Hybrid Printed Rigidity‐Programmable Substrate/Liquid Metal 3D Circuits Toward Stretchable Electronics

Stretchable electronics have the unique capability of 3D (three dimensional) deformation, overcoming the brittleness of traditional inorganic electronics. However, during large deformations, different scale strains between the rigid and stretchable components lead to mismatch, causing interconnect failures. Therefore, the development of the rigidity‐programmable substrate with effective strain shielding capabilities has become a research hotspot. Furthermore, the exponential growth in electronic density presents challenges in the circuit design of stretchable electronics. The urgent need is to develop highly integrated stretchable electronic systems. In this study, a highly integrated stretchable pulse sensor with effective strain shielding capabilities using hybrid 3D printing technology is developed, which comprises electronic chips, a rigidity‐programmable substrate/encapsulation layer printed by using PSC (polydimethylsiloxane/silica‐nanoparticles composite)‐based ink, and LM (liquid metal)‐based 3D circuits. First, the PSC‐based ink is optimized to enhance the strain shielding effectiveness of the rigidity‐programmable substrate. Meanwhile, 3D printing parameters are optimized to achieve high printing precision with minimum line widths below 100 µm. The resulting stretchable pulse sensor demonstrated good mechanical and electrical stability under complex 3D deformations, including bending, twisting, and stretching. The PSC region strain of the sensor is only ≈2% when the global strain is up to ≈65%, which exhibited effective strain shielding capabilities.


Introduction
Stretchable electronics achieve the electrical performance of traditional electronics and exhibit complex 3D (three dimensional) deformation capabilities, including bending, twisting, and stretching.[22] However, integrating rigid electronic components onto stretchable substrates is challenging because of low compatibility.Large deformations result in different scale strains between the rigid and stretchable components, leading to mismatch and detachment.These issues can cause interconnect failures and deteriorate the overall functionality of the device.Therefore, strain isolation with effective strain shielding ability is crucial to developing stretchable electronics.25][26] This method utilizes the substantial difference in mechanical rigidity between soft and hard materials to achieve good strain isolation.However, because the hard material is implanted instead of being grown naturally, the matching at the soft-hard interface is hindered, resulting in system instability.
Recently, rigidity-programming strategies for stretchable electronics have been proposed to address the aforementioned challenges.These approaches utilize hybrids of hard and soft materials with contrasting properties and programmable stiffness.For example, Lee et al. introduced inorganic components on surface relief patterns instead of flat surfaces. [27,28]Xu et al. incorporated liquid-filled cavities into the soft substrate, [29,30] while a rigidity-programmable substrate attained via UV exposure in specific regions was reported Cai et al. [31,32] Another study by Cao showed a strain-isolating polymer achieved through partial oxidation. [33]These strain isolation strategies do not necessitate additional structural modifications to the components of stretchable electronic devices, and they do not induce inherent deformations or distortions in the device's starting condition.Notably, strain is predominantly supported by the substrate when the device is stretched, highlighting the importance of strain isolation design.[33] In addition, the process of preparing rigidity-programmable substrates is intricate.For instance, the liquid-filling method demands the creation of complex microfluidic channels. [29,30]Therefore, while these rigidity-programming strategies offer innovative ways to tailor the mechanical properties of stretchable substrates for electronics, challenges persist.They include intricate optimization, complex manufacturing, and potentially increased manufacturing complexity with the growth in electronic density.Further advancements and research are needed to streamline these strategies for practical application in the field of stretchable electronics.
In light of the need for the simplified preparation of rigidityprogrammable substrates and the growing complexity of circuits, we developed a simple hybrid 3D printing technology, achieving integrated printing of rigidity-programmable substrates and LM (liquid metal)-based 3D interconnected circuits.The rigidityprogrammable substrates with effective strain shielding capabilities were printed by the use of PSC (polydimethylsiloxane/silicananoparticles composite)-based ink.LM-based 3D circuits were printed by the use of LM-based ink based on the low-temperature LM-based 3D printing technology described in our previous work. [34]The formation of the LM-based 3D cross-bridge circuit is of significance for the high-density integrated development of stretchable electronic devices.After being integrated with electronic chips, a stretchable pulse sensor with effective strain shielding capabilities was developed.

