Rémi Bertrand

Hugo Robe
email: h.robe@isgroupe.com

Damien Texier

Yasser Zedan

Eric Feulvarch

Philippe Bocher

Analysis of AA2XXX/AA7XXX friction stir welds

Keywords:

Comparable microstructures and mechanical properties are obtained for the different AA2139-T8/AA7020-T651 joints welded in the range 600-1000 rev. min -1 / 250-550 mm.min -1 as rotation and advance speeds. Joints exhibit tensile properties as good as 77%-79% of the yield strength and 88%-96% of the ultimate tensile strength of the AA7020 base material. Except for the high speed conditions that failed in the nugget, all joint variants failed in the heat affected zone (HAZ), which is also consistent with lower hardness properties. This drop in hardness in the HAZ is produced by reaching and keeping for a short time a critical temperature during welding between 225 and 260 °C. Such a combination of dissimilar alloys offers conservative mechanical properties for a large range of welding parameters suitable for the assembly purpose of high-performance structural components.

Introduction

Transportation applications, both aeronautical and terrestrial, are confronted to severe specifications in terms of lightweight design. For such a purpose, aluminum alloys are suitable materials as they exhibit reasonable mechanical properties for relatively low densities. Due to their high mechanical performances, 7XXX and 2XXX series aluminum alloys are commonly used for structural components in the aeronautic, aerospace, automotive, and defense industries as explained by [START_REF] Jha | Metallurgical analysis of cracked aluminum alloy (AFNOR 7020) components used in satellite launch vehicles[END_REF]. [START_REF] Kumar | Investigations on warm forming of AW-7020-T6 alloy sheet[END_REF] have reported that, among the AA7XXX family, AA7020 is a medium strength Al-Zn-Mg alloy -type, precipitation hardened. On the other hand, [START_REF] Al-Obaisi | Precipitation hardening and statistical modeling of the aging parameters and alloy compositions in Al-Cu-Mg-Ag alloys[END_REF] and [START_REF] Lach | Characterization of Al-Cu-Mg-Ag Alloy RX 226-T8 Plate[END_REF] affirmed that 2XXX series as Al-Cu-Mg alloys are intended to progressively replace 7XXX alloys due to their higher mechanical strength and lower density. [START_REF] Grujicic | Computational investigation of hardness evolution during friction-stir welding of AA5083 and AA2139 aluminum alloys[END_REF] have shown that the Al-Cu-Mg-Ag alloys (AA2139) demonstrate outstanding mechanical properties in terms of ballistics and fatigue because of homogeneous -type precipitation initiated by the formation of Mg-Ag clusters. Despite the strong optimization of the specific properties, the intensive use of AA2139 for structural applications is limited by its production cost. To overcome such an issue, cost-efficient designs of structural components using hybrid designs are promoted by assembling medium-performance aluminum alloys (AA7020 for instance) in most structures with a high-performance aluminum alloy (AA2139 for instance) in specific locations subjected to high mechanical stress levels; however, these alloys cannot be easily assembled by the conventional fusion welding process due to their high thermal diffusivity. As explained by [START_REF] Mishra | Chapter 1 -introduction[END_REF], Friction Stir Welding (FSW), a solid-state welding process, has been found to be a particularly efficient method for assembling these materials.

During FSW, the friction contact of the material with a non-consumable rotating tool together with the large plastic deformations generates sufficient heat to lead materials to a soft state. By moving the pin, it is then possible to create a continuous joint between the materials along the tool path. Another interesting development in FSW processes in regard to conventional welding techniques is the possibility to weld dissimilar materials. [START_REF] Noh | Friction stir welding of F82H steel for fusion applications[END_REF] and [START_REF] Avettand-Fenoel | A review about Friction stir welding of metal matrix composites[END_REF] have highlighted numerous advantages of the FSW in terms of mechanical properties. All these studies, among others, led FSW to be an alternative solution for dissimilar AA7XXX/AA2XXX alloys assemblies. In FSW, the side where the tool rotation direction and the welding direction are the same is called the advancing side (AS). The other side is the retreating side (RS). A number of studies have focused on sample orientation. Yan et al. (2016) showed that this parameter significantly affects the tensile strength and the material flow. [START_REF] Firouzdor | Al-to-Mg friction stir welding: effect of positions of Al and Mg with respect to the welding tool[END_REF] have demonstrated that the heat input and the joint quality also depend on sample orientation. The influence of this parameter on the hardness was reported by [START_REF] Luo | Microstructure evolution and mechanical properties of friction stir welded dissimilar joints of Mg-Zn-Gd and Mg-Al-Zn alloys[END_REF]. All these studies suggest that the material with the highest mechanical properties should be placed in the advancing side. In some specific configurations, it is possible to observe local heterogeneities that form banded structures in the microstructure known as "onion rings". [START_REF] Krishnan | On the formation of onion rings in friction stir welds[END_REF] explains that "onion rings" are due to the heterogeneous process of friction heating due to the rotation of the tool and the forward movement which extrudes the metal around the retreating side of the tool. As demonstrated by Texier et al. (2016), textures and mechanical behaviors also change on the various bands.

Since 2010, the FSW of AA7020 has been studied repeatedly in similar and dissimilar configurations. Several studies have highlighted the influence of advance and rotation speeds on the mechanical and microstructural properties. [START_REF] Heidarzadeh | Tensile properties of friction stir welds of AA 7020 aluminum alloy[END_REF] have shown, for AA7020 similar welding, that decreasing the speed ratio and axial force increases the tensile strength and elongation of the joint, and leads to a more ductile joint. A study conducted by [START_REF] Gaafer | Microstructural and mechanical characteristics of AA7020-O Al plates joined by friction stir welding[END_REF] on AA7020 similar welding has shown that, for a constant welding speed, increasing the tool rotational speed increases the average grain size, the hardness at the center of the welding zone, and the ductility of the joint. It also showed that for a given range of rotational speed, increasing the welding speed increases the tensile strength of the FSWed joints. [START_REF] Giraud | Investigation into the dissimilar friction stir welding of AA7020-T651 and AA6060-T6[END_REF] noticed the influence of the process parameters on material mixing, weld integrity, and mechanical behavior. Masoumi [START_REF] Masoumi Khalilabad | Effect of tool geometry and welding speed on mechanical properties of dissimilar AA2198-AA2024 FSWed joint[END_REF] observed that increasing the traverse speed of the tool increases the global hardness of the joint, but also creates defects like a kissing bond, in the case of an AA2198/AA2024 FSWed joint.

According to [START_REF] Mishra | Chapter 2 -physical metallurgy of 7XXX alloys[END_REF], the sequence of precipitation for most of the alloys from the 7XXX series is:

SSSS → GPZ → η' → η
where SSSS is the supersaturated solid solution, GPZ the Guinier-Preston zones, η' and η are respectively the metastable and stable phases responsible for the hardening. The η'-phase tends to evolve to the η-phase during additional heat treatment. As demonstrated by [START_REF] Kamp | Modelling of friction stir welding of 7xxx aluminium alloys[END_REF], this overaged state is commonly found in the HAZ after welding. The η-precipitates present in the form of coarse intergranular and incoherent precipitates in the aluminum matrix undergo a faster coarsening than the metastable phase η' and lead to hardness decrease. In the welding of aluminum alloys, overaging is a typical consequence of the thermal cycle imposed by the process. [START_REF] Ma | Softening behaviour of Al-Zn-Mg alloys due to welding[END_REF] have shown from experiments and simulations of 7020-T6 alloy arc welding that softening in the HAZ occurs when the peak temperature exceeds 230 °C. In 7050-T651 FSWed similar joints, [START_REF] Mahoney | Properties of friction-stir-welded 7075 T651 aluminum[END_REF] have observed overaged precipitate in the fracture zone, which corresponds to the HAZ. Wu et al. (2015) and Sun et al. (2017) both found that reducing the heat input induced by the tool during welding reduces the minimum hardness reached, the width of the HAZ and its distance from the center line of the joint. The use of high depth-to-width ratio tools has been studied by [START_REF] Ahmed | Effect of tool geometry and heat input on the hardness, grain structure, and crystallographic texture of thick-section friction stir-welded aluminium[END_REF] and [START_REF] Huang | Numerical design of high depth-towidth ratio friction stir welding[END_REF], and both concluded that increasing depth-towidth ratio narrows the HAZ. Similar welded joints with AA2139 were investigated by [START_REF] Prisco | Influence of welding parameters[END_REF] and [START_REF] Eberl | Friction stir welding dissimilar alloys for tailoring properties of aerospace parts[END_REF]. They noticed a softened region, surely in the HAZ, characterized by a drop of hardness for AA2139 as-welded and post-weld heat treated joints. Sree Sabari et al. (2016) have studied the grain size for different rotating speeds and a fixed welding speed on an AA2519 welded joint. For all welding areas, the higher the welding speed is, the bigger the grain size is. Radisavljevic et al. (2013) showed that the best welds are obtained for cold welds, i.e. for high weld pitch, in the case of AA2024-T351 similar welding. [START_REF] Prisco | Influence of welding parameters[END_REF] reported the same conclusions for similar AA2139 welds.

Welding combinations of 2XXX and 7XXX series is of great interest for the Aerospace and Defense industry. As proven by the work of [START_REF] Cavaliere | Mechanical and microstructural behaviour of 2024/7075 aluminium alloy sheets joined by friction stir welding[END_REF] and [START_REF] Khodir | Friction stir welding of dissimilar AA2024 and AA7075 aluminum alloys[END_REF], AA7075/ AA2024 weld has been widely studied for plane applications. In the literature, several studies have been conducted on 7XXX/2XXX joints, but AA7020/AA2139 is an innovative and undocumented combination.

In the present study, various joints were produced with large ranges of industrial welding parameters, i.e. the rotation and advance speeds. The microstructure and the mechanical properties of the dissimilar friction stir welded joint variants made of AA2139-T8 and AA7020-T651 were investigated. Microhardness mapping was conducted to characterize the local variability of the joints whereas macroscopic tensile tests and fractography analyses were performed to evaluate the performance of the joints in comparison to the base metals. The link between the welding parameters and the global and local mechanical behaviors of the various joints was studied in detailed.

