Highly Stretchable, Sensitive, and Transparent Strain Sensors with a

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Highly Stretchable, Sensitive, and Transparent Strain Sensors with a Controllable In-Plane Mesh Structure Zhihui Wang, Ling Zhang,* Jin Liu, and Chunzhong Li* School of Materials Science and Engineering, Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, China

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S Supporting Information *

ABSTRACT: Highly stretchable and transparent strain sensors integrated in-plane meshed structure with silver nanowire conductive networks are rationally designed and prepared by the ultrafast laser cutting technique and facile drop-coating process, which exhibit great promise for efficient large-scale production. The in-plane meshed structure and low-content conductive network enable the sensor to achieve outstanding sensing performance and optical transparency based on simple fabricating processes and simplified components. More impressively, finite element method simulation is used to analyze and predict the influence of multiple structures on properties for selecting optimal meshed structures. Consequently, the strain sensor with optimal meshed structures exhibits high sensitivity in a wide working strain range (gauge factor of 846 at 150% tensile strain), good optical transparency of 88.3%, slight hysteresis, and long-term durability under large-strain cycles. Because of these superior performances, this in-plane mesh-structured strain sensor has great potential as candidates for wearable electronics, especially in skin-mountable devices detecting both subtle physiological signals and large motion information. KEYWORDS: strain sensor, transparent sensor, meshed structure, silver nanowires, elastic composites

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INTRODUCTION Stretchable strain sensors that can convert mechanical deformation stimulus into quantitative electrical signals are attracting considerable interest and used in the applications of human-motion monitoring,1−3 personal physiological detection,4−6 soft robotics,7 and human−mechanical interaction.8,9 One of the crucial characteristic parameters for strain sensors is high sensitivity [i.e., gauge factor (GF)] to provide precise perception capability and high-quality signal output. It is also vital to achieve large stretchability and stability which can build a stable conformal contact with any irregular surfaces and moveable parts. With increasing demands of functional, portable, and wearable devices, there is a vigorous and urgent need to improve the sensitivity of strain sensors under large tensile deformations on the premise of ensuring circuit electrical conductivity. To balance the sensitivity and stretchability, strain sensors are generally composed of active materials and elastic polymer substrates. Owing to the outstanding features including intrinsic high transparency, elasticity, good stability, and biocompatibility, polydimethylsiloxane (PDMS) has been widely used to provide stretchable support for composite films.10,11 As for the conductive materials, several nanomaterials, such as carbon nanotubes,12,13 graphene, and metal nanowires,14−16 have been widely utilized in fabricating stretchable strain sensors. Among them, silver nanowires © XXXX American Chemical Society

(AgNW) have been proposed as a promising candidate for flexible devices because of their excellent conductivity and optical transparency.17−19 Ko et al. reported a highly conductive and stretchable AgNW electrode with low resistance and large stretchability over 460%.20 In addition, Amjadi et al. reported a AgNW-based strain sensor with tunable GFs from 2 to 14.21 It is noteworthy that the AgNW concentration greatly decides the research direction for flexible AgNW-based electronics. High concentration can promote the formation of a dense percolation network to develop stretchable conductors maintaining stable conductivity under deformations, whereas low concentration might render local detachment of the conductive network under the tensile state which yields sensing signals. Notably, the low-density percolation network provides a remarkable electrical response to small strains, but it fails to sustain the circuit conductivity under large tensile strains which severely limit the stretchability and working range of the AgNW-based strain sensor. As a feasible and efficient approach, the structure design has been widely utilized to significantly enhance stretchability of flexible electronics including electrodes and sensors.22−27 A suitable structure can effectively alleviate abrupt or even Received: October 7, 2018 Accepted: December 26, 2018 Published: December 26, 2018 A

DOI: 10.1021/acsami.8b17459 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Three types of omni-stretchable and in-plane structures. Stress distribution of three meshed models in the tensile process by finite modeling analysis simulation: (b) hexagon-arrayed mesh structure, (c) hexagon-r-arrayed mesh structure, and (d) circle-arrayed mesh structure; scale bar is 2 mm.

