# Control-Based 4D Printing: Adaptive 4D-Printed Systems
## 3 Integration of 3D-printed Sensors into 4D Printing
### Mechanical Motion and Deformation Measurements  
Various thermoplastic materials, such as thermoplastic polyurethane (TPU) and polylactic acid (PLA), have been utilized in mixed form with conductive fillers and liquid metals, such as carbon black (CB) [68] and eutectic gallium-indium (EGaIn) [34], to develop strain sensors. These strain gauge sensors were 3D-printed based on the piezo-resistive or capacitive principles. The pressure variation resulting from geometrical changes generates signals indicating measures such as mechanical  
Figure 2: Controllable 4D-printed systems (**a**) A combined sensor and 3D-printed pneumatic muscle (reproduced from [66] with the permission of Frontier under Creative Commons CC-BY licence); (**b**) An embedded piezoelectric sensor in the hydrogel actuator (reproduced [67] with the permission of SPIE); (**c**) A sweating 4D-printed system with thermoregulation pores (reproduced from [61] with the permission of The American Association for the Advancement of Science); (**d**) The 4D-printed bioinspired microneedles with stiffness control (reproduced from [62] with the permission of John Wiley and Sons); (**e**) A buckling-induced 4D-printed active skin (reproduced from [60] with the permission of the Nature Publishing Group). deformations including compression, extension, bending and twisting [69; 70]. The 3D-printing sensor industry has utilized the electrical conductivity enhancement through the introduction of conductive fillers such as graphene, but such sensors may suffer from degradation or stress-strain hysteresis issues related to conductive materials. This led to a new field of investigations in 3D-printed fibre optic sensors with a direct application in 4D-printed systems because of being lightweight, transparent and inexpensive [71]. Three-dimensional-printed displacement sensors, based on the Hall effect and Eddy-current principles, were fabricated to remotely identify mechanical motions [72]. The higher switching rate, more design flexibility and the wider range of material access are prominent advantages of this type of sensors against its capacitive and resistive counterparts. An inductance sensor was also 3D-printed incorporating ferrofluidic and magnetic materials into the 4D-printed system at room temperature [73; 74]. The prototype demonstrated an acceptable success to measure bending as well as axial and lateral strains. The color-based proprioception method was used for the real-time interaction of soft actuators in an uncertain dynamics environment. A color-based sensing approach was developed to instantly translate the dimensional change of the soft structure into changes in color to reflect local deformations [75]. Piezoelectric sensors have also been 3D-printed with a prospect application in 4D-printed systems. Poly(vinylidene fluoride) (PVDF) was studied as a pressure sensor with a piezo-resistive property that could be 3D printed using electric poling-assisted additive manufacturing (EPAM) [76; 77; 78; 79; 80]. A 3D-printed graphite/polydimethylsiloxane (PDMS) sensor was also developed with a capability of detecting small forces in the range 3.5-7.5 mN [81]. Fibre Bragg grating (FBG) was attached to a 3D-printed acrylonitrile butadiene styrene substrate to develop a pressure sensor [82]. Another study demonstrated that the variable impedance properties commonly found in ionic hydrogels could be used for pressure-sensing at a broad range of payloads [67]. Tactile sensors were integrated into 4D-printing to mimic artificial skin by sensing the stimuli resulting from external interactions. These stimuli can be stresses and strains resulting from the mechanical interactions or temperature of the object encountered. Recently, tactile sensors were made from FDM 3D-printing of conductive PLA filament. [83; 84]. A multiaxial force sensor was FDM-3D-printed as a composite of TPU and carbon nanotube (CNT) with a piezo resistive surface. Despite some hysteresis, the sensor demonstrated sensitivity to small deflections [68].