Materials and experimental procedure

Welding process

In this study, the two heat-treatable aluminum alloys, AA2139-T8 (T8: cold worked and artificially aged), and AA7020-T651 (T651: peak strength artificially aged and additional stretched) with a thickness of 5 mm were used, welded, and studied in as-welded condition after a natural aging time of several months. The chemical compositions of each material are listed in Table 1.

The FSW specimens were performed by TRA-C industrie with a CFSW (China Friction Stir Welding Center) LM-B equipment. The AA2139-T8 sheet is positioned on the advancing side (AS). The rolling directions (RD) of the two metal sheets are positioned parallel to the cross-welding direction (CWD), i.e. perpendicular to the welding direction (WD) (see Fig. 1a). Process parameters and sample references were summarized in Table 2. In addition, the speed ratio R (advance speed / rotation speedmm.rev -1 ) is mentioned in order to see its effect on the welds. The tool is composed of a 15 mm diameter scrolled shoulder with a tapered, threaded Triflat ™ 4.8 mm long pin (Fig. 1b), and is made of H13 (heat treated tool steel).

Microstructural analysis

As a reference, samples were extracted from base metals, and their cross-sections were investigated in planes normal to the three process directions: rolling direction (RD), transverse direction (TD), and normal direction (ND). The base metal samples were mounted and manually polished, following a standard polishing procedure with a finish down to 0.05 µm colloidal silica suspension. Samples were then placed in a BUEHLER VibroMet for 17 h with a 0.05 µm colloidal silica suspension. AA2139 samples were finally etched by Keller's reagent [2 mL HF (48%), 3 mL HCl (conc.), 5 mL HNO 3 (conc.), 190 mL H 2 O] for 7 s at room temperature. AA7020 samples were etched by Graff-Sargent etchant [15.5 mL HNO 3 (conc.), 0.5 mL HF (48%), 3 g CrO 3 , 84 mL H 2 O] for 30 s and by Keller's reagent for 7 s at room temperature. Optical macro-and micrographs were conducted by means of an optical/laser confocal microscope. The optical micrographs of both base materials are presented in Fig. 2 in a 3-D view. AA2139-T8 has slightly elongated grains in RD and more equiaxed grains in the other two directions. On the other hand, AA7020-T651 reveals a typical elongated grain structure, due to rolling and stretching performed after the heat treatment stage, as reported by [START_REF] Giraud | Investigation into the dissimilar friction stir welding of AA7020-T651 and AA6060-T6[END_REF]. All FSWed joint variants were examined with a metallographic method similar to the one previously described for the AA7020 base metal samples.

Hardness mapping

For each welding condition, samples were extracted from the joint and mounted in a cold epoxy resin. The samples were polished, using the standard polishing procedure detailed in the Section 2.2. Microstructural analysis. Vickers hardness was measured by means of an automatic microhardness apparatus with a 50gf load applied during 10 s. Maps composed of 39 profiles of 193 indents have been performed in the CWD-ND cross-section plane to form a regularly spaced square pattern (120 µm). The mapping covered all the welding zones over a total area of 23.2 ◊ 4.7 mm 2 . Hardness values were then numerically computed using MATLAB® to plot cross-section profiles and to draw 2D hardness maps.

Tensile tests

Tensile testing was conducted on the base metals and the different friction stir welded joints. Tensile specimens were machined out from both base metals for mechanical property references and others were extracted from the welded plates for the five welding conditions with the longitudinal direction of the specimen being aligned with CWD (the joint being perpendicular to the tensile direction). The joint centerline was centered in the gauge section of the specimen. The specimen geometry follows ASTM-E8 standards and is illustrated in Fig. 3. The tensile tests were performed on a 100-kN servo-hydraulic machine at a nominal strain rate of 2.5 × 10 -4 s -1 and repeated three times. The specimen elongation was continuously recorded using a 25.4 mm gauge length extensometer. After failure, fractography analyses were carried out by means of a scanning electron microscopy (SEM) with a field emission-SEM in a secondary electron mode.

Mid-thickness thermal measurements for extreme conditions

In order to correlate the thermal history and the mechanical properties of the joints, thermal measurements were acquired during welding for extreme conditions: BW03 (hot) and BW06 (cold) (condition BW07 was not stable enough to conduct proper thermal measurements). Several points at various transverse distances from the center line of the joint and at a depth of 2.5 mm (mid-thickness) were chosen for measurements (Fig. 4). The measurements were repeated 4 times. Thin K-type thermocouples with a diameter of 0.2 mm were used. The sampling frequency has been set to 75 Hz.

Experimental results

Microstructure analysis

The evolution of the macrostructure in a CWD-ND cross-section and interface for all the FSWed joint variants is illustrated in Fig. 5. Observations prior and after etching at various magnifications have validated the fact that defect-free joints were produced. A rather similar macrostructure was observed in the range of welding parameters. The nugget region consists of two parts corresponding to each base material flow. The micro-and macrostructure of the joints, and the shape of the interface between the two materials in the nugget are comparable and repeatable for all welding conditions. No real mixing between the two materials is achieved and only a small amount of material from one side is moved onto the other side of the nugget as illustrated with optical micrographs Fig. 5. The classical regular and smooth S-shape of the two-material interface, reported by da [START_REF] Da Silva | Material flow and mechanical behaviour of dissimilar AA2024-T3 and AA7075-T6 aluminium alloys friction stir welds[END_REF], does not appear and, for all welding conditions, continuous "onion rings" are formed on the lower two thirds of the nugget between the two materials.

Depending on the welding speeds, some slight differences were found. Beyond the highest parameters of the BW07 condition (1000 rev. min -1 / 550 mm•min -1 ), defects (voids and chips) are formed on the upper surface during the welding process (Fig. 6). As reported by [START_REF] Jain | Friction stir welding: Scope and recent development[END_REF] and [START_REF] Hartmann | Influence of surface roughness of tools on the friction stir welding process[END_REF], these defects are due to the abnormal sticking of the pasty aluminum and shearing under the shoulder. They are mainly due to an excessive heat input and could be deleted by enhanced welding parameters. Previous studies conducted on AA7020 alloy (similar and dissimilar configurations), such as [START_REF] Giraud | Investigation into the dissimilar friction stir welding of AA7020-T651 and AA6060-T6[END_REF] work, have revealed the ability to reach higher welding parameters without creating defects. This sensitivity in 2XXX/7XXX joints illustrates the limits of the process window. Different regions were identified for each side of the weld on CWD-ND cross-sections: The base metal (BM), the heat affected zone (HAZ), the thermo-mechanical affected zone (TMAZ), the shoulder-affected nugget zone (SANZ), and the pin-affected nugget zone (PANZ). The PANZ corresponds to the nugget area with onion rings (about the lower two thirds of the nugget) while the SANZ corresponds to the rest of the nugget (about the top third of the nugget). The different zones are depicted in Fig. 7 for the BW03 welding condition. When comparing the different zones for various welding parameters (Fig. 5), it can be observed that increasing the advance speed reduces the depth of the SANZ while a more vertical TMAZ is reported (especially in the AS i.e. AA2139-T8). The increase of the rotation speed does not reveal any specific modification of the SANZ depth or width.

As depicted in Fig. 7a, some specific locations within the joint were investigated by means of optical observations. In the nugget, both materials have very small grain size most probably due to dynamic recrystallization due to high speed and high temperatures. The boundary between the nugget and the TMAZ is easily identifiable by the obvious difference of grain size and morphology (see Fig. C,H). However, it is more difficult to precisely define the boundary between the TMAZ and the HAZ based on optical microstructural observation, and impossible to locate the end of HAZ towards the base material.

Mechanical characterization

Microhardness

Vickers hardness measurements were conducted on both base materials for references by taking the average value of 15 arbitrary measurements. The base material hardness is 165 HV 0.05 and 134 HV 0.05 for AA2139-T8 and AA7020-T651, respectively. Microhardness profiles along CWD performed at mid-thickness for the five conditions are presented in Fig. 8. For all conditions, the hardness values progressively decrease from the advancing side (AA2139) down to a plateau zone in the middle of the joint. Minimal values between 9 and 12 mm from the centerline of the joint are reported on the retreating side (AA7020), i.e. in the HAZ region, before going back to the base metal hardness. Even if significant microstructural (Fig. 5) and hardness differences are reported between the two base materials, the hardness in the nugget is relatively homogeneous and the interface between the two materials can only be identified using a microstructural criterion (see the absence of variation around the centerline of the weld (0 mm) in Fig. 8). The average value inside the nugget (around 118 HV 0.05 ) is practically not influenced by the welding conditions with a maximum variation of 20 HV 0.05 . No hardness drop was found on the AA2139 HAZ and except for the coldest welding condition (BW06) which exhibits slightly higher hardness values, all the hardness values in this region follow similar trends at ± 20 HV 0.05 . Things are different on the AA7020 side (RS) as the hardness distribution in the HAZ varies significantly and the position and the minimum values of the lowest hardness regions are a function of the welding conditions. Indeed, the hardness drop is found 12 mm from the weld center in the case of the hottest conditions (BW03), while the coldest condition (BW06) is found at 9 mm. The hottest condition reveals the lowest hardness value at 92 HV 0.05 while the other welding conditions display similar minimal hardness values 8 HV 0.05 higher.

Given the strong microstructural heterogeneities, a single hardness profile in mid-thickness is not sufficient to fully characterize the weld and a more global analysis using 2D hardness maps was performed. The results are displayed in Fig. 9. The microstructural maps and the hardness maps correlate. The transition between the TMAZ + HAZ region and the nugget can be easily distinguished on the AA2139 side (AS) due to a severe drop of hardness properties (ΔHV = 40 HV 0.05 from 165 to 120) across this region, going from yellow/green to blue. Comparatively, this transition is less visible on the AA7020 side (RS) as the initial hardness of the base material is significantly lower and a continuous evolution of the hardness properties along CWD is to be noted.

The position of the minimal hardness was clearly associated with the HAZ on the AA7020 (RS) side. As observed on the filiation of Fig. 8 and on Table 3, the minimal hardness value levels are about 100 HV 0.05 except for the hotter condition (BW03) for which a lower minimum hardness value is found around 92 HV 0.05 . If one defines the minimal hardness region (MHR) as the region of the material with hardness below 110 HV 0.05 , relatively straight bands can be identified on the hardness maps. They do correspond to any particular zone of the microstructure maps and they move away from the joint center line as more heat is generated by the process. The advance speed is the process parameter that most affects the location of the MHR: the lower the advance speed is, the further away the MHR is from the nugget region. The closer to the centerline of the joint the MHR is, the more inclined and parallel to the TMAZ/nugget interface it becomes; far from the centerline of the joint, the MHR is mostly perpendicular to the sheet surface.