whereas PDMS films serve as the support of a strain sensor to improve the composite flexibility. Meanwhile, several in-plane mesh structures with discernable features are prepared through a facile and programmable laser cutting strategy and further theoretically analyzed by finite element method (FEM) simulation, demonstrating that the optical mesh structure can significantly enhance the mechanical performance of the strain sensor. Consequently, our strain sensors exhibit ultrahigh sensitivity with a GF of 846 at 150% strain, high transparency of 88.3% at 550 nm, and large sensing range from 0 to 150% strain. Moreover, negligible hysteresis even in large tensile strain (100%), fast response behavior, and stable cyclic performance are revealed. On the basis of these superior properties, the transparent strain sensor is successfully applied in detecting both subtle physiology signals and the movement of large joints, demonstrating its promising potential in the development of inversible and intelligent wearable devices.

irreversible decline of electrical conductivity when suffering large deformations. Dong et al. developed a macroporous structure to endow the strain sensor a high GF of 35 and wide sensing range up to 85%.28 Kim et al. fabricated a microprismstructured sensor based on metal nanowire/elastomer composites, which presented high sensitivity (GF ≈ 81) at large strain even over 130%.29 This indicated that suitable structures can markedly improve the performance of strain sensors. Nevertheless, most special structures are produced through sophisticated, time-consuming, size-confined, or high temperature processes, such as lithography and chemical vapor deposition. It is urgent to explore a facile and scalable strategy for structuring AgNW-based strain sensors. Currently, various electronics have been progressed into a transparent and stretchable form, including displays,30 heaters,31 electromagnetic shielding,32 PM filters,33 supercapacitors,34 and solar cells,35 and the strain sensor is no exception.36 As a crucial element of wearable devices, superior optical transparency can improve the sensor appearance when attached on visible parts of the human body, such as neck and limbs. Meanwhile, it is preferred to be transparent for integration with other optoelectronic devices. However, good transparency requires strict control in content of opaque components, that is, the AgNW content for AgNW-based sensors, which has an adverse effect on its stretchability. On the other side, despite that out-of-plane structures (such as porous and waved) can considerably enhance the stretchability of strain sensors and light scattering is also generated, tremendously reducing the sensor transmittance.37−40 Therefore, developing a strain sensor which achieves highly sensitivity, transparency, and stretchability simultaneously through integrating the rational structure with proper concentration of an active material still remains challenging. Herein, we successfully fabricate a strain sensor, which synchronously possesses superior stretchability, sensitivity, and transparency, through designing an in-plane meshed structure and simplifying active components. Considering the desired properties, AgNW networks are employed as active components to provide sufficient conductivity and sensing response,

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RESULTS AND DISCUSSION Because the stretchability of composite films primarily depends on the PDMS substrate, designing a stretchable substrate film with a suitable structure is essential for the preparation of strain sensors. As depicted in Figure 1a, we designed three types of omni-stretchable and in-plane structures: hexagon, hexagon with round angles (hexagon-ra), and circular mesh structures. Pore diameter (D) and mesh width (s) are key geometric parameters for those meshed structures. To theoretically analyze and predict the effects of those meshed structures on tensile properties, FEM simulation was performed to reveal the real-time stress distribution in the stretching process. Notably, the actual stress of meshed units along the tensile direction gradually increases as the applied strain increases, whereas the meshed units perpendicular to the tensile direction undergo lower loading. Thus, the ultimate strength and elongation of whole meshed films mainly rely on the stress distribution of segments in the tensile direction. For these meshed structures, the difference of their modeling simulations lies in maximum effect stress and distribution uniformity. As shown in Figure 1d, the circle-meshed structure B


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Figure 2. (a) Schematic illustration of the fabrication process of stretchable transparent strain sensor based on the meshed structure and AgNW networks; (b) photographs of the strain sensor under different tensile strains and other deformations (bending, twisting), as well as attached on the human skin; and (c) top-view FE-SEM image of the AgNW percolation network on elastic PDMS substrate, and the scale bar is 1 μm.

