletons of supramolecular polymers can be depolymerized when exposing to the external stimuli like pH in vivo. On the basis of this property, supramolecular polymers are capable of being a drug carrier. Making use of hydrogen bonding between nucleobases to induce self-assemble into pH-sensitive spherical micelle s, successful controlled drug-delivery research have been achieved by Zhu et al, with even more rapid release rate than covalent polymers.

Gene transfection
Effective and low-toxic nonviral cationic vector s are highly desired in the field of gene therapy. On account of the dynamic and stimuli-responsive properties, supramolecular polymers offer a cogent platform to construct vectors for gene transfection. By combining ferrocene dimer with β-cyclodextrin dimer, Zhu et al. developed a redox-control supramolecular polymers system to act as a vector. In Cos-7 cells, this supramolecular polymersic vector can release enclosed DNA upon exposing to H2O2 and achieve gene transfection.

Others
Rationally designed supramolecular polymers-based polymerss can simultaneously meet the requirements of aqueous compatibility, bio-degradability, biocompatibility, stimuli-responsiveness and other strict criterion. Consequently, supramolecular polymers can be applied to the biomedical field as a robust system. Other than applications mentioned above, other important and fascinating biomedical applications, like protein delivery, bio-imaging and diagnosis  and tissue engineering,  are also well developed.

= References =