Polymer Physics 2.0: Exploiting Programmable Nanomaterials to Control Phase and Material Properties of Soft Matter

Biologically evolved materials are often used as inspiration in the both the development of new materials as well as examinations into the underlying physical principles governing their general behavior. One prominent example stems from the highly dynamic cytoskeleton of eukaryotic cells, where consideration of its biopolymer constituents such as actin and microtubules along with their respective sets of modulatory proteins and motors has inspired a deeper understanding of soft polymer-based materials [1-4]. In this case, a major limitation lies in the molecular toolbox provided by naturally occurring biological systems, which has been highly optimized through evolutionary processes to carry out the necessary functions of cells. The inability to deterministically modulate or "program" basic properties such as biopolymer stiffness and interaction strengths hinders a meticulous examination of parameter space, and the subsequent potential for developing new classes of materials.

Using the semiflexible cytoskeletal filamentous polymer actin as inspiration, we seek to circumvent these limitations using model systems assembled from programmable materials such as DNA and peptides. Nanorods with similar dimensions and mechanical properties as actin filaments can be constructed from small sets of specially designed DNA strands [5]. Properties such as stiffness and inter-filament attraction (i.e. crosslinking) can be controlled through the design of the set of DNA strands [6]. When assembled at concentrations approaching the threshold for liquid crystalline ordering in the presence of a bundle-inducing molecule such as polyethylene glycol (PEG), we observe distinct network-like ordered phases reminiscent to those previously characterized in systems based on actin [3,4] or microtubules [2]. In contrast, in lower concentrated "entangled" regimes, networks displaying viscoelastic properties reminiscent of those arising from natural cytoskeletal biopolymers can be generated and systematically modulated according to choice of network constituents.

Left - bundle-induced ordering of DNA tubes at concentration estimated near nematic threshold (16µM/DNA strand). Right - Elastic (blue) and viscous (red) moduli of DNA tube network in entangled regime (4µM/DNA strand) showing elastic plateau modulus over 4 decades.

References:

1. MacKintosh, F. C., J. Käs, and P. A. Janmey. "Elasticity of semiflexible biopolymer networks." Physical review letters 75.24 (1995): 4425.

2. Nedlec, F. J., et al. "Self-organization of microtubules and motors." Nature 389.6648 (1997): 305-308.

3. Smith, David, et al. "Molecular motor-induced instabilities and cross linkers determine biopolymer organization." Biophysical journal 93.12 (2007): 4445-4452.

4. Huber, Florian, Dan Strehle, and Josef Käs. "Counterion-induced formation of regular actin bundle networks." Soft Matter 8.4 (2012): 931-936.

5. Yin, Peng, et al. "Programming DNA tube circumferences." Science 321.5890 (2008): 824-826.

6. Schiffels, Daniel, Tim Liedl, and Deborah K. Fygenson. "Nanoscale structure and microscale stiffness of DNA nanotubes." ACS nano 7.8 (2013): 6700-6710.