Multisource energy conversion modes in minimally altered plants with soft epicuticular coatings

Living plants have recently been exploited for unusual tasks such as energy conversion 1 – 6 and 10 environmental sensing. 7 – 12 Yet, using plants as small-scale autonomous energy sources 1 – 5 was 11 obstructed by insufficient power outputs for steadily driving even low-power electronics. Moreover, 12 multicable and -electrode installations on the plants made a realization challenging. Here, we show 13 that plants, by a minimal modification of the leaf epicuticular region and by exploiting their intrinsic 14 circuitry, can be transformed into cable-free, fully plant-enabled integrated systems for multisource 15 energy conversion. In detail, leaf contact electrification caused by wind-induced inter-leaf tangency 16 was magnified by a transparent elastomeric coating on one of two interacting leaves for converting 17 wind energy into harvestable electricity. Further, augmentation of the power output is achieved by 18 coupling multi-frequency band radio frequency (RF) energy conversion modes using the same plant as 19 an unmatched Marconi-antenna. In combination, we observed up to 1100 % enhanced energy 20 accumulation respective to single source harvesting and a single plant like ivy could power a 21 commercial sensing platform wirelessly transmitting environmental data. This shows that living plants 22 could autonomously supply application-oriented electronics while maintaining the positive 23 environmental impact 13 by their intrinsic benefits such as O 2 production, CO 2 fixation, self-repair, and 24 many more extremely difficult (if at all possible) to realize in artificial harvesters. 25 Plant-integrated solutions like plant-hybrid sensing platforms 7 – 12 , plant-internal electronic circuits 14,15 , 26 and plant-hybrid robotics 16 , as well as living plant-driven energy harvesting 1 – 3 using wind 5 , rain drops 4 , 27 the root/soil microbiome 17,18

