Laser‐Assisted Direct Ink Writing for High‐Fidelity Fabrication of Elastomeric Complex Structures

Polydimethylsiloxane (PDMS) is extensively applied in the 3D printing field due to its excellent biocompatibility, flexibility, and gas permeability. However, as a thermal curing material, its low strength before curing inevitably causes structural deformation during the printing process and deteriorates the shape accuracy. In this paper, an innovative method, the infrared laser‐assisted direct ink writing (DIW) process, is developed to realize the in situ preheating and curing of silicone materials to improve the strength of PDMS filaments during processing. Therefore, the large‐span spatial structure with high‐fidelity morphology can be produced. First, the feasibility is validated that temperature ascension of the voxels arising from the irradiation energy of the infrared laser can accelerate the cross‐linking reaction and realize the precuring of PDMS; then, the relationship between the parameters of laser irradiation and the processing performance of 3D printing is systematically investigated to find the optimization. Finally, experiments are conducted, including the single silicone filament, silicone foams, and complex 3D features. Their excellent structural preservation proves the proposed method is practical to fabricate the high‐fidelity fabrication of complex structures such as large spans and thin wall features, which will help to broaden the 3D printing capabilities of silicone in the development of flexible electronics and soft robots.


Polydimethylsiloxane
(PDMS) has been widely used in biomedical, [1] flexible electronics, [2] soft robotics, [3] and microfluidics, [4] due to its excellent biocompatibility, flexibility, gas permeability, [5] and chemical/thermal stability. [6]In various application scenarios, PDMS is usually prepared as porous materials, simultaneously achieving a low weight and high stiffness (high specific stiffness).Silicone foam with high porosity is widely used in biological tissue culture, [7] composite lattice skeleton, [8] mechanical protection, [9] and other engineering fields. [10]raditionally, elastic porous structures are prepared by physical templating [11] and chemical foaming methods. [12]Still, they generally demonstrate randomly distributed nonuniform cell units, which may present unpredicted mechanical performance. [13]Recently, 3D printing has attracted significant attention due to its ability to fabricate a highly ordered structure according to model data quickly. [14]As a type of 3D printing, the direct-ink writing (DIW) process has been developed to fabricate silicone foams with a controllable mechanical performance by introducing ordered microstructures in ambient conditions. [15]To realize the fabrication of porous structures with self-supporting(or spanning) features, the PDMS ink must possess appropriate rheological behaviors, including viscoelasticity and shear thinning. [16]Inks with low viscosity will spread on contact with the underlying substrate rather than maintain their filamentary shape.In addition, additional processing steps (such as photopolymerization or thermal curing) may be required in most cases to solidify the printed parts fully. [17]When these steps are decoupled from the printing process, it can be challenging to build truly 3D objects, as the underlying printed layers may only partially support the subsequent layers.Even the inks are suitable for printing, especially when the structure has large span features or many layers.As a typical DIW material, commercial high-viscosity PDMS (DOWSIL-SE1700) is still limited to the structure collapse due to the postheat treatment.
An ideal solution is to cure the material in situ during printing.Some researchers employ the UV-assisted DIW technique to cure materials.However, commercial PDMS materials are generally thermal-cured.Despite some studies adding photosensitive materials to commercial low-viscosity PDMS (SYLGARD-184) to achieve in situ curing, [18] there is degraded mechanical performance with the compress strength.As widely known that heating facilitates chemical reactions.The straightforward solution is to accelerate the heating of the thermally cured materials in situ during the process.External thermal field assisted DIW was proposed to improve the filament collapse of silicone foams by adding a heating substrate. [19]However, the heating transfer range of the substrate is finite.In situ, the dual heating strategy was proposed for rapid 3D printing of thermosets by combining a Joule heater with the DIW printhead to provide a high-temperature environment. [20]Compared to range heating, which inevitably requires the heat resistance of the materials and the equipment, another widely used approach is to achieve localized heating through an infrared laser-assisted process.However, this method was mainly researched for the annealing of silver nanowires. [21]Although the fabrication of optic silicone lenses using infrared laser has been studied, [22] the material's low viscosity limits the fabrication of complex structures such as large spans and thin wall features.Thus, it is necessary to study infrared laser-assisted in situ curing for commercial highviscosity PDMS for fabricating complex spatial structures.
In this study, we develop an infrared laser-assisted DIW system to fabricate high-fidelity structures, including elastic foams with large-span structures and sandglass shape architectures, using commercial PDMS (DOWSIL-SE1700) with high viscosity characteristics.The impacts of process parameters will be investigated.The elastic foams with different sizes of spanning features were fabricated.Compared with the conventional direct writing technique, the deformation of the hanging filaments of the elastic foams printed by the proposed in situ thermal curing strategy decreased with no mechanical performance degradation.We further demonstrate the ability to fabricate the sandglass shape structure.

