Nacreous aramid-mica bulk materials with excellent mechanical properties and environmental stability

Summary

Low density, high strength and toughness, together with good environmental stability are always desirable but hardly to achieve simultaneously for man-made structural materials. Replicating the design motifs of natural nacre clearly provides one promising route to obtain such kind of materials, but fundamental challenges remain. Herein, by choosing aramid nanofibers and mica microplatelets as building blocks, we produce a nacreous aramid-mica bulk material with a favorable combination of low density (∼1.7 g cm⁻³), high strength (∼387 MPa) and toughness (∼14.3 MPa m^1/2), and impressive mechanical stability in some harsh environments, including acid/alkali solutions, strong ultraviolet radiation, boiling water, and liquid nitrogen, standing out from previously reported biomimetic bulk composites. Moreover, the obtained material outperforms other bulk nacre-mimetics and most engineering structural materials in terms of its specific strength (227 MPa/[Mg m⁻³]) and specific toughness (8.4 MPa m^1/2/[Mg m⁻³]), making it a new promising engineering structural material for different technical fields.

Graphical abstract

Highlights

• •Spray, protonation, evaporation, and lamination techniques are combined
• •Aramid nanofibers and mica microplatelets are assembled into nacreous bulk
• •Nacre-inspired design imparts the nacreous bulk with high mechanical performance
• •Nacreous bulk displays favorable mechanical stability in some harsh environments

Biomimetics; Mechanical Property; Biomaterials

Introduction

The rapid development of various engineering fields is putting forward increasing requirements for the overall performances, including density, mechanical properties, environmental stability, etc., of structural materials (Ritchie, 2011; Wegst et al., 2015; Yu et al., 2018). In particular, the stability of structural materials applied in many harsh environments, such as extreme temperature, hydrotherm, acid-bases, etc., is indispensable (Yu et al., 2018). Incredibly, nature organisms have found ways to adapt to the complex living environments by evolving a wide range of amazing biological structural materials based on very limited selection of constituents (Barthelat et al., 2016; Eder et al., 2018; Huang et al., 2019). As a presented example, the shell from a deep-sea hydrothermal vent gastropod (Crysomallon squamiferum) serves not only as mechanical armor from predators, but also as protection from harsh corrosive and dissolutive marine environments (e.g., brackish, cold-water, low-pH conditions) (Yao et al., 2010). In addition, natural nacre from the shell of mollusk, a typical biological structural material, presents a striking trade-off between strength and toughness owing to its hierarchical layered structure and extrinsic toughening mechanisms across multiple scales (Barthelat et al., 2007; Fratzl and Weinkamer, 2007). The unusual mechanical properties combined with its low density predestine it to be one of the most studied models to be imitated for creating advanced structural materials (Peng and Cheng, 2017; Zhao and Guo, 2017).

To date, great achievements have been made in fabricating nacre-mimetic materials with precise control of their structures at multiple scales aiming to enhance their ultimate mechanical performance (Bouville et al., 2014; Le Ferrand et al., 2015; Mao et al., 2016; Pan et al., 2018; Yang et al., 2019b). Among them, organic-inorganic nacre-mimetic composites with macroscale bulk form have shown great potential as lightweight load-bearing structural materials for diverse engineering fields, such as biomedicine, aerospace, military and automotive industries, due to their low density, high specific strength, and high specific toughness (Du et al., 2019; Gao et al., 2017; Magrini et al., 2019; Yin et al., 2019). However, the overall performances of these composite materials are fundamentally restricted by the intrinsic properties of the organic components. For example, some water-soluble polymers, including polyvinyl alcohol (Chen et al., 2019) and sodium alginate (Gao et al., 2017), are commonly used as organic matrixes in nacre-mimetic composites, but this kind of polymers are very sensitive to humidity, making the relevant composites very unstable when subjected to high humidity and water environments. Other commonly used polymers, such as polyurethanes (Podsiadlo et al., 2009), epoxy (Zhao et al., 2016), and polymethyl methacrylate (Bai et al., 2016), are impervious to water, but their thermal stability and mechanical properties are still unsatisfying, largely limiting their practical applications. Overall, it remains challenging to produce organic-inorganic nacre-mimetic bulk materials with desirable combination of lightweight, high strength, high toughness, and good mechanical stability in adverse environments.