Principle of Hybrid 3D Printing
Hydrophobic fumed silica-nanoparticles are a kind of silica, which have hydrophobic groups (alkyl groups or polydimethylsiloxane (PDMS) chains) chemically bonded on their surface.[37][38][39] Subsequently, the viscosity of the silicone ink is significantly increased making it printable.Meanwhile, the modulus of the solid silicone is increased.Hence, hydrophobic fumed silica nanoparticles were incorporated into PDMS to enhance its printability and mechanical characteristics.Hence, hydrophobic fumed silica nanoparticles were incorporated into PDMS, a silicone rubber, to enhance its printability and mechanical characteristics.This modified PDMS can serve as the basis for the PSC-based ink for printing the rigidityprogrammable substrate.The rigidity-programmable substrate consists of two components: one is pure silicone with a lower modulus, and the other is modified silicone with a higher modulus.
Furthermore, EGaIn is an eutectic alloy of Gallium (Ga) and Indium (In) known for its good properties, including a low melting point (15.7 °C), high electrical conductivity (3.4 × 106 S m −1 ), and ultralow vapor pressure. [18,19]The viscosity and plasticity of LM are high at low temperatures.Additionally, when exposed to air, LM spontaneously forms an oxide layer, which envelops its surface.Owing to these characteristics, LM microdroplet ink can be consistently extruded from a nozzle to create LM-based 3D circuits at low temperatures. [34]y integrating the printing of the rigidity-programmable substrate and LM-based 3D circuits, we employ a simple hybrid 3D printing technology to fabricate stretchable pulse sensors by the use of a 3D bio-printer, which comprises electronic chips, a rigidity-programmable substrate/encapsulation layer printed by using PSC (polydimethylsiloxane/silica-nanoparticles composite)-based ink, and LM (liquid-metal)-based 3D circuits by using LM-based ink.The basic principle of hybrid 3D printing technology is depicted in Figure 1.PDMS, PSC, and LM microdroplet ink serve as the ink materials.First, the rigidityprogrammable substrate was printed by the use of the PDMS and PSC ink.As shown in Figure 1a, the rigidity-programmable substrate consists of the transparent PDMS-20 (PDMS with a weight ratio of 20:1 (silicone base to curing agent)) and the colored PSC (PDMS-05 (PDMS with a weight ratio of 20:1 (silicone base to curing agent)) with 25 wt% (weight percentage) silica-nanoparticles).Once cured, LM microdroplet ink is extruded from the nozzle to construct LM-based circuits with a 3D cross-bridge structure, which are constructed on the transparent PDMS-20 region with a smaller modulus in the cold well.Commercial electronic chips are positioned within the colored PSC region with a larger modulus, as illustrated in Figure 1b.The PDMS-20 region has a smaller tensile fracture strength (≈0.6 MPa) applied with a strain of ≈240% and the PSC region has a larger tensile fracture strength (≈7 MPa) applied with a strain of ≈100% as shown Figures 1a,  and 2a, and Figure S3 (Supporting Information).As a result, LMbased 3D circuits and electronic chips are encapsulated using PDMS-20 and PSC-based ink, respectively and the surface of the packaged electronic device is smooth and flat.