Figure 3. (a) Normalized resistance variation of mesh-structured strain sensors with different AgNW contents; (b) the GF in the stretching process; (c) transmittance of the mesh-structured strain sensor in the visible wavelength range from 400 to 800 nm; and (d) transmittance of strain sensors with various AgNW contents and the corresponding sheet resistance. (e) The current−voltage (I−V) curve of strain sensor and (f) hysteresis curve during multiple loading−unloading cycles.

exhibits uniform stress distribution in a wide tensile range of 50−200% and high strength up to ∼5 MPa. On the contrary, the other two structures reveal abrupt stress concentration in the corner regions, which turns into defects inducing fracture under certain loading. The hexagon-mesh structure shows a maximum stress about 1.5 MPa around the sharp angle edge at 100% strain, and the hexagon with the round angle structure presents a maximum stress less than 3 MPa under 175% tensile strain (Figure 1b,c). It is suggested that the meshed structures

with corners show more defects and nonuniformity compared to the meshed structure with smooth edges, resulting in inferior strength and limited stretchability. To confirm theoretic analysis results, mechanical properties of these structures with the same structural parameters were measured and plotted as stress−strain curves (Figure S1, Supporting Information). As a consequence, meshed structure with circle pores exhibits the highest tensile strength and ultimate elongation thanks to the smooth edges buffering C


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Figure 4. (a,b) Static response behaviors under small strains and large strains; (c,d) dynamic response behaviors for various strains at 0.5 and 2 Hz frequency, respectively; (e) 1000 loading−unloading cycles from 0 to 100% strain at a frequency of 0.5 Hz and (f) the magnified resistance curve during loading cycles.

against the stress concentration. Conversely, for hexagon and hexagon-ra structures, salient angles spontaneously turn into a singular point, which will induce stress concentration on them, thereby rendering limited stretchability. Afterward, the relationship between the mechanical properties and geometric parameters in circular structures is explored by testing the yield stress and yield strain of samples with different parameters (Figure S1, Supporting Information). Too large diameters or mesh widths render the film more rigidity and impart lower extensibility. In contrast, too small parameters weaken the material integrity and cause instability in measurement. After overall consideration, the circular meshed structure with optimal parameter (D = 1 mm, s = 0.35 mm) has been adopted for subsequent strain sensor preparation and investigation. Comparied to the flat PDMS substrate, the mesh-structured substrate presents higher elongation which further confirms the beneficial effect of the in-plane mesh structure on substrate stretchability (Figure S2, Supporting Information). Figure 2a illustrates the fabrication procedure of the transparent strain sensor with a meshed structure. Briefly, the PDMS prepolymer was spin-coated as a uniform thin film with a thickness of ∼100 μm and patterned into a certain meshed structure by the programmable laser scribing process. Next, the PDMS film was treated by oxygen plasma to obtain the

hydrophilic surface, followed by the deposition of AgNWs using the drop-coating method. Finally, the conductive wires were connected on the surface of AgNW/PDMS films for signal transmission. In addition, the diluted PDMS solution was drop-coated on the surface of AgNWs to form a meshstructured encapsulating layer, improving the sensor reversibility and retaining the mesh structure. Field-emission scanning electron microscopy (FE-SEM) measurements were used to observe the micromorphology of the AgNW network. As shown in Figure 2c, AgNWs distribute uniformly on a twostep-treated PDMS substrate. First, ultrasonic cleaning was conducted on the patterned substrate to remove impurities produced in the laser process. Then, oxygen plasma was used for hydrophily improvement in favor of AgNW uniform distribution. In contrast, untreated or single-treated substrates give rise to the impurity of the substrate surface and aggregation of AgNWs (Figure S3, Supporting Information). Hence, interconnected AgNW networks integrated with the inplane mesh-structured substrate facilitate the as-prepared transparent strain sensor to accommodate various deformations, including large stretching, bending, and twisting deformations, and maintain a conformal contact with the human skin, with promising potential for wearable device applications (Figure 2b). D


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Figure 5. (a) The morphology changes of AgNW percolation networks based on the mesh-structured substrate under different tensile strains and (b) schematic diagram of the sensing mechanism relying on the in-plane mesh structure and micro conductive networks.