Increasing the advancing speed reduces the width of the MHR. The width of this MHR gets smaller as the MHR gets closer to the joint center, going from 2.7 mm when at a distance of 12 mm to a width of 0.84 mm for a distance of 9 mm. This significant decrease in width by a factor of 3.2 should improve the mechanical property of the joint by a notch strengthening effect; i.e., delaying necking in this region during tensile tests.

Tensile tests

Tensile tests were performed to evaluate the macroscopic tensile properties of all the joint variants concerning the properties of the base metals. The tensile properties of both base metals are presented in Table 4. AA2139-T8 demonstrates significantly stronger tensile properties than AA7020-T651. Indeed, the 0.2% offset yield strength (YS 0.2 ) and tensile strength (TS) are 28% and 19% higher in the RD direction for the AA2139 than for the AA7020 in the same direction. While stronger, AA2139-T8 has also an interestingly higher ductility (+36%) than AA7020.

Fig. 10 shows the stress-strain curves for the five welding conditions compared with both base metals in the rolling direction. The joint behavior is close to base metal 7020-T651. The average tensile properties for each welding condition are listed in Table 5. The elastic domain is the same for all conditions while the plastic domain mainly differs for strains higher than 400 MPa. All samples show significant strain hardening compared to the base metals. Besides the BW07 condition, which failed suddenly at a relatively low strain of about 4%, the other four conditions display final elongations of about 7% and a curve typical of a necking behavior. Similar observations can be made concerning the necking strain, which reaches 3.5% for the BW07 condition and about 7% for the other four conditions. The efficiencies, defined as the ratio of the tensile properties of the joint to the base metal involved (using the weakest material in the case of dissimilar welding) are reported in Table 5. Overall, the efficiencies are rather high going from 77 to 79% for the 0.2% offset yield strength and 88 to 96% for the ultimate tensile strength. Generally, increasing the welding speed tends to simultaneously increase the yield and the ultimate tensile efficiency, whereas increasing the rotation speed reduces them.

The result dispersion is relatively low except for the highest rotation and advance speeds (condition BW07) which present a significant variability in the TS values and systematic low elongation-to-fracture. In fact, except for some samples in the BW07 condition, all joints systematically failed in the HAZ on the AA7020 side (RS) between 7 and 13 mm from the centerline of the joint. For all samples that have failed in the HAZ, the MHR location matches with the maximum necking, with a slight offset due to the sample elongation during the tensile test. necking strain and elongation despite the drop of its TS. The remaining specimens in the BW07 condition failed in the nugget region close to the joint centerline.

Fracture surface analysis

Fracture surface analyses were performed for all processing conditions including base metals. All the joints failing in the HAZ of AA7020 had similar features to the ones displayed in Fig. 12a-c (condition BW03). A mixed mode fracture was found composed of dimples initiated from small intermetallic particles (dimples 10 µm in diameter) and transgranular regions revealing elongated grains. If compared to the fracture surface of the AA7020 base material (Fig. 12d-f) interesting differences can be found. The fracture surface of the AA7020 is mostly composed of relatively large dimples ( 20 µm in diameter) close to each other with only few elongated grain features. The sizes of the intermetallic particles found in the dimples are coarser in the base material than the ones found in the HAZ region. These fracture features are characteristic of highly deformed areas with localized stress triaxiality. This is coherent with the fact that these minimum hardness regions are the locations of significant necking.

The fracture occurred in the nugget region of the joint for one sample of the fastest rotating condition (BW07). The fracture does not particularly follow this interface of the joint on the two thirds of the surface, but on the bottom of the weld, the fracture surface corresponds to the interface between the two materials (in green Fig. 13b). The fracture surface (Fig. 13c) reveals two regions along the WD: one with relatively homogeneous ductile features below the shoulder and related to the SANZ (Fig. 13d), and one deep enough in the material to be mainly controlled by the pin ability to mix the material (PANZ). In the latter region, periodic features that can be described as humps and hollows spaced by a distance equivalent to the advance per revolution of the tool can be observed. These periodic features can also be divided again into two zones, identified as zone I and zone II in Fig. 13b. Zone I is located in the middle of the weld thickness and displays some regions with typical ductile fracture. This zone corresponds to a combination of both materials (part of the red Fig. 13b) and large dimples (Fig. 13c) are found in humps, typical of the ductile behavior of AA7020. On the other hand, hollows correspond to the other material and no ductile features were found, even at high magnification (Fig. 13e). The alternation of these humps and hollows is quite similar to the alternative pattern found when cutting and polishing the sample in the WD-ND plane in the same location (Fig. 14b), justifying the fact that the joint interface is strong and that the crack propagates in both materials independently. Zone II, located in the lower part of the nugget, shows rough features and no sign of ductility at low magnification, even if some can be found locally at high magnification (Fig. 13h). This zone corresponds to the joint interface (green Fig. 13b) and documents the way the materials get mixed in that region. At the extreme bottom, an inclined foliation can be seen (Fig. 13i), documenting the lack of tool penetration. The spatial period between the coarse regularly spaced features is about 500 μm, which correspond to the linear advance for one tool revolution (R ratio = 0.55 mm.rev -1 ).

The periodic features corresponding to the various wavy patterns found in the nugget are well illustrated in both pictures of Fig. 14. A comparison between Figs. 13 and 14b is particularly interesting as an identical morphology is found where the SANZ consists only of AA7020 and the PANZ shows complex periodical features. The joint is defect free and the particular features observed on the lower part of the fracture surface reveal a weak material interface.

Mid-thickness thermal measurements around the MHRs for extreme conditions

Thermal profiles at mid-thickness during welding were performed for extreme conditions at each thermocouple location to see the effect of a given thermal cycle, and more precisely the effect of the peak temperatures and the exposure times, on the local mechanical properties of the joints in the MHRs. Average temperature evolutions as a function of the time for thermocouple 5 (TC5), thermocouple 6 (TC6) and thermocouple 7 (TC7) at 10 mm, 12.5 mm and 15 mm from the center line, respectively, are shown for conditions BW03 and BW06 on Fig. 15. It must be noted that the temporal offset between the curves has been removed to facilitate the analysis.

For a given thermocouple position, each peak value obtained for condition BW03 (hot condition) is 15-36 °C higher than that obtained for condition BW06 (cold condition). Table 6 summarizes the peak temperatures and the exposure times above 200 and 250 °C as measured for each thermocouple during welding for conditions BW03 and BW06. The exposure times obtained for condition BW03 are significantly higher than those obtained for condition BW06.

Since the distance of the MHR from the center line is different for each condition, the relative position of the thermocouple with respect to each MHR changes. For condition BW03, the MHR is between TC5 and TC6 whereas for BW06, the MHR is between TC5 and the center line of the weld. By extrapolation of the results presented in Fig. 15 and in Table 6, it is possible to estimate the thermal cycle undergone in the MHR for conditions BW03 (hot) and BW06 (cold). For the condition BW03, the MHR undergoes a peak temperature from 222 to 254 °C with exposure times from 3.7 to 0.9 s for 225 °C and from 4.3 to 5.9 s for 200 °C, respectively. For the condition BW06, the peak temperature is slightly higher than the one given by TC5, that is to say slightly higher than 234 °C and exposure times are slightly above 1 and 2.2 s for 225 °C

Table 3

Minimal hardness value in the MHR, width of the MHR (considering that the limit of the MHR corresponds to a hardness of 110 HV 0.05 ) and position of the MHR from the center line (at mid thickness) for various conditions.

Discussion

The wide window of welding parameters tested on the dissimilar AA2139-T8 / AA7020-T651 welding configurations showed that similar microstructures were obtained in terms of morphology and hardness. Defect-free joints with a complex mixture that provides good mechanical bonding at the interface between the two materials have been produced. Good mechanical results highlighted the great robustness of the FSW process in these conditions. Despite these relatively homogeneous performances, the extreme welding condition (BW07 condition) revealed a less predictable behavior and subsequent low mechanical results related to the presence of a weak interface between the materials at the bottom of the joint.

To understand the possibilities to improve further the best tensile strength of the joints, the effects of the process heat on the characteristics of the MHR will be discussed below by looking at the thermal cycles seen in the nugget and in the HAZ.

The cycle in the nugget reaches temperatures higher than 260 °C resulting in a medium range of hardness levels (110-120 HV). Thus, the local temperatures are high enough to allow a complete dissolution of the initial precipitates followed by their precipitation thanks to the heat available in this region of the weld. These heat and precipitation cycles enable hardness levels close to the base metal hardness as reported by [START_REF] Mahoney | Properties of friction-stir-welded 7075 T651 aluminum[END_REF].

In the HAZ, the thermal cycle reaches a lower range of temperatures resulting in precipitation coarsening. This temperature range is called "critical temperature range" and result in MHR. According to the thermal results presented in Fig. 15 and Table 6, the critical temperature range for this alloy can be determined: from 225 to 260 °C. In the literature, Ma and den Ouden (1999) identified a slightly higher critical temperature range in the case of AA7020-T6 arc welding (from 260 to 320 °C) whereas [START_REF] Mahoney | Properties of friction-stir-welded 7075 T651 aluminum[END_REF] proposed a higher critical temperature range from 300 to 350 °C in a FSW study of AA7075-T651. These higher critical temperature ranges could be due to a shorter exposure time. In fact, the MHR is not only governed by a range of temperatures but also by the exposure time to these temperatures. The thermal cycles decreasing the hardness vary from one welding condition to another as the features of the welds change, such as the distance from the welding line to the MHR or the width of the MHR, which also change from one condition to another.

As the advancing speed increases, the MHR approaches the center line and so does the critical temperature range. This is due to the fact that, as the advancing speed increases, for a given time period, the same amount of heat is spread over a larger material surface. The resulting cooling rate of the weld is higher as the base material acts as a heat sink, and the critical temperature range moves closer to the center line of the weld. The advance speed plays the leading role in the heat input; in comparison, the rotating speed, and therefore the speed ratio R, do not have any significant effect.