Aramid macrofibers, commonly known as Kevlar, are attractive for their low density (1.44 g cm⁻³), high tensile strength (∼3.6 GPa) and stiffness (∼109 GPa), chemical resistance, and thermal stability (Xu et al., 2018). Aramid nanofibers (ANFs) are solution-processable nanoscale versions of Kevlar. As a new type of polymer nanofibers recently developed, ANFs not only retain exceptional intrinsic properties of their macroscale parent but also gain certain unique characteristics due to their nanoscale morphology (Yang et al., 2020). In the meantime, mica microplatelets, a kind of rich and cheap natural silicate mineral, also have attractive property combination of low density (∼2.8 g cm⁻³) (Perez-Rodriguez et al., 2006), high mechanical performance (tensile strength ∼420 MPa, modulus ∼13.2 GPa), chemical and thermal durability, as well as unique ultraviolet (UV)-shielding property (Pan et al., 2018). These characteristics make ANFs and mica microplatelets ideal organic and inorganic building blocks, respectively, for assembling nacreous layered structure materials.

Herein, by hierarchically integrating ANFs and mica microplatelets together, we successfully produce a new-style organic-inorganic nacreous bulk material (nacreous ANFs-Mica bulk) with simultaneous achievement of low density, high strength and toughness, as well as excellent stability to some harsh environments. It was fabricated via a highly efficient bottom-up strategy proposed here, which combines spray-assisted gelation, solvent exchange, evaporation-induced self-assembly and further lamination (Gao et al., 2017) procedures. By optimizing the component proportion and the interfaces from nanoscale to macroscale, the resultant nacreous ANFs-Mica bulk (containing 40 wt.% of mica) not only exhibits low density (∼1.7 g cm⁻³) but also possesses high strength (∼387 MPa), stiffness (∼18.3 GPa), and fracture toughness (∼14.3 MPa m^1/2), surpassing those of many existing biomimetic bulk composites. Notably, both its specific strength (${\mathrm{\sigma }}_f/\mathrm{\rho }$) and specific toughness ($K_c/\mathrm{\rho }$) are higher than those of previously reported bulk nacre-mimetics, and most traditional engineering structural materials. In particular, the mechanical properties of the nacreous ANFs-Mica bulk can keep stable in a series of adverse environments, including acid and alkali solutions, boiling water, liquid nitrogen, and strong UV radiation.

Results and discussion

Fabrication and environmental stability

Aramid macrofibers were first split into high-quality ANFs by deprotonation (Figure S1) in saturated potassium hydroxide dimethyl sulfoxide (DMSO) solution (Xu et al., 2018). In order to enhance interfacial interaction between mica and ANFs via forming hydrogen bonds (Kim et al., 2014; Yang et al., 2015), mica microplatelets were modified with polyacrylic acid (PAA) on their surface (Figure S2). Afterward, ANFs serving as organic matrix and PAA modified mica microplatelets (PAA-mica) serving as inorganic bricks were assembled together to fabricate nacreous ANFs-Mica films at first via a spray-assisted gelation, solvent exchange and further evaporation processes we proposed here (Figures 1A–1C). In brief, the ANFs-Mica dispersion was sprayed onto a glass plate at room temperature, which quickly turned into gel (Figures 1A, S3A, and S3B) via seizing protons from water vapor in the air (protonation) during this process (Xu et al., 2018). The spray-assisted gelation method developed in this work allowed us to quickly obtain a homogeneous ANFs-Mica-DMSO-H₂O gel with controllable thickness according to the volumes of the sprayed dispersion. Subsequently, in order to further restore the molecular structure of the ANFs by protonation (Yang et al., 2019a), the ANFs-Mica-DMSO-H₂O gel was immersed into deionized water (DIW) to exchange DMSO, forming an ANFs-Mica-H₂O gel (Figures 1B, S3C, and S3D). Then, the expected nacreous ANFs-Mica film could be obtained by drying this hydrogel via evaporating its containing DIW (Figures S3E and S3F). It was supposed that mica microplatelets embedded in the hydrogel were gradually organized in parallel to each other along the surface direction during this evaporation process (Ji et al., 2018), resulting in the typical nacreous “bricks-and-mortar” microstructure (Figure 1C). These films were further cut with equal size, modified with 3-(trimethoxysilyl) propyl methacrylate (γ-MPS) and laminated together with epoxy resin as the interfacial adhesive among the films, forming the expected nacreous ANFs-Mica bulk with desired size (Figure 1D). Fracture morphology of the nacreous ANFs-Mica bulk displayed a typical step-like layered microstructure (Figures 1D and S4), closely resembling that of natural nacre.