Optimization of PSC-Based Ink
As previously mentioned, adding silica-nanoparticles to pure PDMS improves its printability and mechanical properties.In this section, we explore the viscosity characteristics of PSC with varying wt% of silica-nanoparticles, ranging from 0 to 30 wt%, to study its rheological properties (refer to Figure S1a, Supporting Information).When the silica-nanoparticle content reaches 25 wt%, the onset of yielding becomes evident due to shear yield stress. [40]At shear rates higher than 0.2 s −1 , the viscosity decreases with increasing shear rate, indicating non-Newtonian behavior due to long silica−silicone−silica chains in its structure, resulting in shear-thinning behavior at high shear rates. [41]Due to the high viscosity (>3000 Pa s) of the PSC-based ink with 30 wt% of silica-nanoparticles, it can't be extruded from the nozzle (nozzle diameter: 510 μm) to construct PSC patterns even applied with a high pneumatic nozzle pressure of 0.6 MPa (pressure upper limit of the 3D bio-printer: 0.6 MPa).In order to further evaluate the printing capability of the PSC-based ink, PSC lines with a width of ≈800 μm are printed by using different PSC-based inks (PDMS incorporating 5-25 wt% silica-nanoparticles) as shown in Figure S1b,c (Supporting Information).As the wt% of silica-nanoparticles decreases, the ink spreads more easily from variations of PDMS with weight ratios of 5:1, 10:1, 15:1, and 20:1 (silicone base to curing agent), respectively.Based on this analysis, PDMS incorporating 25 wt% silica-nanoparticles demonstrates good printing capability and is a suitable choice for PSCbased ink. Figure 2a  Conversely, the maximum strain increased from ≈140% to 180% with decreasing curing agent ratio.However, compared with uniform substrates described in Figure 2a, their maximum strains decreased proportionally.This is because the strain in Part B changes minimally as the global strain of the PSC-based rigidityprogrammable substrate increases.Specifically, the strain in Part B (PDMS-05 with 25 wt% silica-nanoparticles) only increases from ≈1% to 7% when the global strain ranges from ≈25% to 125%, as shown in Figure 2c.As seen in Figure 2d, the rate of area change in Part B (PDMS-05 with 25 wt% silica-nanoparticles) only increases from ≈1% to 2% as the global area change rate increases from ≈30% to 110%.In light of these discussions, PSC with 25 wt% silica-nanoparticles is identified as a suitable PSC-based ink, while PDMS-20 is chosen as the PDMSbased ink.These materials are used for printing the PDMS region with a smaller tensile fracture strength (≈0.6 MPa) and the PSC region with a larger tensile fracture strength (≈7 MPa), as depicted in Figures 1 and 2a, and Figure S3 (Supporting Information).

Hybrid 3D Printing of Rigidity-Programmable Substrate
In order to quantitatively analyze the printability of the rigidityprogrammable substrate, we investigated how various factors, including nozzle diameter (D), printing speed (v), pneumatic nozzle pressure (p), and the distance (d) between the nozzle and substrate, affect the resolution of the PSC line.The applied pressure propelled the PSC-based ink through the nozzle, ensuring its equilibrium state.The extruded PSC made contact with the substrate, it adhered to the surface.When other parameters remained constant, the flow rate of the extruded PSC-based ink per unit of path length decreased as v increased, resulting in a narrower PSC line (as shown in Figure 3b).The nozzle diameter, D, also affected the accuracy of the printed PSC line.Nozzles with D ranging from 210 to 510 μm were used.Figure 3c illustrates the width of the PSC line constructed with different nozzle diameters.The width of the constructed PSC line decreased as D decreased.After optimization, the width of the PSC line is ≈150 μm with printing parameters (D: 210 μm, d: 0.05 mm, p: 0.1 MPa, and v: 6 mm −1 s).The width of LM-based 3D circuits can be up to ≈50 μm by using low-temperature LMbased 3D printing technology described in previous work. [34]Although the printing resolution of rigid programmable substrates is sufficient for LM-based 3D circuits, as the complexity of future circuits continues to increase, its printing resolution needs to be further improved.As shown Figure 3a−c, the printing resolution of rigid programmable substrates can be improved by decreasing D, p, or d.The width of the PSC line can be up to ≈90 μm when the D is 160 μm, the p is 0.1 MPa, the v is 6 mm −1 s and the d is 0.05 mm.When the nozzle diameter (D) is smaller than 160 μm, the PSC-based ink can't be extruded from the nozzle even when applied with a high pneumatic nozzle pressure of 0.6 MPa (pressure upper limit of the 3D bio-printer: 0.6 MPa) due to the high viscosity.