In addition to their positions with respect to the welding line, the MHRs differ from one condition to another: their minimum values and their widths vary. Based on the thermal measurements performed, their characteristics are directly dependent on the time of exposure to the critical temperature range. The higher the exposure time, the lower the minimum value reached and the wider the MHR and vice versa. As a result, the BW03 (hot) condition has the widest MHR and the lowest minimum hardness values compared to the other conditions. Conversely, condition BW06 (cold) has the lowest MHR and the highest minimum hardness value. Interestingly, these MHR characteristics are not influenced by the peak. As thermal measurements have been made only for conditions BW03 and BW06, the conditions that control the MHR characteristics cannot be verified for all conditions. However, it can be observed on Fig. 9 and Table 3 that the width of the MHR decreases as the advancing speed increases with a minor contribution of the rotation speed. In fact, as the advancing speed increases, the welding process is faster and the exposure time to the critical temperature range decreases; thus, decreasing the width of the MHR. However, there is no specific relationship between the width of the MHRs and the speed ratios R.

To summarize, the thermal measurements show that reaching a temperature that is within the critical temperature range, i.e. from 225 to 260 °C, generates the MHR while the exposure time influences the width and the minimum value of the MHR. The position of the MHR, as well as the exposure time, are mainly influenced by the advance speed of the tool which therefore controls the heat input.

The fact that the width of the MHR is significantly reduced when the temperature of the weld decreases is also an interesting trend as reducing the width of the low MHR may improve the mechanical property by notch strengthening. Indeed, reducing the width of the MHR may reduce the stress triaxiality in this region during the tensile test and could increase the tensile properties of the joints. However, the mechanical results in Fig. 10 and Table 5 show that the mechanical properties do not significantly change with the MHR widths. This suggests that the width of the MHR obtained in the present work may not be small enough to benefit from this mechanical strengthening. Targeting lower heat exposure during the welding process by using faster advancing speeds for instance may further improve the mechanical properties of the joints. This could improve the mechanical properties of the joints produced in two ways: by reducing the width of the MHR further or/and increasing the lower hardness value in the MHR. However, the excessive increase of the speeds leads to the instability of the process that produces defects in the welded joint. In this case, a lack of cohesion at the interface between the two materials in the nugget, itself due to a lack of metallurgical bonding, can be identified for the high speed condition (BW07) and leads to a fracture at the same place. Therefore, the elongation has been halved.

Conclusions

• The welding parameters have globally no influence on the tensile behavior. A satisfactory minimum efficiency of 88% regarding the TS and 77% regarding the 0.2% offset yield strength is reached.

• In the higher advancing and rotation speed condition, a weakness regarding TS and elongation was found; this was attributed to insufficient mixing of the material at the root of the joint due to a too small pin size. • The highest the advancing speed is, the smaller the width of the MHR is and the closer it is to the nugget region. The exposure time to the critical temperature ranges is also mostly dependent on the advancing speed suggesting that with the present tool it is the main parameter to control the heat input.

• The regions whose hardness is below 110 HV 0.05 are located in the HAZ of the AA7020 side for all conditions. The origin of this weak zone was primarily linked to the precipitates coarsening in the base metal due to an exposure to temperatures in the range from 225 to 260 °C. The exposure time to this critical temperature range could also play a role in controlling the hardness decrease and the width of the MHR.

• The fractures in the MHR on the AA7020 side are similar for all joints (dimples and transgranular fracture). In the higher advancing and rotation speed condition, specimens failed in the nugget most likely because of a lack of cohesion at the interface between the two materials in this area. This lack of cohesion is certainly due to an instability of the process for high welding speed and a lack of material mixing in this region which leads to a lack of metallurgical bonding. It could be avoided by changing the tool geometry and increasing the length of the pin for higher welding speed conditions.

Fig. 1 .

 1 Fig. 1. (a) FSW setup and material orientation for the dissimilar welding trials (RD: rolling direction, TD: transverse direction); (b) 5 mm FSW tool with Triflat ™ pin and scrolled shoulder used for the welding trials.

Fig. 2 .

 2 Fig. 2. Pseudo-3D optical micrographs of base metals for: (a) AA2139-T8 and (b) AA7020-T651.

Fig. 3 .

 3 Fig. 3. Tensile specimen geometry used for both base materials and welded joints (all dimensions in mm).

Fig. 4 .

 4 Fig. 4. Position of thermocouples (dimensions in mm) regarding the joint center line for thermal measurements.

Fig. 5 .

 5 Fig. 5. Optical macrographs (CWD-ND plane) matrix for AA2139-T8 / AA7020-T651 configuration.

Fig. 6 .

 6 Fig. 6. Top view of weld surface with defect formation due to inappropriate welding parameters.

  Fig. 7. BW03 welding condition: (a) optical detailed macrograph of the cross-section revealing the various zones inside the joint, (b) zooms on the different specific welding areas.

Fig. 8 .

 8 Fig. 8. Microhardness profiles at mid-thickness for the five welding conditions. Horizontal dot-lines show base material hardness while vertical green and red lines are the two extreme positions of the MHR function of the welding parameters. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 9 .

 9 Fig. 9. Microhardness maps (HV 0.05 ) for the five conditions inside the process window.

Fig. 10 .

 10 Fig. 10. Stress-strain curves for base metals and the five welding conditions.For each condition, one curve among the 3 performed was chosen (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 11 .

 11 Fig. 11. Fracture analysis for the BW03 welding condition: correlation between the position of fracture and the microhardness map.

Fig. 12 .

 12 Fig. 12. SEM fracture surface features at several magnifications. (a, b, c) welded joint (condition BW03): mixed mode fracture composed of dimples initiating from small intermetallic particles and transgranular fracture from elongated grains, (d, e, f) AA7020 base metal -dimples close to each other and only elongated grain features.

Fig. 13 .

 13 Fig. 13. Fracture on a BW07 sample failed in nugget: (a) Position of area "(c)", (b) Cross weld section with green dotted line for the interfacial fracture and red dotted line for the non-interfacial fracture, (c) Fracture surface revealing homogeneous SANZ and periodic PANZ. In PANZ, two regions can be found according to surface morphology (zones I and II), (d, e, f, g and h) Zoom of various areas on the fracture surface in (c).

Fig. 14 .

 14 Fig. 14. (a) Bottom (CWD-WD) and (b) Longitudinal (WD-ND) views of the BW07 welding condition showing the patterns obtained etching. Darker material corresponds to AA2139 and light grey material to AA7020.

Fig. 15 .

 15 Fig. 15. Average mid-thickness thermal measurements measured during welding around the MHRs with locations of the thermocouples: (a) Condition BW03 (hot), (b) Condition BW06 (cold).

Table 1

 1 Chemical composition of 2139-T8, and 7020-T651 aluminum alloys, wt.-%.

	Alloy	Zn	Mg	Fe	Si	Cu	Ag	Cr	Mn	Ti	Ni	Al
	2139-T8 7020-T651	0.041 4.0-5.0	0.43 1.0-1.4	0.07 0.40	0.031 0.35	4.92 0.2	0.33 -	0.0022 0.10-0.35	0.31 0.05-0.5	0.12 -	0.005 -	Bal. Bal.

Table 2

 2 Sample references, welding parameters, and speed ratios for the various AA2139-T8 / AA7020-T651 configurations.

	Samples	Rotation speed, rev. min -1	Advance speed, mm. min -1	Speed ratio R, mm.rev -1
	BW03 BW04 BW05 BW06 BW07	600 600 800 800 1000	250 400 400 550 550	0.42 0.67 0.5 0.69 0.55

Table 4

 4 Room temperature tensile properties of the two base metals.

	Material	YS 0.2 , MPa	TS, MPa	Elongation, %
	AA2139-T8 (RD) AA7020-T651 (RD)	505 395	529 445	15.6 11.5

Table 5

 5 Room temperature tensile properties and efficiencies (compared to AA7020-T651 RD) of welded samples for two different natural aging times. Average value represents average of test results of three samples.

	Samples	YS 0.2 , MPa	TS, MPa	Necking strain %	Elongation, %	Eff. RP 0.2 , %	Eff. TS, %	Eff. Elong., %
	BW03 BW04 BW05 BW06 BW07	305 ± 0 308 ± 0 306 ± 0 310 ± 1 308 ± 1	417 ± 1 425 ± 0 422 ± 0 427 ± 0 389 ± 15	6.8± 0.1 6.1± 0.1 6.2± 0.3 6.8± 0.4 3.5± 0.5	7.2 ± 0.1 6.4 ± 0.4 6.6 ± 0.4 7.2 ± 0.7 3.7 ± 0.4	77 78 78 79 78	94 96 95 96 88	63 55 57 63 28

Acknowledgements

The authors are particularly grateful to the Ministry of Economy, Science and Innovation from Quebec government for the grant attributed via the Research Support Program (PSR). This work was part of a research program supported by the Aluminum Research Centre -REGAL. A part of this work was also supported by the financial support of the French government defense agency, Direction Générale de l'Armement (DGA).

Data availability

The raw and processed data required to reproduce these findings are available to download from Mendeley Data: https://doi.org/10.17632/ 4gc86ddp6s.2.

Table 6

Average peak temperatures measured during welding around the MHRs for conditions BW03 (hot) and BW06 (cold).

Distance from the center line (mm) BW03, hot (600 rev. min -1 / 250 mm.min -1 , R = 0.42 mm.rev -1 ) BW06, cold (800 rev. min -1 / 550 mm.min -1 , R = 0.69 mm.rev


Bhavana Gupta

Bertrand Goudeau

Patrick Garrigue

Alexander Kuhn
email: kuhn@enscbp.fr

Bipolar Conducting Polymer Crawlers Based on Triple Symmetry Breaking

Keywords: Bipolar electrochemistry, conducting polymers, actuation, artificial muscle, robotics

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Introduction

In recent years many efforts have been devoted by the scientific community to explore new mechanisms allowing objects to transform energy from the surrounding medium into motion.