Plant-integrated solutions like plant-hybrid sensing platforms 7-12 , plant-internal electronic circuits 14,15 , 26 and plant-hybrid robotics 16 , as well as living plant-driven energy harvesting 1-3 using wind 5 , rain drops 4 , 27 the root/soil microbiome 17,18 , and sap components 19,20 endow great prospects for connecting plants to 28 man-made digitalized technology and eventually derive new, sustainable concepts to fight climate 29 change. Often, substantial modification of specific species is required like installing specific electrodes, 30 integrated systems like artificial leaves, and multiple cables and maintaining certain conditions (e.g., 31 high humidity, assure enzymatic activity etc.) to realize the plant-hybrid energy harvesting. Plants 32 continuously change their morphology by growing at their apical regions and by exchanging leaves. 33 Hence, preserving complex artificial components in plants is a challenging issue and environmentally 34 questionable on a larger scale. Consequently, approaches that require minimal alteration of the 35 biological component by further integration and elimination of external components as best as 36 possible are required. 37 Our approach consists in modifying plants with the least possible, micrometer-scale variation while 38 rendering them capable to obtain significant electrical outputs using a combination of wind-induced 39 interleaf touching and plant-antenna-based radiofrequency energy conversion. Fig. 1a gives an 40 overview of the plant's simplified electrical structure consisting of the dielectric and purely polymeric 41 cuticular membrane on the leaf surface and the ion-conductive inner cellular tissue and vascular 42 system. A mechanical contact between the cuticle and another material leads to contact electrification 43 generating surface charges that become electrostatically induced into the tissue as detailed in our 44 previous investigations. 1 In this work, we describe for the first time how to fully realize and integrate 45 the mechanical energy conversion in the plant using contacts of two leaves. Given by the well-46 recognized rules of contact electrification 21-23 , two similar materials (M1 = M2) such as two cuticles 47 that touch each other would generate insufficient charges for energy harvesting as they naturally 48 consist of structurally similar lipids and waxes 24 forming a material pair that does not enhance contact 49 electrification. However, by tuning the materials (M1, M2) of the interacting cuticles, higher charging 50 may be achieved and hence we integrated a thin epicuticular dielectric layer of silicone elastomers on 51 the leaf cuticle as one of the best counter materials for efficient contact electrification of plant 52 leaves. 1,3 Fig. 1b illustrates a simplified circuit used to predict the behavior of a coated leaf when 53 transiently touching an uncoated leaf such as during wind fluttering. Fig. 1c shows the voltage 54 amplitude as function of the impedance of the soil (RSoil) between two nearby plants. During transient 55 mechanical contact, the two leaves generate a corresponding alternating current (AC) of opposite 56 polarity. If RSoil is lower than a given threshold, the signals compensate each other through the internal 57 plant-soil circuit and cannot be easily harvested. The equivalent occurs, when two leaves on the same 58 plant touch each other as the intrinsic tissue resistance does not provide sufficient separation of the 59 two electrodes (Fig. 1d). However, at sufficiently high RSoil (>10 8 Ω) the charges generated can be 60 separated and the electricity produced on both leaves can be effectively harvested (Fig. 1c). High RSoil's 61 can simply be achieved by potting plants in isolating pots (common plastic-based pottery), in dry soil, 62 or for example by maintaining long distances between leaves and roots/soil. Moreover, it is expected 63 that multiple leaf pairs generating signals at various frequencies and amplitudes lead to an overall 64 positive power balance (Extended Fig. 2 to affect plant health. This is also supported by the fact that the thin, soft, adaptive, and 83 semipermeable coatings allow high light transmission at wavelength regimes essential for leaf 84 photosynthesis (Fig. 2f). In addition, the coatings on mature leaves (which are fully developed) does 85 not give a substantial mechanical obstruction to further plant growth and development (see new leaf  86 and branch development in 1-year-growth-period Fig. 2c). 87 Fig. 2g and 2h show the strong increase in the voltage amplitudes at given impact force and frequency, 88 respectively after coating when two leaves of two F. microcarpa touch each other. Signals successfully 89 induced in the inner tissue were measured by a single pin-electrode in the stem at a distance of ~15 90 cm of the stimulated leaves. Voltage amplitudes up to 25 V, generated by one leaf pair are produced 91 by combination of coated and uncoated leaves (c-u pair) whereas the contact between uncoated-92 uncoated leaves (u-u pair) show signals in the mV range (details in Extended Data Fig. 1a-c). The 93 experiment reveals a 450-times increase in the generated potential difference for c-u pairs compared 94 to u-u pairs. The current generated by a single c-u pair reaches an amplitude of 0.75 µA (Extended Data 95 Fig. 1d). Fig. 2i shows an arrangement of two plants used to test voltage generation during air-96 flow/wind-excitation. Therein a c-u and a u-u leaf pair were realized at the position where both plants 97 overlap and substantial voltages are obtained expectedly only from c-u pairs (Fig. 2j). Moreover, the 98 output voltage scales with the wind speed bearing a potential for an air-flow sensing capability (Fig.  99 2k). The contact of just two leaves in such a configuration can already charge a 10 µF capacitor in wind-100 susceptive manner (Fig. 2l). For proving that the effect is species-transferable and to analyze a plant species that is especially 118 interesting for urban ecosystems realized by vertical plant growth and greening of buildings 25 , we 119 modified the climber Hedera helix (ivy) with epicuticular coatings on multiple leaves of two 120 intertwining plants to obtain c-u pairs (Fig. 3a) and by coating again the adaxial leaf surfaces with less 121 stomata, Fig. 3b. The generated voltage amplitude of c-u pairs scales with the impact force as well as 122 the area reaching now even up to ~50 V for single leaves ( Fig. 3c and 3d, respectively with 2 cm² contact 123 area and 1 N impact force). The power analysis of the system reveals a peak power at 17 µW/cm² (at 124 1N stimulus and a load resistance of 200 MΩ), which are typical values achieved with plant-hybrid 125 systems 1,5 and many common triboelectric and piezoelectric generators 26-28 in particular at low impact 126 forces. This shows that the combination of the epicuticular coatings and the whole plant as a circuit is 127 competitive despite minimal fabrication effort and can even convert low forces ≤ 1 N into satisfactory 128 electrical energynotwithstanding the expectedly relatively low mechanical-to-electrical energy 129 conversion efficiency at non-optimum conditions (5.6*10^-5 %, for the c-u pair). Indeed, the energy 130 harvested from a single c-u pair is sufficient to supply a commercial temperature sensor with wireless 131 transceiver connected to the plant using the circuit in Fig. 3f. The charging curve of the 136 µF capacitor 132 driving the sensor during c-u pair excitation (1 N, 30 Hz) is depicted in Fig. 3g along with the wirelessly 133 transmitted data and received signal strength. 