Figure 1.

Fabrication of the nacreous ANFs-Mica bulk and its environmental stability

(A–D) Schematic diagram showing the fabrication process of the nacreous ANFs-Mica bulk (40 wt.%). It is assembled from ANFs and mica microplatelets via a mild and scalable bottom-up strategy which combines spray-assisted gelation (A), protonation by solvent exchange (B), evaporation-induced self-assembly (C) and further lamination (D) together.

(E–H) Photographs of the nacreous ANFs-Mica bulk (40 wt.%) immersed into 1 mol L⁻¹ HCl solution, 1 mol L⁻¹ NaOH solution, boiling water and liquid nitrogen, respectively.

(I and J) Surface SEM images of the pure ANFs bulk and nacreous ANFs-Mica bulk (40 wt.%) after treatment by UV radiation (6 days), respectively.

(K and L) Flexural strength and its retention of the nacreous ANFs-Mica bulk (40 wt.%), respectively, after treatment by 1 mol L⁻¹ HCl solution (24 hr), 1 mol L⁻¹ NaOH solution (24 hr), boiling water (24 hr), liquid nitrogen (24 hr), UV radiation (6 days) and 80% relative humidity (RH) (24 hr). Error bars manifest standard deviation (s.d.) of at least six measurements.

See also Figures S1–S6, and Videos S1 and S2

As we expected, the obtained nacreous ANFs-Mica bulk exhibits a good structural and mechanical stability when exposed to several typical harsh environments (Figures 1E–1J, and Videos S1 and S2). We found that the specimens maintained their integrity with invisible morphology change when immersing in acid (1 mol L⁻¹ HCl) and alkali (1 mol L⁻¹ NaOH) solutions, boiling water as well as liquid nitrogen, respectively (Figures 1E–1H). In addition, in contrast to the pure ANFs bulk materials with many microcracks and etching on its surface after exposure to strong UV radiation, the nacreous ANFs-Mica bulk did not show any microstructural change (Figures 1I, 1J, and S5). Further mechanical testing revealed that flexural strength of these specimens still kept at high values and shown slight decreases (Figures 1K and 1L). To be noted, the nacreous ANFs-Mica bulk revealed much better mechanical stability than that of pure ANFs bulk materials (Figure S6). These experimental results indicate the excellent environmental stability of the prepared nacreous ANFs-Mica bulk, which is elusive for many previously reported organic-inorganic nacre-mimetic composites.

Structural and mechanical characterizations

The superior mechanical properties and stability of the nacreous ANFs-Mica bulk in adverse environments can be mainly attributed to the intrinsic merits of the utilized building blocks and the hierarchically structural design. After assembling the two kinds of nanoscale building blocks into nacreous ANFs-Mica films (Figure S7), their intrinsic merits were supposed to be successfully integrated together and transferred to the ANFs-Mica films. The cross-sectional morphology of the ANFs-Mica films shows typical nacreous layered structure (Figures 2A and S8), which was formed via evaporation-induced self-assembly of the building blocks during the drying process of the ANFs-Mica hydrogels (Ji et al., 2018). Moreover, the ANFs-Mica film (40 wt.%) with different thicknesses could be easily fabricated by adjusting the thickness of the deposited gels (Figure S9). The ANFs-Mica film with optimized mechanical enhancement was confirmed to have 40 wt.% of mica microplatelets. The optimal tensile strength, toughness, and Young's modulus were measured to be approximately 220 MPa, 21 MJ m⁻³, and 9.2 GPa, respectively, which were all much higher than those of the pure ANFs films (Figures 2B and S10). These obvious mechanical enhancements were predominantly ascribed to the efficient interfacial interaction between PAA-mica and ANFs mediated by hydrogen bonding at molecular level (Figure S11), and the nacreous layered structure at nano/microlevel (Peng and Cheng, 2017). Theoretical analysis further displayed that there existed an optimum volume fraction of mica microplatelets that maximize the strength and toughness of the nacreous ANFs-Mica film (Figure S12), which is consistent with the experimental investigations. In addition, experimental results demonstrated that the nacreous ANFs-Mica films retained at least 93% of their original tensile strength after stored at a relative high temperature (≤250°C) for 6 hr, outperforming that of pure ANFs films treated with same conditions (Figures 2C and S13). These results indicate that the intrinsic high thermal stability of both mica and ANFs (Hepburn et al., 2000; Xu et al., 2018) contributes to the satisfactory thermal stability of the nacreous ANFs-Mica film. Additionally, the visible transmittance of the resultant films displayed slow reduction with an increasing mica content, whereas their UV transmittance was all completely shielding (Figure 2D) due to the strong UV absorption capacity of ANFs (Patterson and Sodano, 2016). This intrinsic optical property of ANFs commonly causes the decomposition of their molecular chains (Patterson and Sodano, 2016), namely UV aging. Notably, owing to the intrinsic UV-shielding property of mica (Pan et al., 2018), the UV absorbance of those nacreous ANFs-Mica films was enhanced with an increasing mica content (Figure 2E). Therefore, mechanical performance of the nacreous ANFs-Mica film containing 40 wt.% of mica was more stable than that of the pure ANFs film when exposed to strong UV radiation (Figures 2F and S14). These experimental results manifest that the intrinsic mechanical, thermal and chemical properties of mica and ANFs, together with the unique UV-shielding property of mica were successfully integrated into the obtained nacreous films.