To validate the suitability of the hybrid 3D printing technique for stretchable electronics, we demonstrated PSC-based rigidity-programmable substrates with complex patterns, as shown in Figure 3d−g.Figure 3h,i depict the deformation of the rigidity-programmable substrate, consisting of the transparent part A (PDMS-20) and the colored part B (part B-1: PDMS-05 with 25 wt% silica-nanoparticles, part B-2: PDMS-10 with 25 wt% silica-nanoparticles, part B-3: PDMS-15 with 25 wt% silica-nanoparticles and part B-4: PDMS-20 with 25 wt% silicananoparticles), under a strain of ≈50%.Notably, there was minimal change in the shape of the part B-1 region.Furthermore, LM-based 3D circuits were constructed in the PDMS region, and LEDs were integrated into the PSC region.
Based on the manufacturing and structure of LM-based 3D circuits embedded in PDMS, the stretchability is ≈120%. [43]hen the strain exceeds 120%, the LM-based 3D circuit undergoes sudden fracture.The stretchability of the LM-based 3D circuit primarily depends on the substrate material.However, it can be enhanced by further optimizing the manufacturing process and utilizing different polymer substrates, such as Ecoflex, which can achieve a stretchability of ≈380%. [10]The relative resistance change in the LM-based 3D circuit varied with different widths undergoing stretching strain and increased with the increase in strain. [34]After encapsulation with PDMS-based ink and PSC-based ink, the stretchable LED array functioned effectively with 3D deformations (twisting: ≈180°, bending: ≈360°a nd stretching: ≈100%.To further verify the reliability of rigid programmable substrates, the strain shielding capabilities of PSC substrates integrated with LEDs after stretching were characterized, as shown in Figure S4 (Supporting Information).The LED-intergraded PDMS couldn't work applied with a strain of ≈80%.When a global strain of ≈100% was applied, the PDMS substrate broke where the integrated LED was located due to the stress concentration phenomenon.On the other hand, the Figure 3. Hybrid 3D printing of rigidity-programmable substrate.a) PSC line width as a function of pressure for various distances between nozzle and substrate with a printing speed of 6 mm −1 s and a nozzle diameter of 210 μm.b) PSC line width as a function of pressure for various printing speeds when the distance between the nozzle and substrate is 0.05 mm and the nozzle diameter is 210 μm.c) PSC line width as a function of pressure for various nozzle diameters when the distance between the nozzle and substrate is 0.05 mm and the printing speed is 6 mm −1 s. d−g) PSC-based rigidityprogrammable substrates with various complex patterns.h,i) PSC-based rigidity-programmable substrate before/after applying strain.j,k) Stretchable light composed of PSC-based rigidity-programmable substrate, LM-based 3D circuits, and LEDs with bending, twisting, and stretching deformation.PSC, polydimethylsiloxane/silica-nanoparticles composite; LM, liquid metal.
LED-integrated PSC still worked very well even when the global strain of ≈100% was applied.The maximum global strain can be up to ≈160%.The local strain in Part B (LED-integrated PDMS-05 with 25 wt% silica-nanoparticles) only increases from ≈0% to 4% when the global strain ranges from ≈0% to 100%.[33] Additionally, under a strain of ≈60%, the LM-based 3D circuit integrated with LEDs maintains stable electrical properties after undergoing 500 repeated stretches. [34]his stability is attributed to the minimal change in the total resistance value in LED-integrated stretchable circuits. [43]

Stretchable Pulse Sensor
Based on photoplethysmography (PPG), various stretchable pulse sensors have been developed in previous studies. [10,42]owever, the reliability, integrity, and preparation methods of these reported stretchable pulse sensors have been limited.In response, we have developed a highly integrated stretchable pulse sensor with effective strain shielding capabilities, making it suitable for comfortable patient monitoring in personal healthcare settings.This portable and non-invasive sensor comprises a photoelectric converter and a light source (LED) and operates as a reflection-type photoelectric sensor.The schematic diagram of this sensor has been previously described in our previous work. [42]The 3D fabrication process of the stretchable pulse sensor is