A surgical instrument, and methods for its use, is described that includes clamp heads that can be nestled within or extended from a tubular sheath by longitudinal movement of the clamp heads' tines with respect to the tubular sheath. One of the tines includes an arch that slides against a mouth and inside wall of the tubular sheath, causing the clamp heads to open or close. The clamp heads close lightly, to within a predetermined (or zero) distance from one another, gently grasp an ultrathin polymer substrate seeded with cells, and pulls it within the sheath such that the substrate curls and folds to protect the cells.