Mechanisnns for altering the shape of a cell or chamber of a shaft or an actuation drive of an instrument are provided. The mechanisms may selectively rigidize the shaft of surgical or diagnostic instruments. The shaft assembly includes a shaft operatively connectable to a control member, at least one cell or a set of cells defined within the shaft, a shape altering material contained within the cell or cells, and, an activation link operatively connectable to a source of activation energy for delivering activation energy to each cell for activating the shape altering material to selectively rigidize or unrigidize the shaft. An actuator for producing work is also provided that includes an element within a housing that defines a cell or a set of cells. The shape altering material is contained within the cells, and a source of activation energy operatively connected to each cell for activating the shape altering material to expand or contract the cell. The element is operatively connectable to a driving member of an instrument such that the change in the cell is translated to the driving member to facilitate the production of work.