illustrated in Figure 4a-c, while optical images of the fabricated sensor are presented in Figure 4d.When a 2.8 V DC power supply is applied, the LED emits light, and the stretchable pulse sensor performs effectively even under extreme deformation conditions (strain: ≈65%), as demonstrated in Figure 4d−g and Video S1 (Supporting Information).Compared with the PDMS region (encapsulating LM-based 3D circuits) of the rigidityprogrammable substrate, the colored PSC region (encapsulating electronic chips with high integration) exhibits minimal changes in shape.The local strain of the colored PSC in the stretchable pulse sensor system is only ≈2%.Electronic chips used in commercial applications are typically manufactured from silicon, known for its high Young's modulus.The ≈2% strain of the PSC with low Young's modulus in the stretchable pulse sensor system has negligible impact on the functionality of these chips located in the PSC region.Compared with chip-integrated PDMS, the local strain of the chip-integrated PSC is nearly negligible, which possesses good strain shielding capability as shown in Figure S4 (Supporting Information).This underscores the stretchable pulse sensor's remarkable strain shielding capabilities, attributed to the significantly higher tensile fracture strength of the PSC region compared to the PDMS region.Upon placing the fabricated sensor on human skin, such as the finger, the electrical signal obtained from the sensor is transformed into a digital format using an Arduino board.Subsequently, this digital signal is transmitted to a computer with a custom-made graphical user interface, displaying the results (Video S2, Supporting Information).Experimental outcomes for the stretchable pulse sensor attached to a woman's and a man's finger are depicted in Figure 4h,i.Notably, the pulse period increases with exercise intensity.The attached pulse sensor demonstrates consistent performance even under various mechanical stimulations, such as bending (180°, 1000 times), twisting (180°, 1000 times), and stretching (50%, 1000 times), as shown in Figure 4j.In addition, it also demonstrates stable measurement results compared with the commercial pulse sensor as shown in Figure S5 (Supporting Information).In previous works, the maximum stretch rate of the stretchable sensor is less than 30%. [10,42]Here, the maximum strain of the stretchable sensor system fabricated using the hybrid printing technology is increased to ≈65%, while showing stable electrical properties and good mechanical properties.

Conclusion
In this study, we established a straightforward hybrid 3D printing method for creating a highly integrated stretchable pulse sensor with effective strain shielding capabilities.This sensor comprises a rigidity-programmable substrate, LM-based 3D circuits, and chips.Initially, we optimized and prepared the PSC-based ink.Subsequently, we systematically examined the impact of various parameters (p, D, v, and d) on the resolution of the PSC line.To validate its practicality, we demonstrated various rigidityprogrammable substrates integrated with LM-based 3D circuits and the LED array.These components operated well even under substantial deformations (strain: 100%).Finally, by combining our proposed printing technique with the principles of PPG, we developed a non-invasive stretchable pulse sensor for personal pulse monitoring.Remarkably, the stretchable pulse sensor, equipped with effective strain shielding capabilities, exhibited remarkable stability with complex 3D deformations (bending, twisting, and stretching).Our findings demonstrate that hybrid 3D printing technology offers a convenient and comfortable means of producing highly integrated stretchable electronics with effective strain shielding capabilities.This technology holds immense potential for enhancing the electrical and mechanical performance of wearable electronic devices.

Experimental Section
Preparation of the PSC-Based Ink: Polydimethylsiloxane (PDMS) is a type of silicone, which was formed with specific mass ratios of silicone base and curing agents.After mixing, 25 wt% silica-nanoparticles (average particle size: 20 nm) were added to the PDMS solution.Following stirring and refluxing for 6 h, the PSC-based ink was prepared at room temperature.The type of PSC-based ink depended on the mass ratio of the silicone base and curing agents.