Prominent examples are micromotors or microswimmers. [1] The common point of all these approaches is that symmetry needs to be broken in one way or the other in order to generate directed motion, [2][3][4][5][6] thus leading to systems which can be used for various applications ranging from biosensing and drug delivery to environmental remediation. [7][8][9][10] In many of these examples locally modified electrokinetics are responsible for the motion, [6,[11][12][13][14] but also other energy sources such as light or magnetic fields have been studied in this context. [15][16][17] One concept which has emerged as an efficient approach to break the symmetry of chemical systems in a straightforward way, and thus to induce either directly or indirectly motion, is bipolar electrochemistry (BPE). [13] With BPE, redox reactions can be performed in a wireless way on the surface of conducting or semi-conducting objects when they are placed in an electric field between two feeder electrodes. [18][START_REF] Bouffiar | Electroanalytical Chemistry: A Series of Advances[END_REF][START_REF] Fosdick | [END_REF][21] We've recently proposed BPE to actuate conducting polymers in a wireless way. [22] In this case actuation is due to a differential change in volume as a consequence of ion movement in and out of the polymer upon its localized oxidation and reduction. This leads to an artificial muscle type behavior, which has already been explored previously with various conducting polymers such as polypyrrole (PPy), polyaniline (PANI) and poly (3, 4 dioxythiophene) (PEDOT) by classic electrochemical addressing. [23][24][25][26] Since such polymeric films can be obtained as freestanding films, different types of devices have been elaborated, able to move in specific directions. [27][28][29] However, all these devices require a direct connection to an external power supply. BPE is able to circumvent this problem because electrochemical reactions can be carried out on an object without the need of a physical connection to a power supply. In this contribution we suggest to use electrochemistry as the basis for a triple breaking of symmetry:

-morphological asymmetry generation during the synthesis of a freestanding polymer film -polarization asymmetry due to the presence of an electric field -mechanical strength asymmetry obtained via differential post-synthesis reactions In fine, the combination of these three different types of asymmetry allows a conducting polymer object to perform directed crawling motion.

Results and discussion

In order to achieve efficient directed crawling of the conducting polymer object, it is crucial to break the symmetry of the system in three distinct ways. The first type of introduced asymmetry concerns the morphology of the film ensuring its inhomogeneous response to the surrounding environment. [29,30] Thus, we first prepared robust polypyrrole films of different thickness for which the surface morphology at the two faces is different. The film surface which is facing the electrolyte during the synthesis is found to be rough and porous, whereas the opposite side (in contact with the gold substrate) is smooth and compact as shown in Figure 1a and b respectively. As the movement of charge compensating ions into and out of the polymer film strongly depends on the roughness of the polymer, the rough face is more appropriate for ion exchange than the smooth one. Therefore, the as-synthesized film is a good and intrinsic analogue of a classic bilayer film, often used for actuation.

The second symmetry break is related to the unequal incorporation of counterions through the two interfaces when bipolar reduction and oxidation occurs at the two extremities of the object (Figure 1c). The oxidized form of the polymer, as it is obtained during the synthesis without any post-treatment, is rather stiff due to the inter-chain interactions or cross-linking due to the anions immobilized in the polymer matrix. Reduction of the conducting polymer causes either the removal of anions from the polymer or an insertion of cationic species into the polymer matrix in order to maintain charge neutrality. Which pathway is used for charge compensation depends on the size of the anion that is present during the polymerization. If the anions are small, usually they will leave the polymer during reduction and get reintegrated during the oxidation. Thus, reduction leads to shrinkage and oxidation results in swelling.

However, for large anions, like in the present case where we employ bulky dodecyl benzene sulfonate, an opposite effect is encountered. During reduction, cations move into the polymer and induce a volume increase (Scheme 1), whereas upon oxidation cations are released and cause shrinking. [18,[START_REF] Bouffiar | Electroanalytical Chemistry: A Series of Advances[END_REF] Scheme 1 Electroactivity of polypyrrole during reduction (top line) and oxidation (bottom line)

Cations are crossing the polymer/electrolyte interface preferentially at the rough face of the polymer strip due to the significantly higher active surface area. Thus, the rough face tends to swell and the polymer strip is bending upwards (Figure 1d, left side). In contrast to that, the part of the polymer which is facing the negative feeder electrode is undergoing an oxidation reaction. Therefore, cations are expelled through the rough face due to its higher porosity, thus triggering a more pronounced shrinking. Consequently, the polymer strip tends to bend downwards (Figure 1d, right side). first order approximation of the potential gradient in the bipolar cell, which in reality is not linear close to the feeder electrodes and around the bipolar object.

Introduction of a third type of asymmetry is possible if the conducting polymer is already completely oxidized before the bipolar experiment. In this case, further oxidation at the positively polarized extremity during the first polarization step will lead to over-oxidation and a partial degradation of the polymer at this location. The polymer becomes less electroactive and less suitable for ion exchange at this extremity, thus leading to weaker bending. To confirm this third break of symmetry between the two extremities, scanning electron microscopy images of the polymer strip have been recorded after the bipolar experiment (Figure 2). The end of the polymer strip which was facing the negative feeder electrode during the first bipolar cycle shows an extremely different morphology compared to the part of the polymer which was facing the feeder anode. This asymmetry can even be seen at a macroscopic scale (Fig 2a). The polymer extremity participating in the first oxidation reaction shows cracks on the surface (Figure 2b), and their density gradually decreases when going towards the extremity that participated in the reduction reaction. The latter one has almost the same smooth microscopic morphology than the as-synthesized polymer support (Figure 2c), indicating the intrinsic stability of the polymer even after many switching cycles. This additional third asymmetry is a crucial ingredient for the directed motion of the polypyrrole film, because it allows inhomogeneous bending and relaxation.

Most importantly, the bipolar actuation is switchable. After changing the polarity of the feeder electrodes, the direction of polymer bending is reversed. This feature is extremely important for directional motion, however reversibility alone is not enough as it would lead only to a stationary periodic actuation. As mentioned above, the additional requirement for inducing net motion in a specific direction is that the bending of the two extremities during the bipolar electrochemical process is unequal. In the case of an initially fully oxidized polymer, the over-oxidation of the polypyrrole extremity facing the negative feeder electrode during the first polarization cycle introduces this additional asymmetry. The polymer at this extremity undergoes a partial degradation (vide supra), which makes it stiffer and therefore distinguishable from the opposite extremity. At the same time the effective length of the polymer object which is experiencing the electric field is decreased during this first cycle due to the over-oxidized section. Thus, for the subsequent cycles the overall polarisation of the object is less and no over-oxidation occurs at the opposite extremity when this one gets positively polarized. The resulting unequal mechanical properties of the two ends of the polymer strip enable now a net motion in the direction of the polymer extremity which was initially facing the positive feeder electrode, because this end shows stronger bending compared to the over-oxidized end which is stiffer. This mechanism is schematically illustrated in Figure 3. An as-synthesized fully oxidized polymer strip (10x1 mm) is positioned in the centre of the bipolar electrochemical cell at the bottom in such a way that the smooth surface is facing upwards and the rough surface downwards. A potential difference of 20 V is applied between the graphite feeder electrodes placed 5 cm apart from each other. The maximum potential difference V that might be experienced by the two extremities of the polymer strip can be extracted from the following equation: (1) with E being the potential difference between the feeder electrodes, L the distance between the feeder electrodes (5cm) and l the length of the polymer strip (1cm). This leads to a theoretical potential difference of 4V between the two ends of the polymer. However, there are various sources of potential loss in the bipolar cell such the resistance created by the Nafion membranes, which are separating the electrode compartments, and an important potential drop at the feeder electrodes. In order to estimate the real potential drop along the polymer object, electrical connections have been attached at its two extremities and their potential difference was measured while applying 20 V to the feeder electrodes. A typical V value of 1.4-1.6 V was measured in this case. These values are high enough to allow the simultaneous reduction and oxidation of polypyrrole when comparing with a typical cyclic voltammogram of such a polymer film. The extremity of the strip which is facing the positive feeder electrode participates in the reduction reaction, leading to the insertion of cations during the first bipolar cycle. When switching the polarity after 5 s, the polymer extremity, which previously underwent the reduction reaction, is now oxidized, accompanied by the expulsion of cations. The swelling, which was a result of the reduction reaction in the previous cycle, allows a faster expulsion of the cations during oxidation of this extremity.

Hence, the downward bending occurs faster. The previously over-oxidized polymer extremity now undergoes a reduction reaction, but an overall upward bending is not observed due the over-oxidized state of the polymer which introduces irreversible morphological and mechanical changes (vide supra).

In conventional electrochemistry the amplitude of bending depends on thickness. The thickness of the polymer should not be too high because otherwise it resists to bending due to its stiffness, but should also not be too low because otherwise the ion movement is not sufficient to generate enough volume change for efficient bending. [23,31] Therefore, it is crucial for obtaining a bipolar crawler to optimize the thickness of the polymer. In order to illustrate the importance of this parameter we synthesized polymer films with different thickness and studied their dynamic behavior. As an example, we compare here two films which have been obtained after the same reaction time (2 hours), but at different potentials i.e. 0.7 and 0.8 V. Their thickness is 60 and 85 μm, respectively. The bending of the latter one is less pronounced (Figure 4b) compared to the first one (Figure 4e). In addition to the difference in thickness, also another parameter might be influencing the difference in bending amplitude. The polymerization kinetics is slower in the case of 0.7, V which leads to the formation of shorter polymer chains with a smaller degree of cross-linking and therefore higher flexibility. [32] Furthermore, more positive oxidation potentials might also lead to a deeper degradation-cross-linking gradient in the thicker film obtained by electropolymerization at a more anodic (0.8 V) potential than the thinner one (0.7 V). [33] The characteristic response time to a change in polarity of the feeder electrodes is also faster in the case of films obtained at 0.7 V. All these phenomena increase the ability to produce more pronounced crawling motion with the thinner film. Red: positive feeder electrode; blue: negative feeder electrode; gray: no potential at the feeder electrodes. The scale bar in the SEM images is 100 μm.

When comparing the crawling behavior of both polymer strips upon potential switching it becomes obvious that the thinner film shows a more efficient motion. The fast switching and strong bending in the case of the thin film is responsible for the directional crawling illustrated in Figure 5. The polymer is travelling over a distance of ~4 mm when switching the orientation of the electric field 25 times (see Supplementary information video S1). The speed of motion can be modulated by changing different parameters like salt concentration, thickness of the polymer, applied voltage and switching frequency. Therefore, it is rather difficult to make a direct correlation between the applied voltage and the final crawling velocity.