Subsequently, the electrical behavior of the fabricated strain sensor and the influence of the active material content on sensing performance are investigated. As shown in Figure 3a, the normalized resistance changes (ΔR/R0) of the meshstructured strain sensors with different AgNW contents are plotted as the function of the tensile strain. The sensitivity of

structured strain sensors at 550 nm decreases from 93.2 to 71.7% and the electrical resistance decreases from 58.3 to 3.63 Ω/sq (Figure 3c,d). Notably, the mesh-structured strain sensor with 1.4 wt % AgNW exhibits a high transmittance of 88.3% at 550 nm comparable to that of lower content composites, as well as a low sheet resistance of 14.9 Ω/sq, providing sufficient electrical conductivity for strain sensor operation. By contrast, the flat strain sensor (1.4 wt % AgNW content) without the microstructure just shows 78.7% transmittance, much less than that of mesh-structured sensor, indicating that the in-plane mesh structure greatly promotes the optical transparency without breaking the conductive path (Figure S5, Supporting Information). Hence, further research is focused on the meshstructured sensor containing 1.4 wt % AgNW active components by virtue of its integration outstanding sensing performance with high optical transmittance. To investigate the reversibility and reproducibility of this mesh-structured strain sensor, repeated static and dynamic loading tests are carried out in a wide tensile range. As shown in Figures 3e and S6, the current−voltage (I−V) curves of the strain sensor under different tensile strains present good linear Ohmic characteristics, indicating the stable response behavior of our strain sensor to static deformation. In addition, the meshstructured strain sensor exhibits negligible hysteresis during stretching−releasing cycles even in large tensile strains (Figure 3f). In contrast, meshed strain sensor without encapsulation reveals obvious hysteresis behavior (Figure S7, Supporting Information), indicating major contribution of the encapsulating layer to the reversibility and stability in both low and high tensile deformations. Considering the robustness of the composites for practical applications, fast response behaviors and the durability of mesh-structured strain sensor are also evaluated by repeated loading tests in a wide detection range. As shown in Figure 4a,b, the resistance signal relies on applied strains, enabling the strain sensor to precisely distinguish various external stimulus no matter small stimulus or large tensile strain. Moreover, the response behaviors under different tensile strains are explored by carrying out multiple tensile-released tests. As shown in Figure 4c, the mesh-structured strain sensor presents reliable

(R − R ) / R

0 0 , the strain senor, that is, GF, is defined as: GF = ε where R0 and R, respectively, represent the initial resistance and the real-time resistance change, and ε represents the tensile strain. The GF values are further calculated from ΔR/ R0−ε curve slopes and plotted as corresponding curves, as shown in Figure 3b. Thereinto, the strain sensor containing 1.4 wt % AgNW components exhibits outstanding resistance change of 1260% under high sensing strain of 150%. In addition, its maximum GF can reach 846 at 150% strain which is superior than the sensing performance of strain sensors with other concentrations. Lower AgNW content may cause easier fracture of percolation network under tensile strains, greatly increasing ΔR. However, it also renders high initial resistance (R0), even several magnitudes than that of the sensor with higher content, which ultimately induces a small ratio of ΔR/ R0 and low sensitivity (Table S1, Supporting Information). For higher AgNW content, a dense and stable conductive pathway can be formed, then reducing resistance changes of the strain sensor during the stretching process. It is indicated that selecting a suitable active component content is essential for enhancing the sensing range and sensitivity of the strain sensor. Impressively, the unstructured sensor with the same AgNW content just can operate in very low strain (35%) (Figure S4, Supporting Information), which suggested that the in-plane structure greatly improves the sensing range as well as retains the high sensitivity of the strain sensor, thus markedly widening its practicability and applications in monitoring large deformations. To further confirm the advantage of our strain sensor in optoelectronic fields, the transmittance and electrical conductivity of mesh-structured strain sensors with different AgNW contents are also measured. As the AgNW content increases from 0.7 to 3.3 wt %, the transmittance of mesh-

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Figure 6. Time-dependent resistance variation of the transparent strain sensors conformally attached to different parts of the human body: (a,b) attached onto the chest for monitoring coughing and deep breath signals; (c) attached to the neck to detect swallowing activity; (d) attached onto the finger; (e) attached on the elbow; and (f) attached onto the knee. Note that resistivity changes are characteristic for specific external stimuli.

