Figure 4. Three major design principles of GI shape-changing devices.

A. Bulk deformation caused by (i). expansion of porous hydrogel upon intestinal fluid intake, (ii). filling of balloons by gas produced from chemical reactions, or (iii). disintegration or de-swelling triggered by light; B. uniaxial piercing caused by (i). release of a compressed spring paired with a needle, (ii). inflation of a pouch attached with microneedles, or (iii). movement of a needle attached to a magnet in an external magnetic field; C. folding and unfolding driven by (i). residual stress in a bilayer, (ii). a magnetic torsional spring, (iii). differential thermal expansion in a bilayer, or (iv). shape memory effect. Figure created with BioRender.com with publication license.