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Multifunctional Scaffolds

In this area we focus on the building of active materials or machines by organizing smart molecules and complex nanostructures in polymeric scaffolds, liquid crystalline films or solid surfaces. The cytoskeleton of biological cells serves as a prototypical example and as an inspiration for the desired structures. Furthermore, functionalized polymers are investigated with respect to their application as semiconductors, superconductors, in optoelectronics and as supports for catalysts. Accordingly, within the framework of hybrid techniques, immobilized enzymes will be used as catalysts for synthesis as well as for diagnostic purposes.

From biological cells to biomimetic molecular and nano-systems

Recent scientific progress has generated a variety of smart nano-objects and provided a detailed understanding of active biological macromolecules. Nature can teach us how to build novel machines from the aforementioned nano-objects and macromolecules, which entails more than simply adding together these building blocks.

From a materials perspective biological cells can be considered as an array of active and passive nano-sized objects (i.e. proteins) that are organized by polymeric scaffolds (i.e. the cell’s cytoskeleton) to build a multifunctional, adaptable machine. Biological cells move on their own, generate forces, sense their environment, and independently produce new macromolecules. However, some 25,000 genes encode the information of human life, and their subsequent transcription and translation add to the complexity of molecular interactions resulting in an astronomical combinatorial number of relations that are to decipher and to understand. Nevertheless, two synergistic approaches will be used for the challenging knowledge transfer from cells to new materials. In vitro studies of cells’ reconstituted elements identified as independent functional modules – such as the cytoskeletal elements mentioned in the section on hard-soft hybrid materials – will be complemented by research that utilizes whole cells to create smart materials. Eventually, after clarifying the salient features contributed by the cells, the cells could be replaced by a biomimetic solution which replicates these contributions in a highly controllable fashion.

Novel high throughput and high content cytological techniques facilitated the new era of cellomics. Cutting-edge molecular biology, microarray techniques, and microfluidics will be combined with unique laser manipulation tools to isolate and to manipulate often rare cells with unique functions or desired intracellular components. Among these techniques is the molecular marker-free cell elasticity based isolation of cells with the Optical Stretcher, the optomolecular control of cell growth, and the laser beam based dissection of cells and cell organelles. Building on the Universität Leipzig’s track record, hybrid, biomedical sensors that emerge from the synthesis of cells and semiconductor chips as well as the biomimetic reconstruction of the eye’s retina will be a focus of these efforts. Moreover, cellular motion, force generation, and cellular mechanosensitivity will be analyzed with scanning force microscopy, optical traps, and multiphoton microscopy of motile cells transfected with GFP (green fluorescent protein)-constructs of various cytoskeletal elements to understand how nano-sized active elements in a polymer matrix act as mechanical machines. When a material and/or an application is established that integrates specific cells as functional elements, in a second step, as mentioned, the cells will be replaced by a minimal reconstituted system of cellular components and ultimately a stable biomimetic system will be derived.

Hard-soft hybrid systems

By identifying cellular subunits acting as independent functional modules a biological cell’s complexity becomes tractable and the fundamental physical principles of these modules can be studied. A typical example for such a module is the intracellular scaffold known as the cytoskeleton. Nano- and microparticles can be ideally organized in reconstituted cytoskeletal elements as soft scaffolds.

On the other hand, isolated cytoskeletal structures can be perfectly confined and coupled to solid-state and synthetic polymer micro- and nanostructures. The cytoskeleton is the key structural element in cellular organization. It is a compound of highly dynamic polymers and active nano-elements inside biological cells that mechanically senses a cell’s environment. The cytoskeleton generates cellular motion and forces sufficiently strong to push rigid AFM cantilevers out of the way. These forces are generated by molecular motor-based nano-muscles and by polymerization through mechanisms similar to Feynman’s hypothetical thermal ratchet. Further, the nano-sized motors overcome the inherently slow, often glass-like Brownian polymer dynamics. The interaction between cytoskeletal polymeric scaffolds and molecular motors results in novel self-organization, rapid switching between fluid and solid states, and transitions between ordered and unordered states.

Similar to the lamellipodia found in cells, biomimetic thin active polymeric films can be created which move on nano- and microstructured surfaces by polymerization, and nano-muscles created from polymers and molecular motors can exert forces in nanostructured environments.

Reconstituted cytoskeletal elements (similarly used in bead motility assays) can be confined in nano- and microstructured hard wells to form self-sustaining polymer films that move driven by polymerization in a tread milling fashion and that are ideally suited to transport nano-objects.

Furthermore, nanopores – initially formed in soft membrane systems, but ultimately fabricated in hard structures – are ideally suited to obtain information about the molecular properties of DNA when the DNA is sliding through these pores.

Functionalized polymers and porous materials

Despite the large number of known polymers, polymer synthesis and modification remain key issues in building novel, smart materials. Fully conjugated polymers based on polyenes and related structures represent interesting materials for organic field effect transistors, diodes, polymer grid triodes, light-emitting electrochemical cells, or opto-couplers. These polymers are believed to balance sufficient stability versus moisture and oxygen with their ability to decompose. Furthermore, nano-reactors built from soluble block-copolymers (e.g., prepared by the ‘living’ ring-opening polymerization of 2-oxazolines) are under investigation. They are perfectly suited for both organic and inorganic polymer synthesis, allowing for rapid reactions in environmentally friendly media such as water.

Aliphatic polyesters are extensively used as commodity thermoplastics and have significant biomedical applications. Ring-opening polymerization of cyclic esters is a particularly convenient method for the synthesis of polyesters. A special interest is devoted to poly(e-caprolactone) (PCL) and poly(3-hydroxybutyrate) (PHB), which is due to its miscibility with different commercial polymers, its biodegradability and biocompatibility, its adhesive properties at low temperatures, and its ability to disperse pigments. PHB occurs naturally as an isotactic, highly crystalline polymer. This stereoregular polymer is produced in nature by microorganisms, but it has low thermostability, and melt processing is therefore difficult. However, copolymerization with other lactones has produced polymers with improved thermal stability and better processability.

Nanostructured materials built from degradable polymers are anticipated to be key elements in future tissue engineering and regenerative medicine. The appropriate nanostructured surfaces might be used for stem cell differentiation, and degradable polymers might replace polyethyleneglycol (PEG) to improve bioavailability, stability, and uptake kinetics of macromolecular drugs. Testing can be easily performed by conjugation of PHB to bioactive peptides and proteins. Considering the recently discovered magnetic ordering in carbon, research will also focus on the magnetic properties of metal-free carbon-based polymers, such as polymers with C-H-N-O chains, or Kapton, another metal-free polymer that can show magnetic ordering at room temperature. A special emphasis will be given to the study of the effects of ion irradiation on the magnetic properties of polymers. Polymeric carriers including monolithic and micellar ones will be used for the immobilization of mononuclear or heterooligonuclear transition metal complexes to combine the advantages of homogeneous catalysis with those of heterogeneous catalysis.

A novel class of highly porous coordination polymers, Metal-Organic Frameworks (MOFs), can be obtained selectively in organic solvents by self-assembly of suitable molecular building blocks, i.e. metal salts or complexes and the corresponding linkers. Due to the large variety of possible organic linkers with different sizes and different connectivities, MOFs allow for a wider variability of pore volumes and architectures compared to the conventional zeolites and zeolite-like materials and are thus ideally suited for gas storage or separation. Furthermore, by using functionalized linkers, catalytically active metal complex fragments can be integrated into the framework which will then be employed as highly selective heterogeneous catalysts.

last update: 2 October 2008, A. Hildebrand