and stable response behavior in a wide range of applied strains, which may be attributed to the unique mesh structure and encapsulation process effectively enhancing the reversibility and stability. Meanwhile, the influence of stretching frequency on the sensor is studied as well. Figure 4c,d shows that the resistant output signals are consistent at testing frequencies 0.5 and 2 Hz, suggesting that tensile frequency has ignorable impact on response behaviors. The cyclic stretching−releasing tests are also conducted at a frequency of 0.5 Hz. During 1000 cycles at 100% tensile strain, the sensor exhibits stable sensing performance and excellent durability as shown in Figure 4e,f. All the demonstrated properties including superior reversibility, stability, and durability in a wide detection range facilitate the practical applications of the as-prepared strain sensor in wearable devices and healthcare fields. Except for the outstanding enhancement effect on stretchability, the in-plane mesh structure also positively impacts on percolation conductive networks during the tensile process and ultimately improves the sensing performance of the strain sensor. The sensing mechanism is analyzed through investigating micromorphology transformation of percolation networks during the stretching process from 0 to 125% strain. As shown in Figure 5a, the uniform and random AgNWs are rearranged along the tensile direction and disconnected partly under 25% strain, but the overall conductive network does not generate obvious defects or breakage. When the applied strain

increases to 50%, some tiny cracks are generated in the AgNW network. As the tensile strain increases further, the crack spacings increase and more fine cracks are produced at the same time, while multiple AgNW paths still connect cracks to remain the essential conductivity. Under large strain of 125%, cracks are further widened with few lapped AgNWs, resulting in significant resistance variation. To analyze the origin of microcracks, we observed the micromorphology of PDMS substrates with and without plasma treatment as well as the corresponding morphology of the AgNW network under tensile strain. Despite the plasma exposure indeed induces the crack on the PDMS substrate (Figure S8), the microcrack of the AgNW network on the substrate without plasma treatment is still generated under tensile strains and has a larger crack width compared to that of the AgNW network on plasmatreated PDMS (Figure S9, Supporting Information), indicating that microcracks on AgNW networks are mainly related to external tensile loading and interaction between the active layer and substrate. According to these observed results, the transition mechanism of conductive networks on the meshstructured substrate is illustrated to clarify the in-plane structure effect. As shown in Figure 5b, external loading renders the conductive network cracked locally, which transforms initial coherent paths into narrower and winding conductive paths due to the blocking effect of microcracks, resulting in the resistance change and the enhancement of F



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CONCLUSIONS In summary, a highly sensitive, transparent, and stretchable strain sensor based on in-plane mesh structures as well as uniform AgNW active layers is successfully fabricated through combining the programmable laser cutting strategy and dropcoating process. After selecting optimal structural parameters and active materials content, the as-prepared sensor achieves a low initial sheet resistance of 14.9 Ω/sq, high sensitivity with a GF of 846 at 150% strain, high transmittance of 88.3%, reversible mechanical stretchability, and a wide sensing range from 0 to 150% strain. Meanwhile, the mesh-structured sensor shows excellent reversibility with negligible hysteresis, superior durability, and almost no signal attenuation over 1000 stretching cycles from 0 to 100% strain. For exploring the deformation and sensing mechanism, FEM simulation and dynamic micromorphology observation are applied to analyze real-time stress distribution and conductive path variation during tensile processes. Furthermore, it has been demonstrated that the transparent strain sensor can be applied in monitoring respiration rate, physiological information and large joint motions. The reliable sensing performance in conjunction with facile preparation procedures reveals the promising applications of the strain sensor in inversible and wearable next-generation intelligent devices.

