Twisted self-assembly may lead to artificial bone

Under the right conditions, bacteria-infecting viruses known as M13 phages can self-assemble to form a whole host of useful structures, say US scientists. These structures are able to scatter different wavelengths of light and even act as scaffolds for the growth of patterned sheets of cells or bone-like minerals. 

The self-assembling ability of M13 phages is all down to their rather unique shape. M13 phages are long, thin tubes, around 880nm long and 7nm wide. They are covered in proteins that give them a helical structure, such that in cross-section they look like a water-wheel.  

Other long, thin biological molecules with a helical structure, such as collagen, can self-assemble to form a variety of tissues with different properties. Depending on how it self-assembles, which is dictated by environmental conditions, the same collagen molecule can form stretchy skin tissue, transparent eye tissue or tough bone tissue. 

To get M13 phages to self-assemble, a team of bioengineers led by Seung-Wuk Lee at the University of California, Berkeley, simply suspended them in a salt solution, into which they dipped a glass slide. As they pulled the glass slide out of the solution, the phages stuck to it, forming a film. But rather than lining up randomly the phages formed films with defined structures. Furthermore, these structures changed depending on the concentration of phages in the solution and the speed with which the glass slide was pulled out. 

Lee and his team found that the phages self-assembled into one of three basic structures. In the first, the phages formed alternating bands in which their long axes were either aligned parallel to the pulling direction or perpendicular to it. The second structure was similar to the first, but the phages forming bands with their axes aligned parallel to the pulling direction were twisted to form ribbons. Finally, in the third structure the phages formed lots of swirling filaments running down the length of the glass. 

By altering the phage concentration and pulling speed, not only could Lee and his team alternate between these three different structures, but they could also subtly alter each individual structure, such as changing the distance between the bands in the first structure. This allowed the resultant films to act as diffraction gratings, scattering and reflecting light of different wavelengths according to the spacing between the bands. 

By engineering the phages to display specific peptides on their proteins, the bioengineers were able to encourage cells to grow in patterns on films made up of the third structure. They were also able to deposit the mineral calcium phosphate on these films to form a tough, tooth enamel-like material, offering a new way to produce artificial bone. ’These materials are totally biocompatible and can be used for many different types of biomedical applications,’ says Lee. 

’[This] is an interesting twist on self-assembly,’ says Thom LaBean at North Carolina State University, who develops DNA-based self-assembly techniques. ’The structural DNA nanotechnology community is just beginning to work on organising solution-assembled, DNA-based materials onto solid substrates with anything other than random deposition.’ 

Jon Evans