134 The whole-plant H. helix (Fig. 3h) is also capable of converting wind energy into electricity. showed that the output is force-and contact area-dependent, the system is more efficient, the higher 140 the kinetic energy is that is transferred to the leaf surface instead of being dissipated by mechanical 141 deformation of the leaf and the petiole. Thus, a supporting rigid substrate such as a wall on which the 142 plant climbs (a typical support for H. helix, Extended Data windspeed and reach amplitudes up to ~10 V and ~1 µA, and thus 10 µW peaks, respectively. Although 148 this is sufficient to drive low-power electronics, the power output of solely mechanical energy 149 conversion can be theoretically upscaled using more leaf-pairs due to the contact area-output 150 relationship and by achieving a more effective c-u contact as function of leaf orientations and support. 151 However, at still air and very low wind speeds, the system would not provide the same energy and 152 other effects occurring outdoor like wetting of the leaf surface (during rain periods) will affect the 153 signal transiently even if it recovers after leaf-drying (see effect of leaf-wetting on mechanical energy 154 conversion, Extended Data H. helix on a support panel as shown in k) for exposure to wind. 171 Indeed, a further opportunity for augmenting the overall power output arises, if the plant structure, a 172 branched conductor with dielectric outer surface, would be suitable for RF energy harvesting and 173 plants were previously suggested for radio reception and transmission. 29, 30 We thus analyzed the 174 received spectrum using the H. helix as a receiving antenna in an outdoor environment with ambient 175 RF pollution (Fig. 4a). The power spectrum analysis shows that the plant receives signals at multiple 176 frequencies: 100 to 800 kHz, low and medium frequency radio communication; at 87-107 MHz, ultra-177 high frequency FM broadcast radio stations; at ~800 MHz, likely related to Global System for Mobile 178 Communications (GSM); 2.1-2.2 GHz range, fourth generation (4G) broadband cellular network 179 showing broad range RF reception. We thus slightly modified the harvesting circuit by introducing an 180 earth connection at the rectifier providing a potential difference so that the plant can act as vertical 181 receiver of a Marconi-antenna (Fig. 4b). Indeed, the charging kinetics of a 68 µF capacitor with and 182 without connecting the circuit to two H. helix (height ~120 cm) suggests that the signals received by 183 the plant can be converted and stored without the need of any further modifications of the plant. 184 185 We then exposed the H. helix to a . Even if the conversion efficiency is significantly 210 lower than for tuned RF harvesting systems 33 , the simplicity of realization and marginal material 211 consumption in our case rises the overall sustainability. 212 center frequency of nearby radio emitter), circuit and charging curve of a 68 µF capacitor relative to 216 these signals, respectively indicating potential to transduce RF radiation into electricity. c) Exposure of 217 H. helix in dark and light conditions to a switchable RF source resulting in a received main frequency 218 ~400 kHz-center when the RF source is on, when off, the peak diminishes. frequency, (see spectrum analysis in c)), conversion by c-u H. helix pairs as described in Fig. 3 feeding  230 a 136 µF capacitor. Combination of RF and wind leads to a 550%-increase in the energy output 231 compared to RF as single source. 232 An interesting application scenario is exploiting the plants of urban vegetation for powering battery-233 free environmental sensing. We thus modified a commercial sensor and wireless transceiver by the 234 plant-charging circuit shown in Fig. 4b as exclusive energy source and connected it to a H. helix growing 235 on a pine tree in the front yard of a multifamily residence in a suburb of Pisa, Italy, (Fig. 4d) using the 236 house's ground connection as potential difference. Fig. 4d shows the voltage over the 136 µF capacitor 237 powering the system. Indeed, the plant autonomously powers the sensor platform and the wireless 238 transmission telegrams with temperature and humidity data shown in Fig. 4d. 239 Each measurement requires ~75 µC according to the related voltage drop and the average time 240 required to harvest these charges is about 6 seconds in this configuration. The 136 µF capacitor driving 241 the sensing unit is only powered when the plant is connected to the circuit with a species-dependent 242 charging behavior (Extended Data Fig. 7a-i). Analysis of the received signals, charging dynamics, and 243 spectrum analyses of the multiple species-received signals are given in Extended Data Fig. 7a was controlled by adding/removing the electromagnetic shielding. Fig. 4e shows capacitor charging 257 curves under different conditions. The results confirm that both energy sources can be independently 258 and simultaneously harvested. Moreover, when RF and mechanical stimulation are simultaneously 259 applied, the signals add up leading to faster capacitor charging using the same rectifying circuit. The 260 graph shows that the accumulated sum charges of both sources corresponds to the difference 261 obtained when both sources are used independently. The overall enhancement in the energy 262 accumulated by using the plant to convert the combination of mechanical and ambient RF energy in 263 this experiment corresponds to an 11-times (or 1100%) increase compared to only RF energy 264 harvesting. Likewise, a strong enhancement was also observed when H. helix (120 cm) was exposed to 265 wind and RF radiation. Fig 4f shows

