Sci. Adv. 2, e1601462 (2016)

Biological processes such as encapsulation, recognition and transportation are complex. Many methods have been used to translate the efficiency and specificity of these processes into simple and smart functional materials. Now, inspired by the way cells engulf debris and repair damage in the body, researchers in the US have created polymer-based droplets that can recognize, pick up and drop off hydroxyapatite nanoparticles from surfaces.

Hydroxyapatite — the main component in bone — is rich in calcium. To create a droplet that will bind to hydroxyapatite, Todd Emrick and co-workers at the University of Massachusetts functionalized phosphorylcholine-polyolefin with catechol and then shook an aqueous solution of this polymer in an organic solvent. When pumped through a flow cell over hydroxyapatite-coated substrates under laminar flow, the polymer-stabilized oil-in-water emulsion droplets were able to pick up the hydroxyapatite nanoparticles. Pick-up efficiency was lower under basic conditions and in solutions containing calcium ions. Furthermore, substrate composition also affected pick-up: greater efficiency was seen with PDMS and mica over plastic films. Exploiting this substrate-dependent pick-up efficiency, Emrick and colleagues showed that nanoparticles picked up from silicon substrates can be dropped off downstream on a polydopamine-coated silicon substrate. Such surface-to-surface nanoparticle transport is useful for transferring nanoparticle properties between materials.