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Synthesis and Characterization of Bioinspired Longitudinal Polymer Gradient Materials

By Kai Uwe Claußen  (07.03.2013)

The transfer of biological principles opens up pathways for the development of new materials.[1] For example, submarine mussels such as Mytilus galloprovincialis uses a tough yet elastic appendage called a byssus which successfully anchors them in turbulent intertidal habitats.[2] Byssal threads are longitudinal gradient biomaterials, i. e. their composition changes continuously along the fiber that mediates the mussel’s soft interior to the hard surface of rocks. Due to structure property relationships, this compositional gradient causes also a mechanical gradient. The proximal part directly attached to the mussel stem is more elastic and possesses high extensibility whereas at the end of the thread (distal region) the fiber is stiffer to ensure a strong attachment to the rock (Figure 1).[3]

 

Claussen: Figure 1[Bildunterschrift / Subline]: Figure 1. Schematical attachment of a submarine mussel to a stone via the mussel byssus. The proximal part (50 MPa) is more elastic whereas the distal part (500 MPa) is stiffer. A continuously changing composition mediates the soft interior of the mussel (0.2 MPa) to the hard surface of a stone (25.000 MPa).[3] (published by permission of American Chemical Society)

The gradient is established by a continuous change of composition of different collagens, rendering stiff or elastic properties.[4] By continuously changing these different collagens (so-called preCols) in a polyaddition process the mussel prepares a thread of 5-10 cm in minutes.[5] Trimers of preCols form anisotropic bundles already in granules inside of secretory cells. The granules are secreted into a rim of the mussel foot where the proteins are cross-linked, yielding a fiber in a process akin to reaction injection molding.[3,6] The compositional gradient in the fiber is suggested to minimize interfacial stresses, improve energy dissipation, and increase mechanical toughness.[7] The overall mechanical properties of byssus threads are unique, combining high stiffness and elasticity what results in a fracture toughness similar to that of Kevlar®.

Polyaddition reactions and reaction injection molding are very well-known methods in polymer science. In principle, polymer science thus enables the creation of bulk polymer gradient materials (PGMs), mimicking a mussel byssus. The requirements are a suitable polymer system that renders soft and hard materials and a gradient preparation method.
Recently, we published a review article about PGMs.[8] In the scope of this article, we presented a straightforward method of fabricating longitudinal PGMs on the centimeter scale with high reproducibility. The experimental setup is shown in Figure 2.

Claussen Figure 2.1 Claussen Figure 2.2[Bildunterschrift / Subline]: Figure 2. Scheme (top) and photograph (bottom) for the syringe pump setup employed for the preparation of longitudinal PGMs. The flow rate of each syringe (a) is set and controlled by software (b). The syringes are connected via luer-lock ports and disposable tubing (c) to a custom-made mixing head (d). A static mixer (e) is used to mix the components before casting them into the mold (e) on a moving platform (g). The mold movement is synchronized with the flow rate resulting in uniform and reproducible filling of the mold. The shown poly(dimethyl siloxane)system in the syringes consists of two different siloxane prepolymers (component A: red; component B: colourless) and a curing agent (component C: colourless). (published by permission of John Wiley & Sons*)

Three components (A, B, and C) are fed via a syringe pump system through a static mixer into a mold. A gradient can be generated by continuously changing the ratio A:B, whereas the amount of C is kept constant. Then, the gradient is preserved by a polyaddition reaction. If one component is stained with a dye (here: component A), the compositional gradient can be easily visualized (see specimen in Figure 2). In this way, multiple samples with a gradient in the longitudinal direction on a centimeter scale can be produced with high reproducibility. The approach can be easily adapted to a variety of polyaddition polymer system.

The prepared gradients can be analyzed optically by UV/Vis spectroscopy and mechanically by compressive modulus testing. The absorbance (at a certain wavelength) and the compressive modulus is measured in dependency on the sample position. Since the “soft” component A was dyed, the compressive modulus is low at a high amount of component A and vice versa (Figure 3).

Claussen Figure 3.1 Claussen Figure 3.2[Bildunterschrift / Subline]: Figure 3. Top: Poly(dimethyl siloxane) PGM on a 384-well plate. The absorbance maximum at 560 nm of the red dye can be detected as function of the sample position. Bottom: Compressive modulus testing of the same sample. Cylindrical specimens (5 mm diameter, 1 mm height) were punched every 10 mm along the length of the sample and measured. (published by permission of John Wiley & Sons*)

These preparation and characterization methods allow us to prepare tailored polymer gradient samples of different polyaddition polymer systems and to systematically explore the impact of the gradient structure on the mechanical properties.[9]

 

*This is the pre-peer reviewed version of the following article:
“Polymer Gradient Materials: Can Nature Teach Us New Tricks?” by K. U. Claussen, T. Scheibel, H.-W. Schmidt, R. Giesa, Macromol. Mater. Eng. 2012, 297, 938-957, which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/mame.201200032/abstract

 

 

[1]  M. Antonietti, P. Fratzl, Macromol. Chem. Phys. 2010, 211, 166.
[2]  M. J. Harrington, J. H. Waite, Adv. Mater. 2009, 21, 440.
[3]  J. H. Waite, H. C. Lichtenegger, G. D. Stucky, P. Hansma, Biochemistry 2004, 43, 7653.
[4]  J. H. Waite, X. X. Qin, K. Coyne, Matrix Biol., 1998, 17, 93.
[5]  T. Hassenkam, T. Gutsmann, P. Hansma, J. Sagert, J. H. Waite, Biomacromolecules 2004, 5, 1351.
[6]  E. Vaccaro, J. H. Waite, Biomacromolecules 2001, 2, 906.
[7]  J. Gosline, M. Lillie, E. Carrington, P. Guerette, C. Ortlepp, K. Savage, Phil. Trans. R. Soc. Lond. B 2002, 357, 121.
[8]  K. U. Claussen, T. Scheibel, H.-W. Schmidt, R. Giesa, Macromol. Mater. Eng. 2012, 297, 938.
[9]  K. U. Claussen, R. Giesa, T. Scheibel, H.-W. Schmidt, Macromol. Rapid Commun. 2012, 33, 206.


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