Spacers are used in membrane separation processes like ultrafiltration for water purification to accelerate the mass transfer at the membrane walls. These spacers have rigid surfaces and the mass transfer in channels filled with them can be described using the classic Graetz-Nusselt problem. This paper introduces the concept of non-rigid spacers, which are so called because they are soft enough to be deformed by the viscous stresses in the fluid flowing above them, even in the absence of inertia. Electrochemical mass transfer experiments are done in small channels filled with soft gel blocks and the mass transfer coefficients are compared to those obtained using rigid structures of similar geometry. At least 25% enhancement in mass transfer is observed with the soft spacers when a critical dimensionless interfacial stress is reached. This signifies that soft spacers might be better than rigid spacers due to mass transfer enhancement caused by the deformation of their surfaces by the stresses in the fluid flowing above them.