Structure of negative Poisson’s ratio was obtained successfully from the process of heat treatment with 3-D compression for the scaffold. Concave porous structure was created in the scaffold specimens. The cross-sectional microstructure of the PLGA scaffold is shown in Fig. 4. The porous shape of control specimen was observed as a cubical and convex type as shown in Fig. 4(a). In contrast, experimental specimen showed partially concave and dented shapes as shown in Fig. 4(b) and (c). This transformation of porous shape is resulted from changing micro porous structure of the scaffold by tri-axial compression. The porous structure of these recessed and dented shapes enable the scaffold have negative Poisson ratio.Fig. 4Cross-sectional SEM images of PLGA scaffold. a control specimen before compression. b experimental specimen after tri-axial compression (2.4 : 1), (c) experimental specimen after tri-axial compression (3.1 : 1). The arrows indicate difference of pore structure