Figure 2.

Characterization of nacreous ANFs-Mica films

(A) Cross-sectional SEM image of the nacreous ANFs-Mica film containing 40 wt.% mica.

(B) Tensile mechanical properties (strength and toughness) of the nacreous ANFs-Mica films with different mica contents.

(C) Retention of tensile strength of the 40 wt.% nacreous ANFs-Mica film and the pure ANFs film after 200°C, 250°C and 300°C treatment for 6 hr, respectively.

(D and E) UV-visible transmittance and absorption spectra of ~30 μm thick ANFs-Mica films with different mica contents.

(F) Retention of mechanical properties of the nacreous ANFs-Mica film containing 40 wt.% mica and the pure ANFs film, respectively, after treatment by UV radiation (6 days). Error bars manifest s.d. of at least six measurements.

See also Figures S7–S14.

The nacreous ANFs-Mica films with integrated properties were then used as the second-level building blocks to construct the nacreous bulk materials. The fracture surface of the obtained ANFs-Mica bulk exhibits a similar nacreous layered structure at larger length scale compared with that of nacreous ANFs-Mica film (Figures 3A and S15A), and no delamination or cavity was observed (Figure S15B). This result reveals that the laminating procedure did not perturb the ordered structure of these films, while compact these ANFs-Mica films into one dense unity. After further optimizing the preparation conditions (Figure S16), the ultimate flexural strength and stiffness of the nacreous ANFs-Mica bulk reached ∼387 MPa and ∼18.3 GPa (Figures 3B and 3C), respectively. The fracture toughness, K_IC, as an evaluation of the resistance to a crack initiation, was tested to be ∼4.9 MPa m^1/2 (Figure 3D). In addition, its maximum fracture toughness, K_JC, of the nacreous ANFs-Mica bulk increased by more than two times from the crack initiation (∼4.9 MPa m^1/2) to the end of the stable crack propagation (∼14.3 MPa m^1/2) (Figures 3D and 3E). These mechanical properties are all higher than those of the pure ANFs bulk, the disordered ANFs-Mica composite (Figure S15C), the nacreous ANFs-Mica bulk assembled from mica microplatelets without PAA modification, as well as the nacreous ANFs-Mica bulk assembled from ANFs-Mica films without γ-MPS modification (Figures 3C–3E). Furthermore, Vickers hardness of the nacreous ANFs-Mica bulk (∼68 kg mm⁻²) was also distinctly improved relative to the pure ANFs bulk (∼40 kg mm⁻²) (Figure S17). These mechanical enhancements greatly certify the validity of the hierarchically structural design at multiple scales presented here.

Figure 3.

Mechanical performance and multiple extrinsic toughening mechanisms of the nacreous ANFs-Mica bulk

(A) Cross-sectional SEM image of the 40 wt.% nacreous ANFs-Mica bulk.

(B) Flexural stress-strain curves of the 40 wt.% nacreous ANFs-Mica bulk prepared under different conditions of interfacial modification, the pure ANFs bulk and the disordered ANFs-Mica bulk.

