Piezoelectricity of poly(α-amino acids)-based films and fibers

The long-term goal of this research is to fabricate versatile piezoelectric materials (PM) that can be integrated into small transducers and energy harvesting device. Poly(γ-benzyl α,L-glutamate) (PBLG) is a synthetic poly(α-amino acids) reported to possess the highest electric dipole moment among all organic molecules (26, 27, 28). Compared to conventional ceramic based PM which rely on dipoles from highly ordered crystalline lattices, the PBLG’s dipole originates from the hydrogen bonds of helical back bone and is much more resilient to impurities. In addition, PBLG’s extreme solubility in organic solvents allows various chemical processes for attaining a broader range of shapes, thickness, and sizes compared to ceramic PMs typically produced by mechanical foundries.

Charge assessment of PBLG-PMMA composite film.

During the early stage of the research, we worked on fabrication of piezoelectric polymer films and disks composed of piezoelectric PBLG, and a matrix polymer, poly(methylmethacrylate) (PMMA) that governs the mechanical characteristics of the composite material. Both corona-discharge (11) and contact charging in a designed mold (7) were used to pole the PBLG within MMA solution and polymerize the PMMA matrix. Although we observed consistent and reversible piezoelectric response of d33= 17-20 pC/N for films fabricated by corona discharge and 1-2 pC/N for polymer disk produced by contact charging, higher piezoelectricity was unachievable due to difficulty in poling PBLG solution with high concentration.


Schematics of electrospinning and the PBLG poling process.
 After exploring other means of poling PBLG, we discovered for the first time that electrospinning can be used as a one step method to produce polar polymer fibers with electric dipoles permanently poled in the direction of the fiber axis, resulting in high non-linear optical (NLO) activity and thermally stable piezoelectricity (4). We learned that the PBLG’s macroscopic dipole which is pre-aligned in the direction of helical axis can couple synergistically with external electric field and the shear force during electrospinning. The electrospun fibers exhibited a d33 piezoelectric coefficient of 25 pC N-1 which did not deteriorate even after 100 °C thermal treatment for over 24 hr. To the best of our knowledge, this is one of the highest thermally stable piezoelectric coefficients reported for poled polymers. Considering the versatility of the electrospinning process combined with the chemical diversity of α-helical poly(α-amino acids), fibers electrospun from PBLG and related molecules show great promise as new electro-optical and electro-mechanical materials for small sensors and energy harvesting/scavenging devices.

References:

