Self-powered microscale pumps based on analyte-initiated depolymerization reactions.

Microscale pumps that are simultaneously capable of turning “on” in response to specific analytes in solution and of providing precise control over flow rate will be needed for certain types of smart microand nanoscale devices. Access to these types of pumps, however, has remained limited. Two general approaches for designing microscale pumps have been proposed: 1) those that are turned on and off by input from the user; and 2) single-use pumps that are turned on autonomously by exposure to a specific chemical signal, and therefore do not need to be turned off. The ideal pump in this second category would combine sensing and pumping capabilities, and thus would enable pumping that is controlled by the presence and concentration of a specific analyte. Herein we describe the first examples of this second type of microscale pump. We characterize this type of pump, and establish both the chemical and physical–chemical foundation upon which future applied efforts will be based. The pumps consist of insoluble polymer films that depolymerize to release soluble monomeric products when exposed to a specific analyte (Figure 1). Products formed as a result of the depolymerization reaction amplify the signal and create a concentration gradient that pumps fluids (and insoluble particles) away from the bulk polymer owing to a diffusiophoretic mechanism. These pumps are selfpowered, are capable of turning “on” in response to specific analytes, and can be tuned to respond to a variety of analytes ranging from small molecules to enzymes. They also provide pumping velocities ranging from 0.1 mms 1 at low concentrations of analyte up to 11 mms 1 at high concentrations of analyte. Moreover, the pumps are capable of moving fluids and insoluble micrometer-scale particles directionally despite viscous drag in low Reynolds number regions, and despite Brownian randomizations. To demonstrate these types of pumps, we prepared micrometer-scale polymer films on glass slides. The films were composed of a polymer that is capable of depolymerizing when it responds to a specific chemical signal. We tested two types of polymers: tert-butyldimethylsilyl (TBS) endcapped poly(phthalaldehyde) (TBS-PPHA), which depolymerizes in response to fluoride, and commercially available poly(ethyl cyanoacrylate) (PECA), which depolymerizes and degrades in response to base (Figure 1). The type of polymer used for the pumps is important: it affects both the speed and magnitude of the pumping response. For example, typical degradable polymers require a separate reaction with the analyte for release of each repeating unit. TBS-PPHA, in contrast, is a polymer that depolymerizes rapidly (that is, seconds), continuously, and completely from one end of the polymer to the other when the TBS end-cap is cleaved from the polymer by reaction with fluoride. Thus, TBS-PPHA requires only a single equivalent of fluoride to release 133 monomers (the TBS-PPHA polymer used in this study contained about 133 repeating units, although longer polymers could be used), meaning that a single detection event causes a dramatic increase in the concentration (and therefore gradient) of products in soluFigure 1. Design of self-powered micrometer-scale pumps that are based on analyte-induced depolymerization of individual polymers within a polymer film. Films were made from either tert-butyldimethylsilyl (TBS) end-capped poly(phthalaldehyde) (TBS-PPHA), which depolymerizes in response to fluoride, or poly(ethyl cyanoacrylate) (PECA), which responds to base.