  Bead-based micro-swimmers are promising systems for payload delivery on the
micro-scale. However, the principles underlying their optimal design are not
yet fully understood. Here we study a simple device consisting of three
arbitrarily-shaped beads connected by two springs. We analytically determine
the most favorable kinematic parameters for sinusoidal driving, and show how
the swimmer changes from being a pusher to a puller. For cargo carrying
ellipsoidal beads, we perform geometric optimization under the constraint of a
constant total volume or surface area, with the aim of maximizing the device
transport velocity and efficiency. Interestingly, we identify two major
transport regimes, which arise from the competition between the elastic and the
drag forces faced by the swimmer. We construct a phase diagram that indicates
when the fastest swimming emerges because of minimized drag, and when due to
heightened interactions among the beads.