(C–E) Comparison of flexural strength and stiffness (C), fracture toughness (D), and representative R-curves (E), respectively, of the nacreous ANFs-Mica bulk prepared under different conditions of interfacial modification, the pure ANFs bulk and the disordered ANFs-Mica bulk.

(F) Schematic of the crack-bridging model for the nacreous ANFs-Mica bulk with “brick-and-mortar” microstructure.

(G) Molecular structure of Si-OH group from mica, PAA and ANFs, as well as interfacial hydrogen bond networks formed in the neighboring ANFs and PAA-mica.

(H) Comparison of calculated fracture toughness under different interfacial strength based on mechanical model, the experimental measured fracture toughness for no PAA modified bulk (without hydrogen bonds), and the nacreous ANFs-Mica bulk (with hydrogen bonds).

(I–K) SEM images with different magnifications showing the fracture surfaces of the nacreous ANFs-Mica bulk.

(L) The shear stress field distributions show the progressive interface failure included interlayer sliding, cracking bridging and crack deflection based on FE simulation. Error bars manifest s.d. of at least six measurements.

See also Figures S11 and S15–S20, Table S1 and Videos S3 and S4.

Toughening mechanisms analysis

Representative crack-resistance curve (R-curve) of the nacreous ANFs-Mica bulk exhibited an analogous extensive rising R-curve behavior as that of natural nacre (Figure 3E), indicating the increasing fracture resistance of the nacreous ANFs-Mica bulk during crack propagation (Huang et al., 2019). Multiscale mechanical analysis based on crack-bridging model (Budiansky and Amazigo, 1989; Shao et al., 2012) and finite element (FE) analyses further revealed that the interfacial hydrogen bonds between ANFs and mica microplatelets play a pivotal role in the toughening of the nacreous ANFs-Mica bulk (Figures 3E–3L and S18, and Table S1). Based on the experimental investigations and the FE simulations below (Figures 3I–3L), mica microplatelets were pulled out at the front tip of the crack during the crack propagation process, leading to the formation of crack-bridging zone (Figure 3F) (Budiansky and Amazigo, 1989; Shao et al., 2012). As shown in Figure 3G, hydrogen bonds are formed between NH and CO groups of adjacent ANFs chains, and also between ANFs chains and the COOH group on the surface of PAA-mica (Figure S11). As the mica microplatelets begin to slide relative to each other, hydrogen bonds networks between the neighboring platelets can facilely reform after bond breaking due to micaplatelets and interfibers sliding, which improves the interfacial strength and thus result in much enhanced fracture toughness (Figure 3H). The above toughening mechanisms are consistent with previous researches on toughening staggered layered structures by hydrogen bonds interactions (Zhu et al., 2015). In addition, it is worth mentioning that there also exists other interface dominated toughening mechanisms such as micocrack deflection, crack branching, multiple cracking etc., during crack propagation in the nacreous ANFs-Mica bulk, which could be illustrated by the following experimental observations. As shown in Figures 3I–3K, the multiple extrinsic toughening mechanisms acting at different length scales are clearly displayed. Figure 3I shows that the crack in the nacreous ANFs-Mica bulk initiated from the notch and propagated along a characteristic tortuous path, improving interfacial area per unit crack length. Higher-magnification SEM images of the fracture surface further illustrate the presence of crack branching, multiple cracking and crack bridging at the crack tip (Figure S19 and Video S3). Moreover, a typical trapezoidal fracture surface containing abundant pull-outs of mica microplatelets on the fracture surface and extensive delamination of ANFs-Mica interface were observed (Figure 3J). Notably, ANFs layer densely adhered on the surface of mica microplatelets (Figure 3K), indicating strong ANFs-Mica interfacial interaction. These typical multiple extrinsic toughening mechanisms, derived from the hierarchically layered architecture, synergistically dissipated a tremendous amount of mechanical energy at different length scales, leading to an obvious toughness enhancement of the nacreous ANFs-Mica bulk (Bouville et al., 2014; Gao et al., 2017). A three-dimensional (3D) FE simulation was further developed to determine the mechanisms responsible for the mechanical enhancement of the nacreous hierarchical structure. As we expected, similar mechanisms, including interlayer sliding, cracking bridging and crack deflection, as the experiment observed, were also clarified and visualized by the FE simulation (Figures 3L and S20, and Video S4).