Preparation of LM μ-Droplets Ink: First, EGaIn (5 g), which is a commercially available LM consisting of 75.5 wt% Ga and 24.5 wt% In., and alcohol (5 mL, 95%) were mixed in a glass bottle.The probe of the ultrasonic processor (FS-900N) was inserted into the alcohol.With a power output of 50 W for 1 min, some of the EGaIn was transformed into LM microdroplets.LM microdroplets were mixed with residual EGaIn to create an ink of LM microdroplets when the alcohol solution had vaporized.Further details can be found in the prior work. [34]haracterization of Rheology and Mechanical Properties: The viscosity of the PSC-based ink was determined using a plate rheometer (Anton Paar MCR302, Anton Fear, Austria).The tensile strength of the rigidityprogrammable substrate was measured using a tensile machine (AGS-X, Shimadzu, Japan).
Fabrication of Stretchable Pulse Sensor: The fabrication process of the stretchable pulse sensor is illustrated in Figure 4c.In this study, a 3D bioprinter (Architect Sparrow, Hangzhou Genofiber Biotechnology Co., Ltd.) was employed to construct the rigidity-programmable substrate and LMbased 3D circuits.The PSC-based ink extruded from the nozzle in the PSC region creates the PSC part of the rigidity-programmable substrate within a square dish.The parameters used were a nozzle diameter (D) of 210 μm, a pressure (p) of 0.04 MPa, a printing speed (v) of 6 mm −1 s, and a distance between the nozzle and substrate (d) of 50 μm.Subsequently, the PDMS-based ink was printed in the PDMS region to form the PDMS part, using parameters of D (210 μm), p (0.1 MPa), v (6 mm −1 s), and d (50 μm).The printed rigidity-programmable substrate was cured in a cold well at 60 °C for 5 h.After curing, LM-based 3D circuits were constructed on the PSC part surface using the following parameters: T (−5 °C), D (410 μm), p (0.001 MPa), v (0.1 mm −1 s), and d (200 μm).LM-based 3D circuits were frozen at −25 °C after printing.Due to the lower temperature, LM-based 3D circuits could be maintained in solid state. [34]Using the photoplethysmography principle, LM-based 3D circuits were integrated with electronic chips attached to the surface of the PSC part.These included an operational amplifier chip (MCP6001), an optical receiver (APDS-9008), a green LED, diodes, capacitors, and resistors.LM droplet showed wettability on the solid metal substrate. [42,43]Due to that the pins of electronic chips were made of copper, they were easily integrated with LM-based 3D circuits without additional welding.After integration, the circuits and electronic chips were encapsulated with PDMS-and PSC-based ink respectively (−25 °C, 5 h).With the good fluidity of the PDMS and PSC solution, an effective partition was formed between the raised LM-based circuit part and the lower LM-based circuit.After a 2-h wait, the sample was cured in the cold well (60 °C, 5 h) and formed the stretchable pulse sensor.
presents stress-strain curves for different types of PDMS (PDMS-05, PDMS-10, PDMS-15, and PDMS-20) with 25 wt% silica-nanoparticles.The maximum strain decreases from ≈225% to ≈100%, while the stress increases from ≈3 to ≈7 MPa with an increasing curing agent ratio.Compared to pure PDMS, PSC exhibits increased maximum stress.For instance, PDMS-20 with 25 wt% silica-nanoparticles shows a rise in maximum stress from ≈0.6 to ≈3 MPa compared to the PDMS-20 formulation (refer to Figure S3 Supporting Information).To further optimize the PSC-based ink, we designed and printed simple rigidity-programmable substrates composed of Part A (PDMS-20) and Part B (PSC) to analyze their mechanical properties, as depicted in Figure 2b.The maximum stress remained nearly constant at ≈0.6 MPa because Part A of the rigidity-programmable substrate underwent fracture when the stress reached ≈0.6 MPa.
Figure 3a-c depicts the variation in the width of the printed PSC line with changes in p for various d, v, and D. When the v is 6 mm −1 s and D is 210 μm, the width of the constructed PSC line increased as d increased.Compared with a higher d (125 μm), a smaller d (50 μm) resulted in a smaller cross-sectional area of the printed PSC line relative to d/D.Decreasing d caused the extruded PSC to stack up, influencing the flow rate of the PSC-based ink.When the d is increased, the resistance of the extruded PSC-based ink is reduced.The ink gradually spreads from both sides of the PSC line dur-ing printing.Consequently, the PSC line width exceeded the D.