Self-folding particle chains

The synthesis of materials in biological systems is different from the way human engineered materials are produced. Biological materials are predominantly soft materials which exhibit multiple encoded functionalities and can respond to external stimuli. We propose a biomimetic experimental route to the creation of 3D-structured nano- and micromaterials through directed self-folding of colloid strings. Such a general approach to design and construction of 3-D, man-made, functional materials based on self-organization is practically experimentally unexplored. A ubiquitous material structure in nature is found in peptide chains, which form structurally and func- tionally diverse materials out of a limited set of monomers through self-folding of the polymer chain. The folding is guided by the sequence of the differently interacting peptides. Designing strings of dif- ferent colloidal sub-units that can self-fold - in the spirit of proteins and amino acids - is a tantalizing prospect that holds the promise to create materials that explore structure-function relationships in three dimensions. Such materials would have interesting new properties such as encoded structural and functional sensitivity to environmental conditions and the ability to self-repair their structures by re- folding. Recently we showed, using simulations that by linking particles that have directional interactions as well as isotropic interactions into a string, i.e. into a patchy polymer, it is possible to create chains that spontaneously fold into given target structures. This work suggests that directed self-folding can be realized using a wide variety of interaction potentials, patch interactions and numbers of patches. We are therefore confident that we can experimentally realize a toolbox of colloids with well- characterized interaction potentials that match the reductionist criteria in the model. The particles comprise new systems in their own right but are based on existing synthetic concepts. The particles will be linked into linear chains and used to explore design principles to encode 3D structures into folding chains. Our focus is on establishing suitable characterization techniques and protocols to fabri- cate and measure the properties of colloid monomers, colloid chain sequence and the folding into sec- ondary and tertiary structures. The new methodology to characterize folding in real time will be developed by application of ad- vanced microscopy techniques, primarily digital holographic microscopy, which is applicable to the size and time-scale of movement of the colloids under investigation, combined with confocal fluores- cence microscopy. Together with the synthesis of the new colloidal system, these developments will enable experimentally testing the theoretical prediction of designable, self-folding, 3D colloidal mate- rials, based on patchy colloid monomers.

Our main objective was to create an experimental toolbox of colloids with well-characterized interaction potentials that should enable biomimetic, directed self-folding of colloid strings into 3D-structured micromaterials. The synthesis of materials in biology differs fundamentally from how human-engineered materials are produced. Biological materials are predominantly soft materials that exhibit multiple encoded functionalities and can respond to external stimuli. A ubiquitous material structure in nature is found in peptide chains. They form structurally and functionally diverse materials out of a limited set of monomers through folding of the polymer chain. The sequence of the differently interacting amino acids guides the folding. Replicating a similar type of self-assembly with micrometric beads in the role of 'monomer' is a tantalizing perspective because it allows the study of the folding behavior on the individual monomer-particle level. This would, in turn, enable creating materials that explore structure-function relationships in three dimensions. Our focus was on establishing suitable characterization techniques and protocols to fabricate and measure the properties of colloid monomers, colloid chain sequences and the folding into secondary and tertiary structures. To take this step, we also had to improve existing methods to study colloidal motion and interactions in three dimensions, leading to major improvements in digital holographic microscopy. A key step to control the self-folding is to put monomer-particles together in a controlled sequence with a flexible bond in high numbers and with high yield. We developed a technique inspired by state-of-the-art peptide synthesis by which each new particle was added to the next from a surface using dielectrophoresis. Alternating electric fields were used to attract free particles to the end of the strings. Several different surface chemistries were tested to create activated, orthogonal bonds upon contact. A combination of chemical issues and the discovery that particles in such fields are prone to rapid rotation prevented obtaining sufficient yield to make long strings of defined sequences. Thus, we did not yet reach the goal of studying self-folding particle strings. However, we achieved breakthroughs in real-time 3D microscopy and dielectric manipulation of colloids that, in addition to their scientific relevance, led to the filing of several patents and the expected founding of a new company.