Conclusion

We explored the possibility to use bipolar electrochemistry for triggering a crawling motion of a thin polypyrrole film. The synergetic combination of three different types of symmetry breaking is used to induce global locomotion of the object. An initial asymmetric surface roughness of the synthesized polymer allows preferential ion exchange on one side of the film, which in this way constitutes an intrinsic bilayer architecture. The second source of asymmetry is the polarization of the polymer object by the electric field present in solution.

This allows carrying out a reduction reaction at one extremity of the film, whereas the opposite extremity undergoes an oxidation. Finally, the third break of symmetry originates from an irreversible electrochemical transformation of one end of the film due to its overoxidation during the first polarization cycle. This makes the two extremities distinguishable in terms of mechanical behavior in the subsequent polarization cycles and thus leads to directed crawling motion during reversible bending and relaxation. The main advantage of the here presented proof-of-concept experiments is that, in contrast to classic electrochemical actuation of conducting polymers, the movement can be induced in a wireless way. This, combined with a future downscaling of the object's size, opens very interesting perspectives in the frame of polymer based artificial muscles analogs with advanced functionalities.

Experimental Section

Synthesis of polypyrrole

Polypyrrole was synthesized by potentiostatic polymerization in aqueous medium following a literature protocol. [29] A three electrode electrochemical cell (Reference electrode: Ag/AgCl, counter electrode: gold coated glass, working electrode: gold coated glass) was used for the polymerization. 0.2 M pyrrole (Sigma Aldrich) and 0.2 M sodium dodecylbenzene sulfonate (Sigma Aldrich) was dissolved in 10 ml of distilled water. Parallel orientated gold coated electrodes (1.2 x 1.5 cm) were kept at a distance of 2.5 cm. For generating thin and thick polymer films a potential of 0.7 and 0.8 V was applied for polymerization during 2 hours, respectively. After completion of the polymerization process, the polymer was washed thoroughly with distilled water, dried under a N 2 stream and finally peeled off the surface using a sharp blade. For the partial reduction of the polypyrrole film, the polymer was immersed in monomer free 0.1 M sodium dodecylbenzenesulfonate solution and -1 V was applied for 1 hour vs Ag/AgCl. After completion of the reduction process, the film was washed in water, dried and peeled off the working electrode with the help of a sharp blade.

The as-synthesized or partially reduced polypyrrole films were used for the subsequent studies.

Characterization

The surface morphologies of the prepared films were characterized using a tabletop electron microscope (Hitachi, TM-1000) after Au sputtering on the polymer surface to reduce electric charging.

Wireless bipolar motion

Wireless bipolar motion was accomplished in a bipolar cell equipped with two graphite electrodes (1 cm diameter) separated by a distance of 5 cm. The rectangular polymer film (10 x 1 mm 2 ) was kept in the center (laying at the bottom) of the bipolar cell filled with 0.5 M LiClO 4 . A potential of 20 V was applied between the two graphite electrodes, and the polarity of the electrodes was switched every 5 seconds. To avoid parasitic effects of gas bubbles generated at the feeder electrodes, the center part of the bipolar cell containing the polymer strip is separated by two Nafion membranes. Phenomena of bending and generated locomotion were recorded with a macroscope (LEICA Z16 APO) in the video mode. Several dozens of polymer strips were characterized with respect to their dynamic behavior, and in all cases they behave similarly from an qualitative point of view. However, one can observe some variations in the efficiency of motion from one polymer strip to another, because thicknesses and defects are not necessarily completely identical.

Figure 1 a

 1 Figure 1 a) Top face (facing the electrolyte during polymerization), and b) bottom face

Figure 2 a

 2 Figure 2 a) Low magnification SEM image of the polymer strip after bipolar switching at 20

Figure 3 a

 3 Figure 3 a) Asymmetric shape of the polypyrrole strip during potential switching. b)

Figure 4 a

 4 Figure 4 a) Thick polypyrrole film in the absence of electric field b) in the presence of electric field after the 8 th bipolar cycle and c) corresponding SEM image of the film section.

Figure 5 :

 5 Figure 5: Different stages of polypyrrole film deformation during crawling motion a) before

Acknowledgements

This work was supported by the 2016 Post-Doctoral Fellowship Program of the Initiative of Excellence IdEx Bordeaux. A.K. acknowledges financial support from the Institut Universitaire de France. This project has also received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement n° 741251, ERC Advanced grant ELECTRA).


Lin Zhang

Bhavana Gupta

Bertrand Goudeau

Nicolas Mano

Alexander Kuhn

Wireless electromechanical readout of chemical information

Collecting electrochemical information concerning the presence of molecules in a solution is usually achieved by measuring current, potential, resistance or impedance via connection to a power supply. Here, we suggest wireless electromechanical actuation as a straight-forward readout of chemical information. This can be achieved based on the concept of bipolar electrochemistry, which allows measuring the presence of different model species in a quantitative way. We validate the concept by using a free-standing polypyrrole film. Its positively polarized extremity participates in an oxidation of the analyte and delivers electrons to the opposite extremity for the reduction of the polymer. This reduction is accompanied by the insertion of counter ions and thus leads to partial swelling of the film, inducing its bending. The resulting actuation is found to be a linear function of the analyte concentration and also a Michaelis-Menten type correlation is obtained for biochemical analytes. This electromechanical transduction allows an easy optical readout and opens up very interesting perspectives not only in the field of sensing, but also far beyond, such as for the elaboration of self-regulating biomimetic systems.

INTRODUCTION

Conducting polymers are popular candidates to be used as smart materials for the transduction of chemical information, light emission, energy storage, photovoltaics and actuation due to their easy synthesis and interesting electrochemical properties. [START_REF] Aydemir | Conducting polymer based electrochemical biosensors[END_REF][START_REF] Naseri | Recent progress in the development of conducting polymer-based nanocomposites for electrochemical biosensors applications: A Mini-Review[END_REF][START_REF] Zhou | Conducting polymer-based catalysts[END_REF][START_REF] Ibanez | Conducting poly-mers in the fields of energy, environmental remediation, and chemicalchiral sensors[END_REF][START_REF] Ishiguro | Site-controlled application of electric potential on a conducting polymer "canvas[END_REF] Most importantly, their π-conjugated system allows charge transport as well as fast and reversible redox activity, and therefore makes them suitable as amperometric, potentiometric, conductometric and impedometric transducers. [START_REF] Ibanez | Conducting poly-mers in the fields of energy, environmental remediation, and chemicalchiral sensors[END_REF][START_REF] Lindfors | Potassium-selective electrodes with stable and geometrically well-defined internal solid contact based on nanoparticles of polyaniline and plasticized poly(vinyl chloride)[END_REF][START_REF] Vanamo | Instrument-free control of the standard potential of potentiometric solid-contact ion-selective electrodes by shortcircuiting with a conventional reference electrode[END_REF][START_REF] Guziński | Solid-contact ion-selective electrodes with highly selective thioamide derivatives of p-tert-Butylcalix[4]arene for the determination of lead(II) in environmental samples[END_REF] In all these cases the polymer needs to be connected to a power supply or an electric measuring device. Therefore, it is of great interest to explore alternative approaches using a wireless concept. As an example, conductometric wireless polyaniline sensors have been developed recently based on near field communication (NFC) technology. [START_REF] Ma | Highly sensitive, printable nanostructured conductive polymer wireless sensor for food spoilage detection[END_REF] Typical features of conducting polymers such as color and volume change can equally be used as ingredients to develop sensors or biosensors. [START_REF] Ma | Highly sensitive, printable nanostructured conductive polymer wireless sensor for food spoilage detection[END_REF][START_REF] Martinez | Biomimetic dual sensing-actuators: theoretical description. sensing electrolyte concentration and driving current[END_REF][START_REF] Gicevičius | In optical sensors based on electrochromic conducting polymers[END_REF] In this context bipolar electrochemistry (BPE) is an interesting alternative to trigger in a wireless way a change of the above mentioned physical properties of the conducting polymer. [START_REF] Inagi | Bipolar Patterning of Conducting Polymers by Electrochemical Doping and Reaction[END_REF][START_REF] Inagi | Fabrication of gradient polymer surfaces using bipolar electrochemistry[END_REF] BPE is a straight-forward way to break the symmetry of chemical systems and has been used in the past for many applications in various fields ranging from materials science to catalysis and analysis. [START_REF] Fosdick | Bipolar electrochemistry[END_REF][START_REF] Koefoed | Bipolar electrochemistry-A wireless approach for electrode reactions[END_REF][START_REF] Loget | Bipolar electrochemistry: from materials science to motion and beyond[END_REF] It has been employed, among others, to modify conducting or semiconducting objects, [START_REF] Loget | Bipolar electrochemistry: from materials science to motion and beyond[END_REF][START_REF] Tiewcharoen | Anisotropic metal deposition on TiO2 particles by electric-fieldinduced charge separation[END_REF][START_REF] Yadnum | Site-selective synthesis of janus-type metal-organic framework composites[END_REF][START_REF] Loget | True bulk synthesis of janus objects by bipolar electrochemistry[END_REF][START_REF] Bouffier | Recent advances in bipolar electrochemistry[END_REF][START_REF] Loget | Propulsion of microobjects by dynamic bipolar self-regeneration[END_REF][START_REF] Bouffier | Electric fields for generating unconventional motion of small objects[END_REF] allowing an easy access to Janus particles [START_REF] Yadnum | Site-selective synthesis of janus-type metal-organic framework composites[END_REF][START_REF] Bouffier | Recent advances in bipolar electrochemistry[END_REF] , to trigger electrochromism, photoluminescence and electroluminescence in conducting polymers [START_REF] Inagi | Bipolar Patterning of Conducting Polymers by Electrochemical Doping and Reaction[END_REF][START_REF] Inagi | Fabrication of gradient polymer surfaces using bipolar electrochemistry[END_REF][START_REF] Hu | Visualizing the bipolar electrochemistry of electrochemically doped luminescent conjugated polymers[END_REF][START_REF] Hu | Wireless electroluminescence: polymer lightemitting electrochemical cells with inkjet-printed 1D and 2D bipolar electrode arrays[END_REF][START_REF] He | Enhancing the overall performances of blue light-emitting electrochemical cells by using an electroninjecting/transporting ionic additive[END_REF] and to generate directional motion or forced propulsion of particles. [START_REF] Loget | Propulsion of microobjects by dynamic bipolar self-regeneration[END_REF][START_REF] Bouffier | Electric fields for generating unconventional motion of small objects[END_REF] We recently reported the use of BPE also for the wireless actuation of polypyrrole (PPy) based on an asymmetric volume change. [START_REF] Gupta | Wireless electrochemical actuation of conducting polymers[END_REF] An electric field, generated in the electrolyte, breaks the symmetry of the PPy film due to its polarization with respect to the solution. This polarization results in spatially separated oxidation and reduction reactions, located at the two ends of the polymer object. Together with an additional intrinsic asymmetry, related to the different morphologies of the two faces of the film, this allows a directional bending and motion of the PPy film. [START_REF] Gupta | Wireless electrochemical actuation of conducting polymers[END_REF][START_REF] Gupta | Bipolar conducting polymer crawlers based on triple symmetry breaking[END_REF] We demonstrate here with several proof-of-principle experiments that this wireless mechanical deformation of a freestanding polymer layer can be directly correlated in a quantitative way with the concentration of different model molecules such as hydrogen peroxide, hydroquinone and glucose. BPE has been already proposed as an attractive approach to collect information about the chemical composition of a solution. Seminal work by the Crooks group demonstrated that the presence of an analyte can be coupled to either the dissolution of a metal [START_REF] Chow | A sensing platform based on electrodissolution of a Ag bipolar electrode[END_REF][START_REF] Fosdick | Bipolar electrodes for rapid screening of electrocatalysts[END_REF][START_REF] Chow | A large-scale wireless electrochemical bipolar electrode microarray[END_REF] or the generation of electrochemiluminescence (ECL). [START_REF] Chow | Wireless electrochemical DNA microarray sensor[END_REF] The latter concept could also be generalized to ECL emission in the bulk [START_REF] De Poulpiquet | Dual-color electrogenerated chemiluminescence from dispersions of conductive microbeads addressed by bipolar electrochemistry[END_REF][START_REF] De Poulpiquet | Dual enzymatic detection by bulk electrogenerated chemiluminescence[END_REF] or from moving particles. [START_REF] Sentic | Electrochemiluminescent swimmers for dynamic enzymatic sensing[END_REF] Various other interesting approaches have highlighted the outstanding features and advantages of BPE in the context of transduction of chemical information. [START_REF] Bouffier | Imaging redox activity at bipolar electrodes by indirect fluorescence modulation[END_REF][START_REF] Zhang | Bipolar electrodes with 100% Current efficiency for sensors[END_REF][START_REF] Eßmann | Linking glucose oxidation to luminol-based electrochemiluminescence using bipolar electrochemistry[END_REF][START_REF] Qi | Wireless electrochemiluminescence with disposable minidevice[END_REF][START_REF] Zhang | An ITO bipolar array for electrochemiluminescence imaging of H2O2[END_REF][START_REF] Wang | Detection of ferrocenemethanol and molecular oxygen based on electrogenerated chemiluminescence quenching at a bipolar electrode[END_REF][START_REF] Wu | Electrochemiluminescence on bipolar electrodes for visual bioanalysis[END_REF][START_REF] Wu | Integrating bipolar electrochemistry and electrochemiluminescence imaging with microdroplets for chemical analysis[END_REF][START_REF] Guerrette | Fluorescence coupling for direct imaging of electrocatalytic heterogeneity[END_REF] However, the possibility of an electromechanical bipolar readout has not been explored so far. The principal of this concept relies again on the decoupling of the sensing chemistry from the reporting functions (Scheme 1). The bipolar object, a sheet of the conducting polymer, is placed between two graphite feeder electrodes. [START_REF] Loget | Bipolar electrochemistry: from materials science to motion and beyond[END_REF][START_REF] Bouffier | Recent advances in bipolar electrochemistry[END_REF] Upon application of a constant electric field, the PPy strip is polarized in an asymmetric way, the extremity with a polarization δ + acting as an anode, whereas the opposite extremity with a polarization δ -behaving as a cathode. This break of symmetry triggers an oxidation and reduction reaction at the two terminal ends of the PPy, with the peculiarity that PPy can participate itself in the redox reactions. [START_REF] Bryan | Conducting polymers for pseudocapacitive energy storage[END_REF] Oxidized PPy does not undergo further oxidation, thus only the extremity involved in the reduction reaction shows actuation behavior. Reduction of the PPy is accompanied by an intake of cations in order to ensure electroneutrality. This movement of ions is responsible for the swelling of the PPy film. However, as the two faces of the PPy strip have a different morphology and surface roughness, their swelling behavior is not identical. The rough side (facing the electrolyte during preparation of the film by electropolymerization) allows a more efficient uptake of cations compared to the smooth side (facing the substrate during electropolymerization). Therefore, the bending occurs preferentially towards the smooth side and is perfectly reversible upon removal or inversion of the electric field. Since at the anodically polarized extremity the oxidation cannot involve PPy itself, the conversion of a redox active molecule from the electrolyte has to ensure charge equilibration. This enabled us to propose the development of a new analytical readout mechanism described in this contribution.

