  Microcapsules with liquid cores encapsulated by thin membranes have many
applications in science, medicine and industry. In this paper, we design a
suspension of microcapsules which flow and deform like red blood cells (RBCs),
as a valuable tool to investigate microhaemodynamics. A reconfigurable and
easy-to-assemble 3D nested glass capillary device is used to robustly fabricate
water-oil-water double emulsions which are then converted into spherical
microcapsules with hyperelastic membranes by cross-linking the
polydimethylsiloxane (PDMS) layer coating the droplets. The resulting capsules
are monodisperse to within 1% and can be made in a wide range of size and
membrane thickness. We use osmosis to deflate by 36% initially spherical
capsules of diameter 350 {\mu}m and a membrane thickness of 4% of their radius,
in order to match the reduced volume of biconcave RBCs. We compare the
propagation of initially spherical and deflated capsules under constant
volumetric flow in cylindrical capillaries of different confinements. We find
that only deflated capsules deform broadly similarly to RBCs over a similar
range of capillary numbers (Ca) -- the ratio of viscous to elastic forces.
Similarly to the RBCs, the microcapsules transition from a symmetric
'parachute' to an asymmetric 'slipper'-like shape as Ca increases within the
physiological range, demonstrating intriguing confinement-dependent dynamics.
In addition to biomimetic RBC properties, high-throughput fabrication of
tunable ultra-soft microcapsules could be further functionalized and find
applications in other areas of science and engineering
