# Control-Based 4D Printing: Adaptive 4D-Printed Systems
## 5 Discussions and Future Perspectives  
The three-dimensional printing of sensors and actuators have provided a new platform for developing mass-customized systems with autonomous capabilities owing to recent advancements of ML-based control algorithms. This paper has proposed and discussed the new technology of adaptive 4D-printed systems which are printed systems that incorporate 3D-printed sensors and control algorithms and programmed to handle delicate tasks in dynamic environments. It was emphasized that the synthesis and processing of stimuli-responsive materials played a key role for the development of entirely adaptive 4D-printed systems. The recent advancements of 3D-printed sensors for measuring feedback including mechanical motions and deformations as well as physical and chemical proprioceptive information and their specific applications to adaptive 4D-printed systems were discussed. ML-based control algorithms were suggested and reviewed to compensate for the uncertainties pertaining to 4D-printed systems, which originated from non-linear mechanical characteristics and the viscoelastic nature of soft polymeric materials. Although there are materials available to achieve adaptive 4D-printed systems, there is a scope for further investigations on 3D-printable mechanochromic materials that exhibit variable mechano-responsive properties in different environmental conditions. Moreover, promoting the further applications of adaptive 4D-printed systems in key industries, such as automotive, agriculture and aerospace, demands stimuli-responsive and printable materials with a high thermal and chemical stability to function in corrosive, high-temperature and high-pressure, harsh environmental conditions. Piezoresistive and piezoelectric materials, such as PVDF, have exhibited suitable characteristics for integrated sensors and actuators. However, further investigations are required to enhance their printability and piezoelectric performances. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Refs.** & **4D Printing Equipped with Sensors** & **Controller** & **FEM** \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 2: Studies toward adaptive 4D-printed systems. Four-dimensional printing provides a situation for using a random and/or free source of energy in nature to make non-random structures [155]. Studies have shown that energy consumption in 4D printing is the least of all manufacturing processes [14]. Four-dimensional printing could utilize abundant biodegradable materials in nature, for example, chitin from fisheries and cellulose from plants as the most abundant and broadly distributed natural materials in the ecosystem [48]. The current 4D-printed systems do not possess adaptability and longevity and hence are not environmentally friendly. However, the proposed adaptive 4D-printed systems aim to produce systems with a good stability and repeated use, benefiting from the recyclability and low carbon footprint of bio and organic stimuli-responsive polymers. There is still more demand for developing the multiphysics simulation and control platforms for 4D printing to accelerate and support the practical applications of adaptive 4D-printed systems in diverse environmental interactions [156; 157; 158]. The reversible multi-stable compliant mechanisms should be incorporated into 4D printing as sensorless adaptive 4D-printed systems [159]. Four-dimensional printing is often accompanied by considerable bending that may lead to the rupture of particularly soft materials. Therefore, the development of self-healing and shape-changing materials can be one of the future directions of development. The amphibian 4D-printed systems with the adaptability to operate in various ambiances is another direction of future research [160]. They are still unsolved problems in practical applications of 4D-printed systems, particularly in the high-frequency bandwidth in terms of control complexities.