Integrated 3D printing of flexible electroluminescent devices and soft robots

Flexible and stretchable light emitting devices are driving innovation in myriad applications, such as wearable and functional electronics, displays and soft robotics. However, the development of flexible electroluminescent devices via conventional techniques remains laborious and cost-prohibitive. Here, we report a facile and easily-accessible route for fabricating a class of flexible electroluminescent devices and soft robotics via direct ink writing-based 3D printing. 3D printable ion conducting, electroluminescent and insulating dielectric inks were developed, enabling facile and on-demand creation of flexible and stretchable electroluminescent devices with good fidelity. Robust interfacial adhesion with the multilayer electroluminescent devices endowed the 3D printed devices with attractive electroluminescent performance. Integrated our 3D printed electroluminescent devices with a soft quadrupedal robot and sensing units, an artificial camouflage that can instantly self-adapt to the environment by displaying matching color was fabricated, laying an efficient framework for the next generation soft camouflages.

UV radiation from a high-pressure mercury light source. A UV light source (Exfo Omnicure S2000, wavelengths of 365 nm) with a triggering cable was attached to the DHR Rheometer.
Once the UV radiation was on, time-dependent evolution of the mechanical properties (G and G ) was measured at a constant amplitude of 0.5% and frequency of 10 rad s −1 . All the measurements were carried out at room temperature, and all results were processed using the TA Instruments TRIOS software.

Mechanical Testing.
The mechanical properties were measured using a Cellscale's Ustretch tensile machine, which was equipped with a 44-N load cell. Dog-bone samples with a dimension of 5 mm in width, 10 mm in gauge length, and 0.6 mm in thickness were stretched at room temperature.
180-degree peeling tests were carried out to quantify the interfacial toughness of joints between the 3D printed layers of different inks. The 3D printed samples were fabricated with a dimension of 50 (L) × 10 (W ) mm. Tearing tests were also conducted to quantify the toughness of the ICE, ELE and IDE samples. The samples were fabricated with a dimension of 50 (L) × 10 (W ) mm. All the tests were carried out at a deformation rate of 20 mm min −1 .

Characterisation of the Electrical Properties of the EL units.
Resistance of the ICE samples was measured with a TH2832 LCR meter. To track the change in the resistance upon mechanical deformation, the samples were connected to the LCR meter using conductive Ag pastes prior to unidirectional stretching.
Dielectric frequency spectra of the insulating dielectric elastomer (IDE) layer and the PDMS dielectric polymers were measured using a LCR meter (E4980A) with a DMS-2000 dielectric temperature spectrum system. In order to prepare the double-sided electrode films for the measurement, the samples with a dimension of 10 mm (L) × 10 mm (W ) × 0.5 mm (t) were laminated with PET masks that had a 6 mm diameter punched-through hole.
A thin film of platinum was then deposited over the sample using ion sputtering (Mciooo ion sputter, HITACHI). The permittivity measurements of the samples were performed at a frequency range from 20 to 10 6 Hz at room temperature.
Imaging and Image Analysis.
Photos and videos in this work were taken with a MILC SONY camera (α7R4, Japan).
Green dye (fluorescein) was used to stain the 3D printed samples for better visualization effect. Optical microscope (Mshot MD30) was used to assess the micro-structural features of the 3D printed patterns. Inverted fluorescence microscope (Olympus IX73) was used to evaluate the fidelity of the 3D printed ELE filaments and the conformity of the interfaces between different layers of inks. The blue luminescence of the ELE ink was resulted under an excitation wavelength λ ex of 530 nm. The ICE layers were stained by Nile Red (0.1 wt.%) with an excitation wavelength λ ex of 530 nm, and an emission wavelength λ em of 570 nm. The morphologies of the 3D printed multi-layer interface were also assessed using a field emission scanning electron microscopy (Merlin, Zeiss) at a current of 100 pA and an acceleration voltage of 5 kV. Prior to SEM observation, the 3DP multi-layered samples (ca. 11 layers, ca. 2.5 mm in thickness) were sliced at a thickness of 200 µm using a freezing microtome (Leica CM1950 Cryostat).

Luminance Quantification.
The luminance and chromaticity of the 3D printed EL devices were measured with an integrating sphere (FOIS-1) and an QE Pro spectrometer, respectively. A high-voltage AC power supply (6705 linear programmable AC power source, ECC industries), coupled with voltage amplifier (OPT.624, output voltage 600 V) and frequency amplifier (OPT. 625, output frequency 1 kHz), was employed to drive the electroluminescent devices. Optoelectronic characteristics of the EL devices were quantified using a QE Pro spectrometer by gradually increasing the voltage from 50 V to 600 V. The amplifier provides monitoring terminals output voltages (1 V/1 kV) and current (1 V/4 mA), which were recorded with a Keysight DSOX2024A oscilloscope.

Finite Element Analysis (FEA) Simulation.
A commercial software (ABAQUS 2018, SIMULIA) was used to study the stress and strain distributions within the 3DP samples and the physically-laminated samples (control) when a 35% strain was applied to the samples. The sample, which consists of ICE and ELE layer, has a dimension of 150 mm (length) × 10 mm (width) × 2.2 mm (height).
The simulation was performed using the elastic-plastic constitutive calculation model in ABAQUS. The interfacial toughness between the ICE and ELE layers of the 3DP sample was set as 150 J m −2 in the simulation, whereas the friction coefficient between layers of the physically-laminated sample was set as 0.45.  . (b). G' and G" values of the the ELE inks without ZnS and/or SiO 2 nanofillers measured from dynamic room-temperature oscillation sweeping with the oscillation stress ranging from 10 −1 to 10 4 Pa. A slight change in the rheological properties was occurred when ZnS particles were added, and no yielding occurred for the ELE ink without SiO 2 and the ELE ink without ZnS/SiO 2 . Viscoelastic inks were obtained when silica nanofillers were added (ELE ink), featuring distinct stress yielding which were highly desirable for DIW. Phase diagram illustrating the regions of SiO 2 content and applied pressure for a printable ICE inks. The ink can be printed into structures when the SiO 2 concentration ranged between 5 and 25 wt.% and the pressure was above a certain threshold as indicated in green in the graph. When SiO 2 concentration was below 5 wt.% or above 25 wt.%, the ink cannot be printed due to liquid spreading or particle clogging, respectively; and when the printing pressure was below the threshold value, the ink cannot flow out from the nozzle. (b). Phase diagram illustrating the printable region of the ELE inks. Without the addition of SiO 2 particles, the dielectric inks with or without ZnS particles flowed readily from the nozzle due to liquid spreading, while the addition of SiO 2 particles effectively improved the printability. The inks were printable at various nozzle diameters (i.e. 160, 210, 260 and 340 µm). Supplementary Figure 31: Control logic of the multi-color EL display. The control system of the multi-color EL display was composed of a microcontroller, a light sensor and three relays, which independently control the AC voltage supplied to the three light-emitting EL devices. The sensor converted the information of the environmental color to electrical information. According to the electrical signal, the microcontroller controlled the ON/OFF of the individual relays, hence the AC voltage supply of the three light-emitting devices.