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Detachment of water: Light triggered water droplet release from biomimetic honeycomb-structured polymer surfaces

Periodic Reporting for period 1 - DEW (Detachment of water: Light triggered water droplet release from biomimetic honeycomb-structured polymer surfaces)

Reporting period: 2016-03-01 to 2017-08-31

Inspired by the Stenoccara beetle, which uses ingenious surface chemistry to collect water from the desert air, the long-term vision for the DEW project was the development of biomimetic smart polymer surfaces which can capture water from humid night air and release it upon exposure to the morning sun through a light-triggered mechanism. In moving towards this goal, new honeycomb-structured porous films (HC films) were synthesised from light-responsive polymers. By controlling both the topography and the chemical nature of the films, we hoped to manipulate the behavior of water droplets on the surfaces by using light.

The originality of the project centred on the fact that 1) light-responsive HC films had not been reported, and would provide a new means for manipulating wettability beyond using previously-reported pH value and temperature; 2) new polymers containing molecules which can be switched using benign visible light (rather than potentially harmful UV-light) would open up new applications of HC films in which UV light is precluded; and 3) hierarchical structuration would exaggerate the light-triggered change in surface polarity.

The project was conducted at the EPCP group (IPREM, UPPA, Pau, France) supervised by Prof. Laurent Billon and supported by the ECP team led by Dr. Sylvie Lacombe, Prof. Ross Brown, and Dr. Sylvie Blanc. In addition to exploring light-responsive HC films, light-responsive biomaterials were developed in a 3 month secondment at the Dynamic Biomaterials group at the Institute for New Materials (INM, Saarbruecken, Germany) led by Prof. Aranzazu del Campo.
The work performed in the DEW project can be divided into four components: 1) UV-responsive HC films; 2) a visible-responsive azobenzene photoswitch; 3) a light-responsive cell-adhesion peptide; and 4) sugar-based biomaterials.

1) UV-responsive HC films formed the core of the DEW project. HC films are phenomenal materials formed in the same way that your breath condenses on a cold window. Subjecting a polymer solution to humid air can result in micro-droplets of water condensing on the cold solution surface, introducing highly-ordered pores in the resulting film. The topography of HC films resembles some natural surfaces, such as rose petals, which are well adapted for water capture and release. The DEW project produced HC films from polymers containing light-responsive units known as azobenzenes. The polarity of azobenzenes can be reversibly switched between two states, known as “trans” and “cis”, by using particular wavelengths of light. This affects how water droplets interact with the surface, and allows light-triggered control of wetting.

A range of azobenzene-based polymers were developed to successfully produce HC films displaying i) a long-range, well-ordered porous structure without surface defects, and ii) a reversible shift in wettability when irradiated with particular wavelengths of light. In addition to developing these new light-responsive HC films, insights into the structural features which influence the magnitude of the light response were determined. These results were presented at the recent European Polymer Federation (EPF) congress in Lyon, France, and will be published shortly.

The essence of this work is captured in Fig. 1, which shows a) the UV/Vis absorption spectra of an azobenzene polymer after irradiation to switch the azobenzenes from their initial trans state, to their more polar cis state, then back to a trans-rich state. Photos of the solution (inset) show a beautiful colour change, from yellow (trans) to red (cis), courtesy of the different absorption characteristics of the two isomers. HC films made from an azobenzene-based polymer show highly regular porous structure, and can even be cast on water (Figure 1b) or converted to the distinctive “pincushion” topography by peeling off the top layer of the film (Figure 1c).

2) A new visible-responsive azobenzene, meaning it can be switched from the trans to cis state using visible (rather than UV) light, was developed using an improved synthesis compared with similar visible-responsive azobenzenes in the literature. The relatively small shift in polarity of this species precluded its use in HC films, but it is being explored as a switchable component of biological substrates, where the change in shape (rather than polarity) is exploited. A paper describing this new azobenzene is in preparation, and the collaboration investigating the biological subtrate is ongoing.

The DEW project included a 3 month secondment in the Dynamic Biomaterials (DB) group at the Institute for New Materials (INM, Saarbrueken, Germany). The DB group develops biomaterials, typically hydrogels, which emulate natural systems and whose functions can be controlled by light. Two projects were conducted during this stay.

3) The first DB project developed an azobenzene-based peptide containing an amino acid sequence critical to cell binding. A core azobenzene unit was designed and incorporated into the peptide by solid phase peptide synthesis (SPPS), and its photophysical properties were studied. Reversible presentation of the active ligand courtesy of the azobenzene function is hoped to provide additional functionality not accesible using established approaches in the DB group.

4) The second DB project involved the development of sugar-based polymers which interact specifically with certain cells to form biologically-active substrates.
The DEW project has extended the state of the art in several domains. The new light-responsive HC films are not only useful materials for potential water capture and release, but may also be applied in other light-responsive systems involving the control of wettability such as microfluidics and self-cleaning surfaces. The new visible-responsive azobenzene molecule provides a high-yield alternative to existing visible photoswitches, which show exciting potential in areas ranging from sunlight-harvesting to photopharmacology. The light-responsive biomaterials developed in the DB group provide a platform for probing cell functions in response to on-demand changes to their environment. Understanding such fundamental biological processes can have wide-reaching implications, particularly in disease progression and prevention.
"Fig. 1a) UV/Vis spectra of azobenzene polymer; SEM images a) water-cast and b) ""pincushion"" HC films"