Results and Discussion
Getting heat into the silicone is critical to rapidly increasing material strength.Heat-curing silicone materials usually take a period in a high-temperature environment to achieve a complete cross-linking reaction to cure.And the viscosity of the silicone also gradually increases during this process.Laser radiation offers a high peak intensity heat stimulus in the objective lens's focal region to modify the local silicone material properties.Figure 1 shows the layout of the home-built laser-assisted DIW system.During printing, a 10.6-μm IR laser is focused on a spot beneath the stainless-steel nozzle, where the silicone ink is deposited.Therefore, the material extruded from the nozzle is rapidly heated by the focused laser to achieve an in situ precuring of the silicone filament.See the experimental setup and laser adjustment for the specific equipment composition and the laser focusing process.

Material Properties and Laser Parameters
The SE1700 is a two-part, thermally curable PDMS-based resin cured at 150 °C for 30 min.In our experience, the higher the temperature, the faster the reaction rate, and the curing process will be completed in a shorter time.A differential scanning calorimeter (DSC) test was conducted to investigate the cross-linking reaction temperature.As shown in Figure 2a, the cross-linking behavior of the PDMS appeared when the temperature was increased to 90 °C; until 131.7 °C, the peak cross-linking reaction rate was reached.A thermogravimetric (TG) test was also conducted to analyze the depolymerization of the silicone.As shown in Figure 2b, when the temperature reaches 430 °C, the mass of the material drops sharply, which means the depolymerization or degradation of silicone.
Excessively high temperatures (higher than 430 °C) caused by excessive laser power will lead to depolymerization and degradation of the material.At the same time, a relatively lower temperature (lower than 90 °C) does not contribute to facilitating the cross-linking reaction.Therefore, there are optimal laser parameters and printing parameters.
The principle of infrared laser heating is the absorption of laser energy by the material.Therefore, the wavelength and power of the laser have an essential impact on the temperature rise of the material.Polymers, such as PDMS, experience an absorption loss, mainly caused by the vibrational overtone and combination bands of the CH 3 -groups of the polymer.According to the research of Li et al., [22] different infrared laser wavelengths have great influence in the heating effect of materials.The FT-IR measurement of PDMS material was carried out to confirm the suitability of the laser wavelength applied for material heating.The resulting curve of transmittance versus wavenumber is shown in Figure 2c.The typical peaks of the C-H methyl stretch at 2962 cm −1 , the silicon-methyl bond at 1258 cm −1 , and the broad polymer backbone absorption band between 1130 and 1000 cm −1 are found. [23]The transmission valleys denote the strong absorption of the incident energy at the corresponding wavelengths.According the previous study, [22] at the laser wavelength where the transmittance is almost 0, the temperature at the center of the spot ascends dramatically, which is not conducive to the adjust-ment of laser energy.While at the laser wavelength where the transmittance is close to 100%, there is almost no heating effect.On the one hand, we hope that the selected laser has a certain penetration to the material to pursue uniform curing of a single filament.On the other hand, it has reasonable absorption to facilitate controllable curing of the material instead of damaging the material.Here, a carbon dioxide laser with a wavelength of 10.6 μm was selected.At 943 cm −1 (10.6 μm), the transmittance is 85.5%, keeping a relative low level of absorption, which is conducive to the adjustment of laser parameters.
It can be seen that the absorption peaks of the material are mainly concentrated in the infrared band.According to our experience, if the absorption rate is too high, the curing will be mainly concentrated on the material's surface.In addition, the material's response to the laser parameters is more sensitive and challenging to adjust.On the other hand, too low absorptivity makes it difficult to heat the material.Here, a carbon dioxide laser with a wavelength of 10.6 μm was selected.The absorption rate of the material in this band will not be too high, which is conducive to the adjustment of laser parameters.
The laser output is controlled by the pulse wave generated by the waveform generator.The laser power can be controlled by adjusting the pulse frequency and duty cycle.Thorlabs' PM320M optical power meter and S175C optical power meter probe are used to measure the laser power under different duty cycles.It can be seen from Figure 2d,e that the frequency had little effect on the laser power.Thus the laser power was mainly adjusted by the duty cycle.
Since the material determines all thermophysical parameters, the variable in the 3D printing process is the focused laser parameter.Therefore, the effect of laser power on curing was investigated.The frequency of the pulse waveform that triggers the laser is set to 1 kHz, and the first layer of silicone filaments are cured online with different duty cycles.To ensure the printed filaments' width is consistent, keep the printing speed and supply air pressure unchanged.The printing speed is 8 mm s −1 .