Pre-and post-coating transpiration analysis 299
Whole plant transpiration analysis pre-and post-coating was performed in a transparent, gas tight 300 chamber equipped with internal CO 2 and humidity sensors (SCD30, Sensirion AG, Switzerland, sampling 301 rate 2s) which was placed in a grow room with a 16 hours day and 8 hours night lighting cycle. External 302 CO2 and humidity sensors (SCD30, Sensirion AG, Switzerland, sampling rate 2s) served as reference. 303

Setup for controlled mechanical and air-flow stimulation of c-u and u-u leaf pairs 304
To apply controlled mechanical stimuli between c-u and u-u leaf pairs, a previously described setup 305 consisting

RF sources and controlled exposure 319
During outdoor experiments, the plants were exposed to normal ambient RF radiation from, e.g., radio 320 stations, building supplies, telecommunication etc. For in depth analysis of RF harvesting, the plants 321 were exposed to a controllable, on/off-switchable RF source constituted by the fluorescent lighting 322 system in the plant grow room consisting of 128 fluorescent lamps (MASTER TL5 HO 54W/840, Philips, 323 The Netherlands) that generated a ~390 kHz center frequency RF signal received by the plants during 324 the experiments when turned on. The generated electric field was analyzed in detail in terms of field 325 strength and radiated power as function of distance from the source (Extended Data Fig. 5). Faraday 326 caging made of plain square weave copper mesh with a density of 6.3 strands per cm (PSY406, 327 Thorlabs, Germany) was used to shield RF radiation.