output electrical signals (Figure 3a). Meanwhile, the confined surface area and cambered edge of the mesh structure effectively prevent cracks from extending, thereby retaining overall circuit connected in large tensile strain. Therefore, during the elongation process, the mesh structure transfers sectional external loading into in-plane structural changes, thereby buffering the tensile strain of conductive networks and avoiding dramatical propagation of irreversible cracks. To further comprehend the advantage of the in-plane mesh structure, we observed the micromorphology change of AgNW networks in unstructured composite films. As a result, large cracks, perpendicular to tensile direction, are generated at low strain, which divide the whole conductive network into irregular pieces with few connections. When the strain increases to 30%, cracks widen and increase, seriously blocking the conductive path and finally causing the drastic decline of conductivity (Figure S10, Supporting Information). It demonstrated that the mesh structure can effectively avoid abrupt changes of conductivity and the generation of irreparable cracks, thereby significantly extending the working range of the strain sensor. Attributing to the buffering effect of the mesh structure and interconnected AgNW networks, the strain sensor possesses high stretchability and maintains high sensitivity in a wide deformation range. Considering the sharing effect of the encapsulating layer, optical microscope observation was conducted on the encapsulated strain sensor (Figure S11, Supporting Information). As a result, cracks gradually increase with increasing strains, suggested that the encapsulation does not prevent the crack generation but partly alleviates the widening of cracks. Moreover, the morphology of conductive networks in the mesh-structured sensor during the releasing process from 125 to 0% is also observed. As shown in Figure S12, width of cracks gradually decreases and some small cracks close. When back to the initial state, AgNW networks recover to the uniform and interconnected percolation paths, proving the superior reversibility and stability of the meshstructured strain sensor. The excellent sensing performance and mechanical stretchability allow the strain sensor tremendous potential applications in wearable devices for real-time recognizing and recording human activities. As shown in Figure 6a,b, when the strain sensor is mounted on the trachea, both the deep breathing rate and coughing information can be recorded as distinct signals, indicating its capability of perceiving subtle movement and vibration. The sensor also can be attached on the neck to monitor saliva swallowing motion caused by skin and throat movement (Figure 6c). Notably, the high transmittance greatly improves the aesthetics of the strain sensor especially when attached on the exposed skin. Except for these subtle motions, the strain sensor is able to monitor large motions. Figure 6d shows the step-by-step characteristic signals detected by the sensor affixed on the bending finger. With the increase of the finger bending angle, the sensing signal curves rise steadily. Additionally, the structured sensor also can track and discriminate the motions of the elbow joint and knee joint, manifesting superior recognizing deformation ability and reliable stability. These results demonstrate that our mesh-structured strain sensor possesses high sensitivity in detecting both subtle motions and large activities. The widerange detection ability and transparent appearance have significant meaning in developing human-machine communication, disease diagnosis, and physiological signal monitoring, especially “invisible” components in wearable devices.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Fabrication of the Mesh-Structured PDMS Substrate. PDMS elastomer (Sylgard 184, Dow Corning) and curing agent were mixed uniformly at a ratio of 10:1 and placed into a vacuum oven to exhaust bubbles. Next, the PDMS mixture was poured on a cleaned glass slide and spin-coated at a speed of 900 rpm. After curing at 60 °C for 2 h, the uniform PDMS film was peeled off from the substrate. Then, PDMS films were patterned into designed three types of mesh structures by laser cutting (commercial ultraviolet laser with 3 W power, laser beam diameter 10 μm, cutting speed 50 mm/s). To enhance the adhesion between active materials and the PDMS substrate, the surface of patterned PDMS films were cleaned by the ultrasonic process for 20 min and treated by O2 plasma treatment to hydrophilic state. Preparation of the Strain Sensor. AgNW solution was diluted by isopropanol into certain concentration and drop-coated onto the treated PDMS substrate and placed in 50 °C oven to enable isopropanol evaporated completely. The AgNW contents in composite films are controlled by the concentration of AgNW dispersion and drop-coating times. Copper wires were stuck on two ends of the conductive layer by a silver colloid for electrical signal transmission. When performing tests, the strain sensor was clamped from two ends and connected to measuring devices by copper wires which greatly avoided measurement interference (Figure S13, Supporting Information). Then, the mixture solution of PDMS liquid, curing agent, and vinyl acetate as a ratio of 10:1:5 was dropcoated on the surface of AgNW film, and excess solution flew through the mesh pores, which formed another mesh layer after curing at 60 °C for 2 h. Characterization. The morphology of strain sensors was observed by FE-SEM (Hitachi S-4800). Optical transmittance was measured by the UV−vis spectrophotometer (UV-2600, Shimadzu). The stretch and release curves of the strain sensor were tested and recorded by Mark-10 dynamometer. The digital source meter Keithley 2400 system was used to collect the real-time electrical signal of the strain sensors. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b17459. G


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Mechanical properties, influence of the treatment process, initial resistance, unstructured strain sensor characterization, current−voltage curve, encapsulation effect, and morphology during the releasing process (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 21 64252055 (L.Z.). *E-mail: [email protected] (C.L.) ORCID

Chunzhong Li: 0000-0001-7897-5850 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51673063, 21878092, 21838003), the Shanghai Scientific and Technological Innovation Project (18JC1410500), the Innovation Program of Shanghai Municipal Education Commission, the Fundamental Research Funds for the Central Universities (222201718002).

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REFERENCES

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