Simulation of Temperature Distribution Along Silicone Filament
To confirm the appropriate range of laser power parameters so that the temperature of the silicone in the irradiated area is within the acceptable range (131.7 °C-430 °C), the laser scanning process was simulated by Comsol.Silicone filaments are simplified to cuboids with a 400 μm broad square cross-section.The laser is reduced to a Gaussian beam to simulate its energy distribu-tion.Here, the laser spot size is a crucial parameter affecting energy distribution.Due to the excessive energy at its focal point, a conventional beam analyzer cannot measure this long-wave infrared laser.The more traditional and generally applicable knifeedge method is used here to calculate the spot size at the focal point.The equipment of the knife-edge test is shown in Figure 3a.The measured laser power accumulation and position curves are shown in Figure 3b.By conversion, The obtained spot diameter is about 714 μm.
Silicone surface temperature increases linearly with increasing laser power.The influence of different laser power on the temperature of silicone material was simulated, as shown in Figure 3c,d, in which w is the beam radius.When the laser power was 0.128 W and the scanning speed was 8 mm s −1 , the maximum temperature of the silicone filament was about 184 °C.To verify the validity of the simulation results, an infrared camera d) The optical image of silicone filaments scratched with a tweezer.e) The differential scanning calorimeter (DSC) stacked profiles.f) The apparent curing degree of silicone dealt with different laser power.The apparent curing degree was calculated from the integrated area of the exothermic peak in Figure 4e.
was used to measure the temperature distribution cloud image of the material during the printing process, as shown in Figure 3e.The laser power was set as 0.128 W.During the laser scanning, the temperature change over time was recorded and plotted as a frequency profile shown in Figure 3f.It can be seen that the surface temperature of the silicone is usually between 150 °C and 200 °C during the laser scanning process.The average temperature was 183 °C, above the curing temperature and below the degradation temperature.This result is consistent with the simulation.

