Fig. 4.

Heterogeneous stiffness characterization and inclusions detection. a Characteristic of the objects with different upper thickness and underneath stiffness. b The encapsulated objects detection illustration. c, d The output signals generated from pressing the objects with encapsulated different stiffness materials, and encapsulated 3D-printed materials of different sizes, shapes and amounts. e The block diagram of the data processing procedure of the heterogeneous stiffness application. f (I) The confusion matrix of the machine learning outcome for recognizing the 16 different 3D printed objects encapsulated. (II) The confusion matrix of the machine learning outcome for recognizing the soft object with encapsulated different stiffness