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Mussel-Inspired Polyglycerols as Universal Bioinert and Multifunctional Coatings

Muschel-inspirierte Polyglycerole als universelle bioinerte und multifunktionale Beschichtungen
 
: Wei, Qiang
: Haag, Rainer; Hartwig, Andreas

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Berlin, 2014, 205 S.
Berlin, Univ., Diss., 2014
URN: urn:nbn:de:kobv:188-fudissthesis000000098105-2
Englisch
Dissertation
Fraunhofer IFAM ()

Abstract
In this thesis, a set of universal polymer coatings with tunable chemical activity and bioinertness were developed by mussel-inspired catecholic hyperbranched polyglycerols (hPGs) via simple dip coating. A wide range of material surfaces, including metal oxides, noble metals, ceramics, and polymers, were successfully modified by these new coatings to achieve versatile biomedical applications. Under weak basic condition, parts of the catechol groups in the catecholic hPGs can be spontaneously oxidized to quinones which can crosslink with each other. The remaining catechol groups still anchored the polymers to the substrates, while the crosslinking of the quinones caused the formation of multilayer coatings. In the case of acidic conditions, the oxidation can be avoided, thus catechol groups can only serve as anchors to result in monolayer coatings. However, monolayer coatings are only stable on metal and metal oxide surfaces, on which catechols form coordinative bonds with the surface metal atoms. Moreover, a single catechol group even failed to efficiently anchor hPG molecules on metal oxide surface due to the oxidative detachment. Therefore, it is necessary to employ multivalent anchoring for long-term stable coatings. As a result, only the multivalently anchored and crosslinked multilayer coatings can be stabilized on various surfaces. Bioinert surface coatings can be directly prepared by catecholic hPGs with appropriate amount of catechol groups. Thirty percent of catechol functionalization switched the bioinert hPG to a protein-adhesive molecule, because quinones strongly interact with amine and thiol groups in proteins. As mentioned above, single catechol anchor group suffers oxidative detachment. Our results revealed that hPGs with 5 to 10 percent of catechol functional degree showed an excellent antifouling performance, and at the same time, can generate stable multilayer coatings. However, there were still a few free catechol groups exposed on the surface of the coatings. In order to generate perfect bioinert hPG surfaces, hierarchical hPG multilayer coatings were developed. In this case, mono-catecholic hPGs were used to terminate all of the free catechol groups and to construct a flexible bioinert top layer via quinone crosslinking. In addition, an extra chemically active catecholic hPG foundation layer can stabilize coatings even on chemically inert substrates including polytetrafluoroethylene (PTFE). This foundation layer can be further shielded by the above mentioned bioinert catecholic hPGs, and the mono- catecholic hPG terminal layer via the same chemistry. As a result, the chemical activity of this new type of coatings gradually decreases and the bioinert property gradually increases from bottom to top. With these characteristics, this new architecture was employed to form a highly stable material-independent surface coating with outstanding antifouling properties. The highly adhesive catecholic hPG that used as foundation layer contains 40% of catechol groups and 60% of amine groups. Both two kinds of functional groups are abundant in mussel foot proteins (mfps) and play key roles in the rapid formation of mussel byssus. Furthermore, the molecular weight of this catecholic hPG reaches 10 kDa, which is similar to the most adhesive mussel foot protein mfp-5 (about 9 kDa). Also, the dendritic structure, exhibits a relatively distinct “interior”, and exposes its functional groups on the surface of the polymer, while natural proteins exhibit important domains on the surface as well. Based on the mimicry of functional groups, molecular weight, and molecular structure, this new mussel-inspired hPG formed a considerably stable coating on virtually any type of material surface within 10 min or a micrometer scale coating in hours, which is comparable to the formation of mussel byssal threads in nature. Functional molecules, like collagen A and rhodamine B, can be post-functionalized or pre-functionalized to the coatings to generate different kinds of functional biosurfaces. Additionally, the controllable surface roughness resulted in superhydrophilic or superhydrophobic surface properties for self-cleaning applications. This bioinspired copy of mussel foot proteins reaches a new level of functional mimicry. What more can we learn from these proteins to design synthetic molecules for material surface modification? Mussels employ thiol-rich mfp-6 to reduce quinones in the adhesive interface back to catechols to enhance its adhesion, while inside catechols can be oxidized to quinones to enhance cohesion. How to achieve controllable redox balance of catechol groups in coatings remains a tremendous challenge and can be a direction to develop next generation of mussel-inspired coatings. Besides catecholic surface chemistry, mussels also employ hydrophobic aromatic sequences, mainly present in mfp-3 “slow”, on the one hand to retard oxidation of catechols by shielding the groups from aqueous solution, on the other hand to increase the hydrophobic interaction which is not pH dependent. Combining this hydrophobic interaction with catecholic chemistry, universal polymer coatings with well defined thickness and surface morphology may be achieved.

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