Effects of Laser-Assisted Curing
In order to test the effect of laser-assisted curing, rheological characterizations were performed on the materials treated with different laser pulse duty factors.In order to obtain enough materials for rheological testing, a 0.4 mm stainless steel metal needle was used to deposit silicone layer by layer on the glass slide substrates at a printing speed of 8 mm s −1 , and the silicone was filled the entire glass slide in the XY direction.The silicone filaments are dealt in situ with different laser pulse duty factors during the printing process.Then the materials on the glass slides were smeared onto the stage of the rheometer for rheological performance testing.
The viscosities and storage modulus of materials dealt with different laser power are shown in Figure 4a,b.The sample labeled 0% indicates that it is raw material.Its viscosity decreases with the increase of share rate, reflecting the characteristics of shear thinning, which is suitable for DIW 3D printing.With the laser power increasing, the apparent viscosity and storage modulus of silicone both increase due to the precuring of the material.Due to the unidirectionality of laser irradiation, uneven curing of materials is inevitable.When the silicone was irradiated by laser light, the surface of the material was firstly and locally cured compared to the rest part of the filaments.
To further illustrate the curing situation of silicone, zig-zag filaments were printed on the substrate using a 0.4 mm stainless steel metal needles and treated in situ with different laser pulse duty factors.A tweezer was used to scratch silicone filaments vertically from top to bottom to visually check the curing of the silicone filaments, as shown in Figure 4d.Apparently, the bottom of the silicone filament was still in the gel state at various laser powers, while the top is cured to some extent first at higher laser powers.
In order to verify and examine the curing degree of the material on the surface of the silicone filaments, and further determine the appropriate laser power, the surface modulus of single silicone filament was tested using the force-displacement mode in AFM.The modulus of silicone cured by the oven is compared.The samples used here were the fore-mentioned zig-zag filaments.The degree of curing should be as large as possible.At the same time, the material will not be ablated due to excessive power.Five regions were selected for each sample, the region size was 5 × 5 μm, and five uniformly distributed points on the top surface of silicone filaments were chosen for each region for measurement.The probe model used was CONTSCR, with a force constant of 0.2 N m −1 and a probe radius of 10 nm.The modulus calculation model adopted is the DMT model.
It can be seen from Figure 4c that the larger the pulse duty cycle, the larger the surface modulus.When the pulse ratio is less than 8% (corresponding laser power is 0.128 W), AFM cannot measure the modulus because the silicone surface is not cured.Therefore, when laser-assisted direct writing is used to print silicone materials at a pulse frequency of 1 kHz and a printing speed of 8 mm s −1 , at least an 8% pulse duty cycle is required to cure the surface of the silicone filaments.
In addition, the apparent curing degree of silicone should be clarified to quantitatively evaluate the effect of the laser irradiation on in situ heating.The DSC tests could be a suitable method for the purpose. [20]The DSC stacked profiles of silicone treated with different laser pulse duty factors are shown in Figure 4e.With the increase of laser power, the decrease of exothermic peak area indicates the deepening of curing degree.For the oven cured silicone, the DSC profile shows no exothermic peak, indicating a totally cured condition.While for the freshly prepared PDMS, there is a largest peak in the profile.The apparent curing degree was calculated according to variation of the exothermic peak area and drawn in Figure 4f.With the increase of laser power, the apparent curing degree increased accordingly.

Collapse of a Single Filament
The real-time curing of the silicone during printing increases its modulus, which will facilitate the printing of suspended structures without support and structural preservation.A formpreserving embodiment is the degree of collapse of a single filament at a specific span.Usually, under a certain span, the forming filament tends to collapse significantly due to its gravity's influence and small modulus.Here, collapse characterization was performed on filaments cured with different laser powers.The schematic diagram of the printing process is shown in Figure 5b.Scaffolds with spans ranging from 0.6 to 7.2 mm were designed, fabricated by machining, and had a rough surface of Ra3.2.Collapse features were characterized by ultra-depthof-field microscopy (DVM6, Leica, Germany).Here, the maxi-mum fluctuation distance of the upper surface of the filament is extracted as the collapsed size.The printing speed is kept at 8 mm s −1 .The pulse duty factor that controls the laser power varies from 0% to 10%.To verify the effectiveness of the process for different feature resolutions, collapse experiments with varying filament widths were performed.
The optical images of the collapsed features in different cases are shown in Figure 5a.Here, the collapse parameters of the maximum span (7.2 mm) for different filament width sizes under different laser signal duty cycles were extracted, as shown in Figure 5c.It can be seen that with the increase in span, the collapsed size shows an increasing trend.As the laser power increases, filament collapse decreases for all feature resolutions.Especially for thicker filaments (400-800 μm), the collapse reduction is pronounced.Since thicker filaments are more susceptible to gravity to produce more significant collapse, a slight amelioration in material modulus substantially improves the degree of failure.When the duty cycle of the laser signal was more important than 8%, the collapse did not improve significantly.In addition, it should be noted that a duty cycle greater than 8% will result in noticeable ablation marks on the silicone surface, which would be undesirable.The optical and SEM images of the surface of filaments with a diameter of 440 μm under different laser power conditions are shown in Figure 6a-c.
Therefore, a duty factor of 8% would be a preferred laser parameter, which can significantly improve the large-span collapse without causing significant surface ablation.In addition, crosssections of the filaments were observed to understand the effect of the laser on the filament topography.As shown in Figure 6d, when the pulse duty factor is less than or equal to 8%, there is no noticeable change in the cross-sectional morphology of different filament diameters.The laser power at the 10% pulse duty factor results in ablation marks on the top of silicone filaments.Finally, the optimal parameters were obtained, which have less influence on the surface and cross-sectional morphology of silicone and minor deflection at large spans.

