**Droplets for Sampling and Transport of Chemical Signals in Biosensing: A Review**

**Shilun Feng 1,2,\*, Elham Shirani 1 and David W. Inglis 1,2,\***


Received: 2 April 2019; Accepted: 17 June 2019; Published: 20 June 2019

**Abstract:** The chemical, temporal, and spatial resolution of chemical signals that are sampled and transported with continuous flow is limited because of Taylor dispersion. Droplets have been used to solve this problem by digitizing chemical signals into discrete segments that can be transported for a long distance or a long time without loss of chemical, temporal or spatial precision. In this review, we describe Taylor dispersion, sampling theory, and Laplace pressure, and give examples of sampling probes that have used droplets to sample or/and transport fluid from a continuous medium, such as cell culture or nerve tissue, for external analysis. The examples are categorized, as follows: (1) Aqueous-phase sampling with downstream droplet formation; (2) preformed droplets for sampling; and (3) droplets formed near the analyte source. Finally, strategies for downstream sample recovery for conventional analysis are described.

**Keywords:** microfluidic probe; droplet; sampling; Taylor dispersion