1. Y. Li, C. A. Foss, D. D. Summerfield, M. G. Pomper and S. M. Yu (2011) Targeting Collagen Strands by Photo-Triggered Triple Helix Hybridization. submitted to Nature Materials.
2. C. Kelliher, S. Chakravarti, N. Vij, S. Mazur; P.  J. Stahl, C. Engler, M. Matthaei, S. M. Yu, A. S. Jun (2011) A Cellular Model for the Investigation of Fuchs' Endothelial Dystophy. Exp. Eye. Res., in press.
3. T. R. Chan, P. J. Stahl, and S. M. Yu (2011)  Matrix-Bound VEGF Mimetic Peptides: Design and Endothelial Cell Activation in Collagen Scaffolds. Adv. Funct. Mater., in press. DOI: 10.1002/adfm.201101163
4. D. Farrar, K. Ren, D. Cheng, S. Kim, W. Moon, W. L. Wilson, J. E. West, and S. M. Yu (2011) Permanent Polarity and Piezoelectricity of Electrospun α-Helical Poly(α-amino acid) Fibers. Adv. Mater. 23, 3954.
5. S. M. Yu, Y. Li, and D. Kim (2011) Collagen Mimetic Peptides: Progress Towards Functional Applications. Soft Matter 7, 7927.
6. Y, Li, X. Mo, D, Kim, and S. M. Yu, (2011) Template-Tethered Collagen Mimetic Peptides for Studying Heterotrimeric Triple-Helical Interactions Biopolymers 95, 94.
7. Y. Hwang, D. Farrar, J. E. West, S. M. Yu and W. Moon (2011) Piezoelectric Properties of Polypeptide-PMMA Molecular Composites Fabricated by Contact Charging. Polymer 52, 2723.
8. P. Stahl, N. Romano, D. Wirtz, and S. M. Yu, (2010) “PEG-Based Hydrogels with Collagen Mimetic Peptide-Mediated and Tunable Physical Crosslinks” Biomacromolecules 11, 2336.
9. Lee, J. H., Yu, C., Chansakul, T., Hwang, N. S., Varghese, S., Yu, S. M., and Elisseeff J. H. (Enhanced Chondrogenesis of Mesenchymal Stem Cells in Collagen Mimetic Peptide-Mediated Microenvironment. Advanced Tissue Engineering (2010), Chapt. 15, Eds P.C. Johnson and A.G. Mikos.
10. Wang, A. Y., Leong, S., Liang Y.-C., Huang, R. C., Chen, C. S.,  and Yu, S. M. (2008) Immobilization of growth factors on collagen scaffolds mediated by polyanionic collagen mimetic peptides and its effect on endothelial cell morphology. Biomacromoleulces 9, 2929.
11. Farrar, D., West, J. W., Busch-Vishniac, I. J., and Yu, S. M. (2008) Fabrication of Polypeptide-Based Piezoelectric Composite Polymer Film. Scripta Materialia 59, 1051.
12. Wang, A. Y., Foss, C. A., Leong, S., Mo, X., Pomper, M. G., and Yu, S. M. (2008) Spatio-temporal modification of collagen scaffolds mediated by triple helical propensity. Biomacromoleulces 9, 1755.
13. Lee, J. H., Yu, C., Chansakul, T., Hwang, N. S., Varghese, S., Yu, S. M., and Elisseeff J. H. (2008) Enhanced Chondrogenesis of Mesenchymal Stem Cells in Collagen Mimetic Peptide-Mediated Microenvironment. Tissue Engineering 14, 1843.
14. Mo, X., An, Y. J., Yun, C. S., and Yu, S. M. (2006) Nanoparticle-assisted visualization of binding interactions between collagen mimetic peptide and collagen fibers. Angewandte Chemie-International Edition 45, 2267-2270.
15. Lee, J. H., Lee, J.-S., Chansakul, T., Yu, C., Elisseeff, J. H., and Yu, S. M. (2006) Collagen mimetic peptide-conjugated photopolymerizable PEG hydrogel. Biomaterials 27, 5268-5276.
16. Mo, X., Krebs, M. P., and Yu, S. M. (2006) Directed synthesis and assembly of nanoparticles using purple membrane. Small 2, 526-529.
17. Wang, A. Y., Mo, X., Chen, C. S., and Yu, S. M. (2005) Facile modification of collagen directed by collagen mimetic peptides. Journal of the American Chemical Society 127, 4130-4131.
18. Fukuto, M., Heilmann, R. K., Pershan, P. S., Yu, S. J. M., Soto, C. M., and Tirrell, D. A. (2003) Internal segregation and side chain ordering in hairy-rod polypeptide monolayers at the gas/water interface: An x-ray scattering study. Journal of Chemical Physics 119, 6253-6270.
19. Fukuto, M., Heilmann, R. K., Pershan, P. S., Yu, S. M., Soto, C. M., and Tirrell, D. A. (2002) Confinement-induced order of tethered alkyl chains at the water/vapor interface. Physical Review E 66.
20. Yu, S. M., McQuade, D. T., Quinn, M. A., Hackenberger, C. P. R., Krebs, M. P., Polans, A. S., and Gellman, S. H. (2001) An improved tripod amphiphile for membrane protein solubilization.  Protein Science 10, 1089-1089.
21. Yu, S. J. M., and Tirrell, D. A. (2000) Thermal and structural properties of biologically derived monodisperse hairy-rod polymers. Biomacromolecules 1, 310-312.
22. Yu, S. J. M., Soto, C. M., and Tirrell, D. A. (2000) Nanometer-scale smectic ordering of genetically engineered rodlike polymers: Synthesis and characterization of monodisperse derivatives of poly(gamma-benzyl alpha,L-glutamate). Journal of the American Chemical Society 122, 6552-6559.
23. McQuade, D. T., Quinn, M. A., Yu, S. M., Polans, A. S., Krebs, M. P., and Gellman, S. H. (2000) Rigid amphiphiles for membrane protein manipulation. Angewandte Chemie-International Edition 39, 758.
24. Fukuto, M., Heilmann, R. K., Pershan, P. S., Yu, S. J. M., Griffiths, J. A., and Tirrell, D. A. (1999) Structure of poly(gamma-benzyl-L-glutamate) monolayers at the gas-water interface: A Brewster angle microscopy and x-ray scattering study. Journal of Chemical Physics 111, 9761-9777.
25. Song, J.-J., Yoon, S.-C., Yu, M. S., and Lenz, R. W. (1998) Differential Scanning Calorimetric Study of Poly(3-hydroxyoctanoate) Inclusions in Bacterial Cells. International Journal of Biological Macromolecules 23, 165.
26. He, S. J., Lee, C., Gido, S. P., Yu, S. J. M., and Tirrell, D. A. (1998) A twist grain boundary-like twisted smectic phase in monodisperse poly(gamma-benzyl alpha,L-glutamate) produced by recombinant DNA techniques. Macromolecules 31, 9387-9389.
27. Fukuto, M., Heilmann, R. K., Pershan, P. S., Griffiths, J. A., Yu, S. J. M., and Tirrell, D. A. (1998) X-ray measurements of noncapillary spatial fluctuations from a liquid surface. Physical Review Letters 81, 3455-3458.
28. Yu, S. J. M., Conticello, V. P., Zhang, G. H., Kayser, C., Fournier, M. J., Mason, T. L., and Tirrell, D. A. (1997) Smectic ordering in solutions and films of a rod-like polymer owing to monodispersity of chain length. Nature 389, 167-170.
29. Jin, J.-I., and Yu, S. M. (1995) New Polyarylates Prepared from 2,5-Bis(α-phenyl-isopropyl)hydroquinone, Terephthalic Acid and Isophthalic Acid. Bull. Korean Chem.  Soc. 16, 17.