Printing of Silicone Foams with Large Span Feature
Based on the optimized laser parameter (8% duty factor), the cross-sectional morphology of the laser-assisted foam structure samples was studied and compared with those obtained without the laser-assisted process.Figure 7a shows foam samples with different characteristic filament widths, where the filament spacings were ten times the typical filament width.Cross-sections of the lattice structure were measured to characterize the collapse of the filament before compression.As shown in Figure 7a, intuitively, obviously, the laser-assisted design has minor collapse under various filament width and span conditions, which contributes to the assurance of structural accuracy.Similarly, as with the collapse of a single filament, the improvement in collapse for the sample obtained with larger inner diameter needles was more apparent, as shown in Figure 7b.

Printing of Complex 3D Structure with Thin Wall Feature
To demonstrate the advantages of the laser-assisted direct writing process, the sandglass shape structure with a thin wall  feature, as shown in Figure 8a,b, was printed.The left image in Figure 8a,b are samples fabricated by the traditional DIW process, while the right image is the laser-assisted process.The samples in Figure 8a have a diameter of 20 mm at the bottom and top layer and a diameter of 3 mm at the neck in size.The angle between the cone bus and substrate is about 63°.The thin wall of the structures was built up of only single filament width, making it vulnerable to collapse, combined with the small size of the neck.
Compared to the traditional DIW process, the benefit of precuring can significantly improve the modulus of silicone materials to effectively resist the structural deformation for the target with large-span spatial structure no matter during the printing process (Figure 8a, Movie S1, Supporting Information) nor the subsequent curing process in an oven (Figure 8b), which proves the effectiveness of the method.In addition, a single pillar was fabricated in Figure 8c by elevating the nozzle during extruding material, which can reflect the ability of the material to resist deformation.It can be found that the online precuring gives the material higher rigidity and contributes to the architecture's retention, which means that the production of a structure with a high aspect ratio is feasible.

Conclusion
In this paper, we have developed an infrared laser-assisted direct ink writing process to fabricate high-fidelity PDMS struc-tures with commercial thermally cured DOWSIL-SE1700.The increase of the viscosities and modulus of silicone materials were observed after treatment with an infrared laser, which validates the feasibility of the method.The accelerated cross-linking reaction arising from the irradiation energy of the laser significantly improves the collapse of large-span architecture.The optimized parameters are suitable for the different filament widths from 0.2 to 0.8 mm in the DIW process.The observed ablation marks on the filament surface at the high laser power supply mean that there should be an optimal laser parameter to accelerate the cross-linking reaction without degrading the material.The pulse duty factor of 8% was confirmed to be an optimized laser parameter in our cases.The fabrication of complex structures with a thin wall, such as the sandglass shape and a single pillar with large aspect ratio features, indicates that the proposed infrared laser-assisted DIW process is capable of realizing excellent shape retention compared with the traditional DIW process.The online curing process for commercial thermally cured silicone materials is an effective method to realize the production of large-span and thin-wall structures and will help to broaden the 3D printing capabilities of silicone in the development of flexible electronics and soft robots.This method is theoretically feasible for more general thermally curable materials.