With regard to structural materials, besides mechanical properties and environmental stability, density is another main factor, determining their practical applications¹. Based on the low density of both the building blocks and the designed hierarchical nacreous structure, the nacreous bulk material achieved a striking trade-off among density, strength and toughness, which is hard to accomplish simultaneously for engineering structural materials and demonstrates impressive superiority compared with previously reported biomimetic bulk materials (Figures 4A and 4B) (Bouville et al., 2014; Chen et al., 2018; Estili et al., 2012; Gao et al., 2017; Grossman et al., 2017; Guan et al., 2020a, 2020b; Le Ferrand et al., 2015; Libanori et al., 2013; Liu et al., 2013; Mao et al., 2016; Morits et al., 2017; Munch et al., 2008; Naglieri et al., 2015; Nakagaito et al., 2005; Song et al., 2018; Yang et al., 2017; Zhao et al., 2016). As shown in Figure 4C and Table S2, the specific strength, ${\mathrm{\sigma }}_f/\mathrm{\rho }$ (227 MPa/(Mg m⁻³)) and specific toughness, $K_c/\mathrm{\rho }$ (8.4 MPa m^1/2/(Mg m⁻³)) of our nacreous bulk material are both higher than those of the other bulk nacre-mimetics (Bouville et al., 2014; Chen et al., 2018; Estili et al., 2012; Gao et al., 2017; Grossman et al., 2017; Guan et al., 2020a; Le Ferrand et al., 2015; Libanori et al., 2013; Liu et al., 2013; Mao et al., 2016; Morits et al., 2017; Munch et al., 2008; Naglieri et al., 2015) (Song et al., 2018). Notably, both the ${\mathrm{\sigma }}_f/\mathrm{\rho }$ and $K_c/\mathrm{\rho }$ of our nacreous bulk material are higher than those of nearly all biological structural materials and most engineering structural materials including high-performance ceramics, metallic alloys and even glass-fiber-reinforced polymers (Figure 4D and Table S2) (Bouville et al., 2014; Estili et al., 2012; Gao et al., 2017). Consequently, the nacreous ANFs-Mica bulk presented here is confirmed to be a typical new-style lightweight, strong, tough, and environmentally stable structural material.

Figure 4.

Comparison of mechanical performance of the nacreous ANFs-Mica bulk with relevant bulk structural materials

(A) Ashby chart plotting flexural strength versus density for our nacreous ANFs-Mica bulk and other biomimetic bulk composites.

(B) Ashby chart plotting fracture toughness versus density of our nacreous ANFs-Mica bulk and other biomimetic bulk composites.

(C and D) Ashby diagrams plotting the specific toughness versus specific strength for our nacreous ANFs-Mica bulk compared with other nacre-mimetic bulk composites (C), as well as natural and engineering structural materials (D). Numbers in the charts stand for relevant references in Table S2.

See also Table S2.

Conclusion

In conclusion, a high-performance nacreous bulk material was facilely produced via a mild and scalable bottom-up assembly strategy. Based on the hierarchically structural design and the superior intrinsic properties of the utilized building blocks, the obtained nacreous ANFs-Mica bulk exhibits an impressive combination of low density (1.7 g cm⁻³), high strength (387 MPa), and high fracture toughness (14.3 MPa m^1/2). In particular, its mechanical properties can maintain stable in several typical adverse environments, including acid and alkali solutions, boiling water, liquid nitrogen, and strong UV radiation. With respect to its overall property combinations, the nacreous ANFs-Mica bulk possesses great superiority compared with biological structural materials, currently existing bulk nacre-mimetic composites and most engineering structural materials. As a new-style biomimetic structural material, this nacreous ANFs-Mica bulk is expected to offer an applicable choice as a brand new structural material for application in some adverse environments.

Limitations of the study

Nacreous aramid-mica bulk materials with excellent mechanical properties and environmental stability were fabricated under laboratory conditions, but its larger scale preparation still needs to be further investigated.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the Lead Contact, S.-H. Y. (Email: shyu@ustc.edu.cn).

Materials availability

This study did not generate new unique reagents.

Data and code availability

All relevant data are available from the authors upon request.

Methods

All methods can be found in the accompanying Transparent methods supplemental file.

Published: January 22, 2021