Electric field analysis 343
Electric field strength and power was measured using a laser-powered, high-speed, low-noise 3D 344 electric field probe operating in a frequency range from 10 Hz -8.2 GHz (LS Probe 1.2, LUMILOOP, 345 GmbH, Germany). 346

Impedance spectroscopy 347
Impedance spectroscopy was performed in freshly cut branches of H. helix applying a 1 V bias between 348 two pin electrodes inserted in the inner tissue at the indicated distances and frequencies using the 349 precision LCR Meter (E4980A, Keysight Technologies, USA). 350

Energy harvesting circuits, sensing and wireless data transmission 351
The components for assembling the energy harvesting, sensing and data transmission circuits were 352 low leakage diodes (BAS416, Nexperia, The Netherlands), ceramic capacitors (Taiyo Yuden Co. LTD., 353 Japan) with indicated capacitances, and the temperature and humidity sensing and 868 MHz RF  354  transmitter module (STM 330 & HSM100, EnOcean GmbH, Germany) of which the solar cell, battery  355  and capacitor was removed and replaced with the plants using the indicated circuits for energy  356 harvesting, sensing, and data transmission. The plants were typically connected using coaxial cables 357 (SMA RG142U, RS PRO, UK) by connecting the central pin to the pin electrode penetrating the plant 358 tissue at the base of the stem. 359

Simulation of multiple leaf energy harvesting circuit 360
Circuit simulation to estimate requirements for electrical insulation between two leaves to obtain a 361 positive power balance during energy harvesting was done in Matlab/Simulink (Version R2019b) using 362 the circuit depicted in Fig. 1 and assuming a tissue resistance of 100 kΩ and a 10 V, 5 Hz sinusoidal 363 alternating voltage signal generated by the leaves. The analysis of the sum signal of multiple 364 overlapping signals randomly generated by multiple leaves was done in Matlab using a code that builds 365 the cumulative sum of a given number of sinusoidal functions of arbitrary phase and amplitude 366 (Extended Data Fig 2). 367

Further instrumentation and methods 368
Leaf surfaces and manually cut cross-sections of leaves with epicuticular coatings were imaged with a 369 digital microscope (KH-8700, Hirox, USA). Transmission spectra of epicuticular coatings were measured 370 in a UV-Vis spectrophotometer (Lambda 45, Perkin Elmer, USA). The H. helix support panel was cut 371 from 5 mm PMMA sheets using a laser cutter (VersaLaser VLS2.30, Universal Laser Systems, Austria). 372

Plant's mechanical energy conversion efficiency 373
To estimate the efficiency to convert mechanical energy applied to a c-u leaf pair (1) 377 where is the current produced by the c-u leaf pair, is the load resistance (10 MΩ), the potential 378 energy ℎ, = ℎ, where is the mass of a weight (here 7 g) with the dimension of 1 cm² onto 379 which an uncoated leaf was fixed, that was dropped from a fixed height ℎ (1 cm) on to a coated leaf 380 supported by a rigid surface, and is the gravitational acceleration. Currents across the two leaves 381 connected through during the current peak time interval from , to , measured by an 382 electrometer were analyzed. 383

Analysis of capacitor charging dynamics by plant converting environmental RF energy 384
To estimate the efficiency of the plants charging the capacitor by converting RF energy at the 385 specific given conditions, we considered the power balance between the RF input power (2) 389 where is the capacitance, is the time after which over the capacitor was reached, and , = 390 √ 2 + 2 + 2 where , , are the x-, y-, and z-components of the electric field power averaged 391 over the full plant height in z direction, measured in the vicinity of the plant by an electric field probe. 392 The short circuit current related to capacitor charging was calculated by equation (3)

Charges required for sensing and wireless transmission cycle 414
In order to estimate the charges required for a measurement cycle (charge required for sensing 415 the humidity and temperature and wirelessly transmitting the data to the receiver), the variation in 416 the voltage , across the capacitor with capacitance during a measurement cycle was 417 analyzed in the corresponding time interval using equation (7): 418 = ( ( 1 ) − ( 2 )) (7) 419 Where 1 and 2 are the timepoints of begin and end of the measurement cycle, respectively. 420

Statistical methods 421
Averages and standard deviations of typically five measurements are reported. Analysis with 422 controlled mechanical stimulation are averages and standard deviations of forty to sixty tests per 423 condition. The data was analyzed in Matlab (Version 2019b). 424