Experimental Section
Materials: The commercial two-part PDMS adhesive named DOWSILTM SE 1700 containing a clear base and the catalyst was purchased from Dow Corning.The ink was prepared by mixing the clear base, catalyst, and inhibitor (3-butyn-1-ol) at a weight ratio of 100:10:1 using a planetary mixer (ZYMC180V, ZYE Technology Co. Ltd.) at 1500 rpm for 150 s.After that, the ink was loaded into 10 cc syringes (Nordson EFD) and centrifuged at 5000 rpm for 5 min to remove any bubbles.The inhibitor was used to slow down the curing rate and extend the shelf-life.
Experimental Setup: The setup of the IR laser-assisted DIW is shown in Figure 1.The laser system and the feeding system are mounted on an optical breadboard, which itself is mounted onto a homemade gantry-type 3D motion platform.Here, a 5 W, 10.6 μm diode laser generator (L5, AC-CESS LASER) is connected to the laser microscope using an optical fiber and focused via a 12 mm objective lens with a focal length of 35 mm.The RF laser driver (ACCESS LASER), which controls the laser generator on and off, is modulated by a pulse signal from a waveform generator (DG812, RIGOL) to enable the precise variation of the laser pulse power.The feeding system consists of an air pressure regulator, a 10 cc syringe, metal needles with different inner diameters (from 200 to 800 μm), and an air conveying pipe.In the traditional DIW process, the material in the syringe is deposited layer by layer in a programmed path with an appropriate pressure controlled by the computer to fabricate desired models.The resolution and the morphology of the parts are depended on the nozzle diameter and the printing parameters.Usually, the silicone structure obtained by the DIW process requires postheat treatment to cure the material, leading to inevitable collapse during the printing process, affecting the structural accuracy and further affecting the structure's mechanical properties.Hence, the IR laser is applied here to solidify the local material during printing to improve the structural collapse.
A laser mode screen is used to develop the invisible infrared spot to align the laser spot and the silicone filament extruded from the nozzle.When illuminated with the long wave UV developing lamp, the phosphors of the laser mode screen fluoresce.When an IR laser beam strikes the fluorescent surface, the absorbed energy raises the surface temperature.It quenches the fluorescence, producing a dark thermal image of the crosssection of the laser beam in intensity.By adjusting the distance between the laser head and the substrate to reach the focal length of the laser objective lens, the laser can be focused on the substrate, and obvious black spots will be displayed on the laser mode screen.The Tri-axis Fine-adjuster adjusts the relative position between the laser spot and the needle head to ensure the laser's focus on the printing filaments.
Laser Adjustment: The position between the laser head and needle tip is fixed.If the laser spot is offset by a certain distance from the needle tip, the material deposited in one direction cannot be cured in time.Therefore, the laser spot must be precisely focused directly underneath the needle tip so that the laser immediately irradiates the extruded material.Here, the laser spot is focused to a filament width below the needle tip, also the inner diameter.It should be noted that when the print head moves in the reverse direction, the laser directly irradiates the silicone filament.In contrast, in the forward movement, it cannot directly irradiate the filament.However, this difference can be negligible considering that infrared laser possesses excellent penetration into the ink.
The angle between the laser outlet and nozzle is set to 60°, and the needle is perpendicular to the substrate.First, move the needle upwards by adjusting the stage installed between the needle and the laser head.The needle tip deviates from the laser path to avoid blockage caused by direct injection to the nozzle after the laser is turned on.Then, trigger the laser to emit light with a pulse signal with a lower duty cycle (5%) at 1 Hz frequency and slowly lower the platform.At this time, the laser spot moves on the laser mode screen and becomes clear from blurring.Until the position of the laser objective is close to the focal length (35 mm), an apparent black spot is formed near the focal length, which means the laser spot position.After that, the nozzle tip and the laser spot are aligned horizontally through the tri-axis fine-adjuster between the laser head and the needle so the laser spot is directly under the needle tip.This process uses a USB camera to better observe the position of the spot.The captured aligned picture is shown in the illustration of Figure 1.Then turn off the laser and lower the needle.A feeler gauge controls the distance between the needle and the developing plate at a filament width.After the silicone material leaves the nozzle during the printing process, it is rapidly heated and solidified by the focused laser.Patterned features are formed according to the motion path.
Simulation of Heat Transfer: Heat transfer of freshly extruded silicone filament during the infrared laser scanning along the filament direction was conducted using the commercial software COMSOL Multiphysics.The action of the laser on the material was simplified to a moving heat source on the silicone filament.The Gaussian distribution of laser energy was adopted from reference. [22]ata Measurement: The cross-linking reaction of the silicone material was determined by differential scanning calorimetry (DSC 2500, TA Instruments, America).The test was performed with air gas and increased from room temperature to 300 °C at a ramp rate of 10 °C min −1 .The apparent curing degree K was calculated according to the following formula: where ΔH s and ΔH u stands for integrated exothermic peak area of sample and totally uncured material.TG analysis was conducted to investigate the degradation temperature of the PDMS using a thermobalance (TG 209 F1 Libra, NETZSCH, Germany).The test was performed with air gas and increased from room temperature to 1000 °C at a ramp rate of 10 °C min −1 .
The beam radius was measured by the knife-edge technique.The focal length of the objective lens is 35 mm.An optical power meter (S175C, Thorlabs) was used to measure the power of the laser beam.For the obtained power accumulation versus position curve, take the difference between the positions where the power is 90% and 10% of the maximum value to get the relative distance.The coefficient 0.7803 times the close distance is the beam radius. [24]he morphology of the fabricated samples was examined using an optical microscope (Leica DVM6 A, Leica Microsystems GmbH, Switzerland).The viscosities of the silicone materials treated with lasers of different powers were measured by a rotational rheometer (HR-10, TA Instruments).The samples were prepared by extruding materials under laser irradiation as in actual printing and then mixed.A 40 mm parallel plate geometry was used and the shear rate was swept from 0.1 to 100 s −1 .A stress sweep from 10 to 3 × 10 3 Pa at a constant frequency of 1 Hz was performed to measure the storage moduli at different sweep stresses.The gap was set as 1 mm.The samples were characterized using a Park XE7 atomic force microscope in F/D mode, which can be used to measure the surface modulus regardless of the shape of the material.The model used for modulus calculation is the DMT model.Considering the small size of the filament width, an area of 5 × 5 square microns was selected here for characterization.

