A necessary function for microfluidic devices used in biology is the ability to "hold" a cell or other object (e.g., embryo) in a known physical location while maintaining the flow around it. This is particularly useful in bioproduction and applications such as incubation/maturation, infection, fertilization, or chemical treatment of a biological object. Although lithographic etching can be used to construct the holding areas (i.e., constriction regions), this method is expensive and complex. This technology uses a simple and inexpensive method -- polymerization -- to construct flow constriction regions. A prepolymer mixture if flowed into the microfluidic channel and polymerized using an ultraviolet source. As it is polymerized, the polymer shrinks, creating a small gap at the top of the channel. This gap allows the fluid flow to continue while the cell/object is held in place. Thus, this technology enables microfluidic devices to be constructed more cost-efficiently.