RESULTS AND DISCUSSION

If the experiment depicted in Scheme 1 is carried out in an aqueous medium, water is the most abundant molecule which can be oxidized in order to provide the electrons needed for PPy reduction at the opposite end. The simultaneous, spatially separated, PPy reduction and water oxidation requires a threshold electric field which needs to be applied by the feeder electrodes. For the present set-up the critical experimental value is 1.8V/cm. This is higher than the value calculated from the cyclic voltammogram in Figure S1 A (0.7V/cm), among others due to overpotentials at the feeder electrodes and the bipolar electrode. If the electrolyte contains redox species which can be oxidized more easily than water, actuation will occur already at a lower electric field. Moreover, for an electric field with a given amplitude, actuation increases with increasing concentration of analyte. Therefore, an electric field which is slightly below the threshold value necessary for water oxidation is imposed on the bipolar electrode (<1.8 V/cm). In this case no bending is observed. Only after addition of the more reactive chemical species bending starts to occur and varies as a function of time. In a first set of proof-of-principle experiments we tested hydrogen peroxide and hydroquinone as target molecules to validate the general concept.

Hydrogen peroxide is known for its catalytic and electrocatalytic oxidation on metal surfaces which is easier to achieve than water oxidation. [START_REF] Orozco | Molecularly imprinted polymer-based catalytic micromotors for selective protein transport[END_REF][START_REF] Ding | An on-chip electrical transport spectroscopy approach for in situ monitoring electrochemical interfaces[END_REF] However, since H 2 O 2 can also be reduced at relatively low overpotentials, it is necessary to evaluate the possible competition between the reduction of PPy and H 2 O 2 , which might interfere with the bending mechanism. Cyclic voltammograms of PPy with and without H 2 O 2 have been recorded (Figure S1 A). In the presence of 75 mM H 2 O 2 , higher currents are observed for potentials more positive than +0.4 V compared to the experiment in the absence of H 2 O 2 . This is due to the electrooxidation of H 2 O 2 which occurs before water oxidation. On the other hand, the reduction current is also increased, however without a significant potential shift with respect to the reduction of PPy. Thus, the charge generated during H 2 O 2 oxidation at the positively polarized extremity can be at least partially used for PPy reduction. The favored H 2 O 2 oxidation will help in decreasing the threshold electric field value for bipolar actuation and the bending will depend on H 2 O 2 concentration. To prove this assumption, we performed BPE with different H 2 O 2 concentrations (Figure 1). The minimum concentration required to achieve a significant bending at 1.6 V/cm is 10 mM. In the absence of H 2 O 2 the same amplitude of bending can only be obtained when we apply a field of 1.8V/cm between the two feeder electrodes. In this case a current of 30 µA is passing through the PPy film, which is enough to deliver a sufficient amount of charge at the time scale of minutes to obtain visible bending. When further increasing the H 2 O 2 concentration, the degree of bending at a readout time of 60s increases linearly up to 75 mM (Figure 1C). From these results it is clear that a redox active analyte which is easier to oxidize than H 2 O 2 should be able to decrease even more the threshold potential for actuation. To confirm this idea, hydroquinone has been selected. Similar to H 2 O 2 , the oxidation of hydroquinone is first examined by cyclic voltammetry (Figure S1 B). In the presence of 20 mM hydroquinone, the current starts increasing for potentials more positive than 0.0 V. This indicates that the electrochemical oxidation of hydroquinone is much easier than water or H 2 O 2 oxidation. On the other hand, the reduction of benzoquinone and PPy occur in a similar potential range. The expected threshold of electric field for triggering the actuation in the presence of hydroquinone can be again calculated by cyclic voltammetry (Figure S1 B) and a value of 0.2V/cm is obtained. However, in the bipolar set-up a higher minimum field of 1.2 V/cm is neces-sary for the same reasons as mentioned above. In order to be able to make a comparison between H 2 O 2 and hydroquinone in terms bending efficiency, we've chosen also 1.6 V/cm for the bipolar actuation in the presence of hydroquinone. The fact that hydroquinone oxidation is much easier results in a more pronounced deformation of PPy. A linear dependence of the bending on concentration is obtained from 5 to 20 mM (R 2 =0.998) (Figure S2).