Figure 1 .
Figure 1.Experimental equipment of the laser-assisted direct ink writing (DIW) system.The upper left inset shows that the laser spot (black dot) was located directly below the tip of the nozzle.

Figure 2 .
Figure 2. a) Differential scanning calorimeter (DSC) curve of uncured SE1700 and b) thermogravimetric (TG) profile of SE1700.c) Infrared spectroscopy of SE1700.d) Laser power with different signal frequencies.e) Laser power with various signal pulse duty factors.

Figure 3 .
Figure 3. Measurement of spot diameter by knife-edge method.a) Equipment setup for measuring laser power.b) The values of accumulating laser power are measured by the optical power meter when the knife edge at the waist position scans the focused beam.c) Temperature contour plot.d) Maximum temperature curves obtained by simulation under different laser powers.e) Temperature distribution during printing captured by an infrared camera.f) Frequency distribution chart of the maximum temperature captured by the infrared camera over time.

Figure 4 .
Figure 4. a) The viscosity b) storage moduli of materials dealt with different laser pulse duty factors.c) Surface moduli of single silicone filament.d) The optical image of silicone filaments scratched with a tweezer.e) The differential scanning calorimeter (DSC) stacked profiles.f) The apparent curing degree of silicone dealt with different laser power.The apparent curing degree was calculated from the integrated area of the exothermic peak in Figure 4e.

Figure 5 .
Figure 5. a) Optical image of the collapse degree of a single filament.The scale bar was 1 mm.b) Schematic printing process diagram.c) Collapse degree with different conditions (filament size and laser power).

Figure 6 .
Figure 6.Optical images of the filament surface dealt with different laser power at a) 340 and b) 800 times magnification.c) SEM image of ablation topography of silicone surface with varying powers of the layer at 120 times magnification.The scale bar of (a-c) was 250 μm.d) Sectional morphology of silicone filaments with different diameters and laser conditions (signal pulse duty factor).The scale bar of (d) was 100 μm.

Figure 7 .
Figure 7. a) Samples for measurements and optical images of the collapse of the cellular structure in different conditions.b) Comparison of the collapse degree of silicone foams.

Figure 8 .
Figure 8. a) The sandglass shape structures with a narrow neck in size.The right sample was fabricated with a laser-assisted process.b) The sandglass shape structures after curing in an oven.The right sample was fabricated with a laser-assisted process.c) Pillars fabricated by vertically elevating nozzle.