Digital light processing 3D printing of flexible devices: actuators, sensors and energy devices

Abstract

Flexible devices are increasingly crucial in various aspects of our lives, including healthcare devices and human-machine interface systems, revolutionizing human life. As technology evolves rapidly, there is a high demand for innovative manufacturing methods that enable rapid prototyping of custom and multifunctional flexible devices with high quality. Recently, digital light processing (DLP) 3D printing has emerged as a promising manufacturing approach due to its capabilities of creating intricate customized structures, high fabrication speed, low-cost technology and widespread adoption. This review provides a state-of-the-art overview of the recent advances in the creation of flexible devices using DLP printing, with a focus on soft actuators, flexible sensors and flexible energy devices. We emphasize how DLP printing and the development of DLP printable materials enhance the structural design, sensitivity, mechanical performance, and overall functionality of these devices. Finally, we discuss the challenges and perspectives associated with DLP-printed flexible devices. We anticipate that the continued advancements in DLP printing will foster the development of smarter flexible devices, shortening the design-to-manufacturing cycles.

Fig. 1. Overview of DLP 3D printing for flexible devices, including soft actuators, flexible sensors and energy devices.

The figure illustrates the printing process of a bottom-up DLP 3D printing approach. EMG electromyogram, ECG electrocardiogram, EEG electroencephalogram. The photos of the photocurable soft materials are reproduced with permission from refs. ^,,–

Fig. 2. DLP printed soft pneumatic actuators.

a g-DLP printed pneumatic actuators with heterogeneous stiffness, capable of performing extension, contraction, bending and twisting motions. Reproduced with permission from Springer Nature (2023). b A miniature soft gripper and a miniature soft inflatable actuator composed of stiff and soft materials. Reproduced with permission from Wiley (2019). c A DLP-printed soft pneumatic actuator with heterogeneous materials capable of detecting both positive and negative bending. Reproduced with permission from American Association for the Advancement of Science (2021). d Centrifugal multi-material 3D printing for producing soft actuators integrated with multiple sensors. Reproduced with permission from Springer Nature (2022)

Fig. 3. DLP-printed muscle-like actuators with LCEs.

a A hybrid printed LCE-based actuator with tunable structural stability fabricated through DIW and DLP printing techniques. Reproduced with permission from Wiley (2022). b Shear alignment of LCEs caused by the shear induced by cyclic rotation of the resin tray. The printed actuators can achieve weightlifting and object grasping. Reproduced with permission from American Association for the Advancement of Science (2021). c Fabrication of LCEs actuators based on a through-plane light attenuation method, which induces crosslinking gradient for guiding a gradient of mesogen alignment during solvent evaporation. Reproduced with permission from Wiley (2021). d Actuators based on stretching-induced alignment of liquid crystal organogels to achieve erasable and reprogrammable actuation. Reproduced with permission from Wiley (2022)

Fig. 4. DLP-printed soft actuators based on SMPs.

a A 3D printed actuator based on SMPs capable of exhibiting different reversible body movements. Reproduced with permission from Elsevier (2021). b 4D printed reconfigurable smart gripper and flat hinge based on mechanically robust CAN-SMPs. Reproduced with permission from American Association for the Advancement of Science (2024). c Cold-programmable 3D lattice structure produced using g-DLP. Reproduced with permission from Springer Nature (2023). d Machine learning model to facilitate inverse design of material distribution of 4D printed composites with active materials to attain the desired actuation shape. Reproduced with permission from Springer Nature (2024)

Fig. 5. DLP-printed flexible resistive strain/pressure sensors.

a A DLP printable polymerizable rotaxane hydrogel assembled by precise host-guest recognition that can be used for creating resistive sensors with fatigue resistance. Reproduced with permission from Springer Nature (2023). b DLP printing of conductive liquid metal patterns, which can be used for various applications, such as resistive sensors, heaters and electrodes for electrography. Reproduced with permission from Wiley (2024). c A resistive sensor and functional construct produced based on recyclable ion-conductive Δ-Valerolactone thermoset ink. Reproduced with permission from Wiley (2024). d Structural design of a flexible pressure sensor to realize tensile strain insensitivity. Reproduced with permission from Wiley (2024)

Fig. 6. DLP-printed capacitive strain/pressure sensors and electrodes for electrography.

a Capacitive sensors featuring DLP-printed domes with a gradient height to enable a high sensitivity, produced from a highly conductive and stretchable nanostructured ionogel. Reproduced with permission from Springer Nature (2024). b Capacitive sensors with stable bonding interface produced from acrylic-based dielectric materials and conductive hydrogels using a dual-material DLP printing strategy. Reproduced with permission from Wiley (2019). c An ionotronic capacitive sensor with multi-mode sensing capabilities that can be harnessed to control the flight of a drone wirelessly. Reproduced with permission from Springer Nature (2023). d Capacitive sensors composed of silk fibroin-based biocompatible hydrogel electrodes with different surface structures for superior adhesive properties. Reproduced with permission from Wiley (2024). e Multiple electrodes made from PSS:PEDOT eutectogels can be sewn into the textile for body surface potential mapping of the forearm. Reproduced with permission from Elsevier (2024). f DLP printed Au electrodes based on anion-assisted photochemical deposition with high conductivity and conformability for electrophysiological signal monitoring. Reproduced with permission from Wiley (2022)

Fig. 7. DLP-printed energy devices.

a A 3D-printed rotating TENG and a TENG with thimble geometry for mechanical energy harvesting. Reproduced with permission from American Chemical Society (2023). b An auxetic structure-assisted PENG that can harvest energy in the bending mode. Reproduced with permission from Wiley (2023). c A SC with adjustable capacitance based on urchin-like Cu(OH)₂ lattice electrodes fabricated by DLP and electro-oxidation method. Reproduced with permission from Wiley (2019). d A structural SC in the shape of a watchband produced through DLP and dipping processes. The device is designed for lightweight and miniaturization, suitable for powering an electronic watch. Reproduced with permission from Wiley (2023). e An impact-resistant SC with self-healable hydrogel electrolyte-infused lattice electrodes that can work under harsh environments. Reproduced with permission from Wiley (2024). f A 3D structured electrochromic SC can visually monitor the energy storage levels by a color change. Reproduced with permission from Wiley (2021)

Fig. 8

Perspective on DLP printing of flexible devices