Silk micro-rockets deliver

C&I Issue 7, 2016

A tiny self-propelling rocket made from silk has been produced by inkjet printing – and shows potential for use in  future drug delivery systems.

The micro-rocket consists of a silk scaffold doped with the enzyme catalase and polyethylene glycol (Small, doi: 10.1002/smll.201600921). To make the device, the Sheffield University researchers in the UK blended a solution of the silk protein fibroin with the enzyme catalase, which catalyses the reduction of hydrogen peroxide.

Using reactive inkjet printing, they printed the silk ‘ink’ onto a silicon wafer. To convert the printed material back into a water-insoluble rigid scaffold – in order to manufacture solid micro-rockets – they alternated each layer with a layer of methanol, which triggers a change from soluble to insoluble silk.

When the device was put in a solution of hydrogen peroxide, the ensuing reaction – initiated by catalase locked in the silk scaffold – generated bubbles of oxygen that propelled the device around. Controlling the distribution of the catalyst allows them to control its movement. If the catalyst is spread evenly through the structure, bubbles are released randomly and motion is more unpredictable.

If the catalyst is distributed unevenly, the device moves faster and with a straighter trajectory. Inkjet printing can produce different shapes and structures, and this could also be used to alter how the rockets move.

Sheffield researcher Xiubo Zhao says the rockets have great potential because many materials, such as drugs, enzymes, and antibodies, could be encapsulated within the silk scaffold. If the enzymatic reactions to power the rockets weren’t reliant on peroxide fuel, they could potentially be used in the body, to carry drugs or repair tissues, or even antibodies that could track down circulating tumour cells, he says.

Current lithographic or metal evaporation processes to make similar devices are time-consuming and expensive. Also, many devices use platinum metal as the motion-producing catalyst, but its catalytic activity is highly sensitive to surface contaminants such as hydrocarbons and thiols, and is also reduced by surface absorption of constituents of some biological fluids.

Instead, says Zhao, advances in printable materials and printing technology allows ‘rapid, scalable manufacture of digitally defined micro-rockets’, which, because they use a silk scaffold, show promising biocompatibility.

The use of inkjet printing for microjets is new, says Samuel Sanchez of the Max Planck Institute for Intelligent Systems in Stuttgart, Germany. While enzymes have been used for propulsion as an alternative to common metallic catalysts for several years, he hopes the authors can expand on their idea using other enzymes to make ‘truly bio-friendly’ fuels, ‘although the shape and size of the structures may become a problem’.

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