Since H 2 O 2 is the product of many enzymatic reactions and quinones are popular mediators in bioelectrochemical systems, [START_REF] Katz | A Biofuel Cell with Electrochemically Switchable and Tunable Power Output[END_REF] the presented approach might also be useful for biosensing. In such a case, the electromechanical readout would be based on a polymer strip modified with the corresponding enzyme. To validate this idea from a fundamental point of view, glucose has been chosen as a target molecule. As for H 2 O 2 , the intention here is not to propose a new glucose sensor, but to provide a general demonstration that a coupling between an enzymatic reaction and a mechanical deformation is possible. This allows envisioning self-regulating biomimetic systems, based on electrochemical feed-back mechanisms, such as stimuli-responsive valves or intelligent drug-delivery systems. Classical electroenzymatic glucose sensing has been well developed in the past decades. [START_REF] Wang | Electrochemical glucose biosensors[END_REF][START_REF] Zayats | Electrical contacting of flavoenzymes and NAD(P)+-dependent enzymes by reconstitution and affinity interactions on phenylboronic acid monolayers associated with Au-electrodes[END_REF][START_REF] Etienne | Mesoporous materials-based electrochemical enzymatic biosensors[END_REF] For glucose oxidase (GOx), which can oxidize glucose to gluconolactone, a mediator is needed to shuttle electrons between the FAD/FADH 2 redox center of GOx and the electrode surface. In this work, we immobilized the organic charge transfer complex (CTC) tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) as a mediator along with glucose oxidase on the rough face of the PPy strip. [START_REF] Wang | Electrochemical glucose biosensors[END_REF][START_REF] Khan | Design of a stable charge transfer complex electrode for a third-generation amperometric glucose sensor[END_REF] TTF-TCNQ is well known for its redox activity around +0.1V. [START_REF] Calvo-Marzal | Determination of reduced glutathione using an amperometric carbon paste electrode chemically modified with TTF-TCNQ[END_REF] To avoid leaching of the enzymes from the PPy surface, a gelatin layer is casted on top. Following a strategy analogous to what has been reported in the literature, [START_REF] Zhan | Electrochemical sensing in microfluidic systems using electrogenerated chemiluminescence as a photonic reporter of redox reactions[END_REF] it is possible to enhance the current originating from glucose oxidation by increasing the width of the positively polarized enzyme modified extremity (Figure 2A). The 8 times bigger surface area of this anodic extremity thus allows a higher loading with GOx and CTC, which therefore can provide comparatively more charges for the reduction of PPy and improve the actuation amplitude. The GOx-CTC modified side is fixed on a larger support in order to avoid bending purely due to the higher mass of conducting polymer, enzyme, mediator and protection layer. An electric field of 1.3V/cm is used in this case to make sure that bending can only occur when the enzymatic reaction is involved. The deflection resulting from electroactuation of the negatively polarized side increases with increasing glucose concentration at a readout time of 1 min as shown in Figure 2 B. Relative errors have been calculated by measuring the deflection three times and are on average around 5% with an associated detection limit of 5mM. As the intention of this work is not to develop a new type of glucose sensor, the performance of the device has not been optimized, however the range of measurable concentrations is nevertheless compatible with classic values in blood (5-10mM). Electroenzymatic catalysis obviously continues after 1 min, but the direction of deflection starts to reverse. We monitored this peculiar long term evolution of the bending in the presence of 50 mM glucose (see detailed explanation in SI, Figure S3 and Video 1). The maximum deflection at 60 sec has been chosen to analyze quantitatively the actuation as a function of glucose concentration (Figure 3A). It needs to be mentioned that if the oxidation potential of the analyte is close to the one of PPy, a slow degradation of PPy might occur due to its partial overoxidation. In the present GOx-CTC system, since the oxidation potential of CTC (~0.05 V) is much lower than that of PPy (~0.4V), no obvious degradation of PPy was observed. At room temperature, the deflection experiment can be again carried out again with a comparable performance even after 6 h.

In parallel, the classic electrocatalytic response of a GOx-CTC modified PPy film was also evaluated by cyclic voltammetry (Figure 3B). For both calibration curves we observe a typical Michaelis-Menten type behavior. The apparent K M for glucose was estimated to be 33 ± 2 mM with the classic electrochemical readout, compared to 53 ± 3 mM for the bipolar electromechanical readout. Taking into account the completely different measurement concepts, these values are quite comparable and in good agreement with the literature, indicating a K M of 33-110 mM. [START_REF] Swoboda | Purification and properties of the glucose oxidase from aspergillus niger[END_REF][START_REF] Gibson | Kinetics and mechanism of action of glucose oxidase[END_REF]

CONCLUSION

We demonstrate the possibility to achieve a quantitative wireless electromechanical readout of the presence of several chemical species in aqueous solution using free-standing PPy films in a bipolar electrochemical cell. In the case of an initially oxidized PPy strip, its anodically polarized side acts as a center for chemical conversion and the cathodic side for the signal transduction in the form of actuation. Modulating the relative size of the anodic extremity with respect to the cathodic one allows controlling the amplitude of the readout. In a set of proof-of-principal experiments we were able to validate this concept of bipolar sensing for different molecules, including electroenzymatic conversion schemes. The main advantages of the approach are related to the intrinsic features of bipolar electrochemistry, namely a wireless transduction, the possibility of massive parallel screening and low cost. The readout concept might also be extended to the reductive detection of molecules under the condition to employ at least partially reduced PPy. In that case the optical detection should take place at the anodically polarized side due to the oxidation and subsequent bending of the polymer.

The presented straight-forward approach can be generalized to a large variety of chemical or biological analytes, as long as they are easier to oxidize than water. Slightly adapting the device should also enable for example immunosensing, DNA analysis or enantioselective detection. This makes it a promising tool for various applications, not only in the field of sensing, but even for more sophisticated concepts, such as biomimetic electrochemical feed-back systems, allowing the design of stimuli responsive valves, intelligent drug delivery schemes or Boolean logic gates.

EXPERIMENTAL SECTION

Electrochemical synthesis of polypyrrole film. For the synthesis of the polypyrrole film, 25mL of pyrrole (0.2 M) (Sigma Aldrich) are dissolved in Milli Q water along with 0.25 M dodecyl benzene sulfonate (Sigma Aldrich). After complete dissolution of both components, two gold coated glass plates (3 cm x 1.5 cm) are fixed in parallel in the beaker. Ag/AgCl was used as a reference electrode. A fixed current of 7.75 mA is applied for 5200s for the polymerization of pyrrole. After completion of the polymerization, the polypyrrole coated gold surface is washed with distilled water. The polymer is then peeled off from the surface with the help of a sharp knife. The wet polymer film is kept between two glass slides overnight for further use. The polypyrrole surface is characterized using scanning electron microscopy (Hitachi, TM-1000) after Au sputtering the polymer surface to reduce electric charging.

Synthesis of tetrathiafulvalenetetracyanoquinodimethane charge transfer crystals (TTF-TCNQ CTC).

For the synthesis of TTF-TCNQ CTC, TTF (0.1M) and TCNQ (0.1 M) is dissolved in 10 ml acetonitrile in two separate beakers. After complete dissolution, the TTF solution is added to the TCNQ solution under stirring. Black CTC are formed immediately. After 30 minutes of mixing, CTC are washed with acetonitrile followed by diethyl ether. Finally, the resulting CTC are dried in a hood at room temperature for 12 h. After drying the CTC are collected in a closed vial and used for immobilization.

Fabrication of glucose oxidase (GOx) modified polypyrrole. A polypyrrole strip with a size of 1.0 cm x 0.4 cm is cut. 6 mg TTF-TCNQ crystals are grinded in a pastel mortar, then mixed with 6mg of GOx (from Aspergillus niger, Sigma Aldrich) predissolved in 100 μl 0.1 M phosphate buffer (pH 7.0). 10 μl of the mixture are drop casted onto the rough face of one half of the polypyrrole strip (roughly 0.5 cm x 0.4 cm). After complete drying, 2 μl of a preheated gelatin solution (12.5 mg in 250 μl) is casted on top of the enzyme layer. After drying, the width of the unmodified part of the polypyrrole strip is reduced to 0.05 cm. Such a GOx-CTC modified polypyrrole strip is used for glucose sensing in 0.1 M PBS (pH 7.0) by bipolar electrochemistry.

Bipolar electromechanical readout. For bipolar electromechanical readout of hydrogen peroxide and quinone, a pristine polypyrrole strip with a size of 10 mm x 1 mm is positioned at the center axis of the bipolar cell. For the electroenzymatic analysis of glucose, the GOx-CTC modified polypyrrole part is fixed at the top of a holder in the bipolar cell. Two graphite feeder electrodes are placed 5 cm apart on the right and left side of the polypyrrole strip. In all bipolar experiments, 0.1 M PBS (pH 7.0) is used as electrolyte to provide a sufficient amount of ions during the process of bipolar electrochemistry. Different concentrations of hydrogen peroxide, hydroquinone and glucose are added to the buffer solution in order to measure the degree of bending of the polypyrrole strip. Actuation was recorded using a macroscope (LEICA Z16 APO) in video mode.

ASSOCIATED CONTENT Supporting Information

Cyclic voltammograms and deflection pictures of polypyrrole in the analyte medium. Video of polypyrrole actuation in the presence of analyte.

Scheme 1 :

 1 Scheme 1: Schematic illustration of the bipolar electrochemical cell used for electromechanical readout. The free-standing polypyrrole (PPy) film is placed between two feeder electrodes. δ + and δ -indicate the polarization potential at the two extremities. Scanning electron microscopic images of the two faces of the PPy film are shown below. Scale bars: 30 μm.

Figure 1 :

 1 Figure 1: (A) Bipolar electromechanical set-up to measure H 2 O 2 concentration in 0.1 M PBS buffer. Blue and red borders denote the negative and positive feeder electrodes, respectively. (B) Zoom at the negatively polarized PPy extremity in the presence of (a) 10, (b) 25, (c) 50 and (d) 75 mM H 2 O 2 . The green colored PPy strip indicates its initial position and red the position after 60 sec. (C) Calibration curve for different H 2 O 2 concentrations. The electric field between the feeder electrodes is 1.6 V/cm for all measurements.

Figure 2 :

 2 Figure 2: (A) Bipolar electromechanical readout to estimate glucose concentration in 0.1 M PBS buffer using a CTC-GOx/gel modified PPy strip. Blue and red borders denote the negative and positive feeder electrodes, respectively. (B) Zoom of the negatively polarized PPy extremity in the presence of (a) 5, (b) 10, (c) 20, (d), 30 and (e) 50 mM glucose. Green color of the PPy strip represents its initial position and red the position after 60 sec. The electric field between the feeder electrodes is 1.3 V/cm.

Figure 3 :

 3 Figure 3: Calibration curves for (A) bipolar electromechanical readout at t=1min and (B) conventional electrochemical measurement at +0.35 vs Ag/AgCl using a CTC-GOx/gel modified PPy strip as a biosensor. Inset: cyclic voltammograms of PPy/CTC-GOx/gel in the presence of different concentrations of glucose in PBS at a scan rate of 10 mV/s.

ACKNOWLEDGMENTS

This study has received financial support from the French State in the framework of the "Investments for the future" Program, IdEx Bordeaux, reference ANR-10-IDEX-03-02. It has also been funded by the ANR project BIO3 (ANR-16-CE19-0001-03). A.K. acknowledges financial support from the Institut Universitaire de France. The work has also received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement n° 741251, ERC Advanced grant ELECTRA).

AUTHOR INFORMATION

Notes

The authors declare no competing financial interests.