Data availability 425
The data supporting the findings of this study are available from the corresponding authors on 426 reasonable request. (additional publication in an online repository before publication planned). 427 Acknowledgements 428 (in the limits of = 1 to 10 V, = 1 to 20 Hz, and = 0 to 0.05 , respectively) were generated. c) 450 and d) show the direct and "rectified" sum signal (dark blue curves), respectively of the 100 input 451 curves (light blue) indicating that the overlapping signals from multiple leaves may sum up leading to 452 voltage spikes that can be more than 10 times larger than the maximal amplitude of a single leaf. increasing mechanical contact for energy conversion between the overlapping leaves. b) We tested 461 how the material used for support influences the voltage generation using the depicted arrangement 462 in which an uncoated leaf was actuated onto a coated/uncoated leaf kept at a given distance from a 463 support material. The graph below shows the generated voltage signals (10 Hz stimulus) using PMMA, 464 wood, PDMS, PTFE, and another leaf as support material when analyzing c-u and u-u pairs. PDMS and 465 PTFE enhance the signal which can be explained by an additional contact charging due to contact of 466 the leaf with this surface, PMMA, wood, and leaf as a substrate does not significantly change the signal. 467 Using the u-u pair, the signal is expectedly lower and only slightly increased by the substrate and the 468 main contribution for generated voltage is the c-u and u-u leaf pair. c) Depicts the PMMA support panel 469 used for wind-exposure experiments in which multiple leaves of two branches of two H. helix plants 470 (one coated, one uncoated) can be fixed at the petiole creating a c-u pair free to move in the wind 471 while being connected to the main plant and supported by the PMMA panel. in a front garden and on a balcony of a multifamily residence. e) to h) Hanning-windowed frequency 500 spectrum analysis (dbV, full range, left; V, >10 kHz, right) of voltage signals received by different plants 501 (H. helix, Y. elephantipes, and P. peltatum). The house' ground was used as reference point/potential 502 difference. Insets are zoom-ins into smaller peaks of the spectrum. Plants strongly increase signal 503 reception at multiple frequencies starting from super low mains noise. Spectrum analysis of H. helix 504 and Y. elephantipes shows among other frequencies also signals related to a nearby 657 kHz MW radio 505 emitter (8 km distance). i) Plant-dependent charging dynamics of a 136 µF capacitor. Wet soil 506 (electrode inserted 20 cm) and no plant, respectively are plotted as reference. P. pinea results in 507 highest instantaneously transferred power of ~10 µW and H. helix in 6.8 µW, P. peltatum in 0.8 µW 508 showing a plant-dependent behavior likely due to plant size and branching that forms the receiving 509 antenna. j) Analysis of 136 µF capacitor charging dynamics by H. helix when the mains circuit of the 510 near-by multifamily residence is turned off, revealing that the energy output is strongly affected by the 511 building mains. 512 Figure 1 Overview and model of living plant structure-based multisource energy harvesting. a) Inner cellular and vascular tissue are ionic conductors; the cuticle on the plant surface is a dielectric polymeric layer. When a leaf with epicuticular coating comes in contact with an uncoated leaf, contact electri cation generates only considerable voltages U in the tissue if the dielectric material M1 and M2 inhere a material pair that speci cally enhances contact electri cation, typically M1≠M2. b) Illustration of basic circuitry established by the plants in a wind and RF energy harvesting scenario. c) Circuit modelling reveals that during mechanical interaction of two leaves from different plants, voltages in the tissue build up only when Rsoil is su ciently high. d) Signals of the two leaves cancel out when the leaves are on the same plant, due to too low tissue impedances (typically ~0.1-1 MΩ).

Figure 2
Leaf-epiculticular coating, in uence on plant viability, and enhancement of mechanical energy harvesting. a) Cross-section of F. microcarpa leaf with ~100 μm epicuticular silicone rubber coating on adaxial surface. b) Microscopy images representing stomatal density on adaxial and abaxial leaf surface. c) One-