An example system includes an array of retaining features in a microfluidic cavity, an array of thermally controlled releasing features, and a controller coupled to each releasing feature in the array of releasing feature. Each retaining feature in the array of retaining features is to position capsules at a predetermined location, the capsules having a thermally degradable shell enclosing a biological reagent therein. Each releasing feature in the array of releasing features corresponds to a retaining feature and is to selectively cause degradation of the shell of a capsule. Each releasing feature is to generate thermal energy to facilitate degradation of the shell. The controller is to selectively activate at least one releasing feature in the array of thermally controlled releasing features to release the biological reagent in the capsules positioned at the retaining feature corresponding to the activated releasing feature.