Introduction

Backscatter communication, characterized by its low-power and low-cost features, plays a crucial role in managing the increasing power consumption of wirelessly connected devices in Internet of things (IoT) systems1. The development of flexible backscatter communication devices and systems, using additive manufacturing and inkjet printing, has opened up possibilities for applications in body sensor networks (BSNs)2,3,4,5. The integration of backscatter communication with energy harvesting (EH) techniques enables battery-free operations, a crucial step towards sustainable device development. Due to the typically lower sensitivity and efficiency of radio frequency (RF) EH compared to wireless communication6, integrating additional energy sources into EH systems has attracted significant interests7,8,9,10,11,12. Imaging technologies, central to applications like environmental monitoring, object detection, and law enforcement13,14,15,16,17,18, have notably advanced through the integration of backscatter communication and EH8,9,10,11, leading to the development of battery-free wireless grayscale imaging solutions. However, implementing wireless color imaging in wearable devices poses challenges. Compared to grayscale, the primary issue is the higher power requirement of color imaging19, which complicates the already challenging task of balancing power density with the compactness needed for wearable applications.

RF, ambient light, thermoelectric and biomechanical energy sources are increasingly sought-after in wearable battery-free applications12. While hybrid RF-solar EH is promising9,20, its compact integration with backscatter communication for wearable is challenging. Advancements in transparent stacked hybrid RF-solar EH, using transparent materials, have enabled complete solar cell ulilization21. Additionally, antenna multiplexing techniques such as polarization splitting22, power splitting23, and frequency splitting (FS)24,25 are increasingly employed for simultaneous wireless information and power transfer (SWIPT)22,23,24,25,26. The use of a shared aperture design allows antennas operating at different frequencies to coexist in a compact space, optimizing space utilization27. Moreover, in wearable applications, addressing the specific absorption rate (SAR) is crucial. Thus, implementing shielding between antennas and the human body is an effective strategy to reduce SAR exposure28.

Here, we propose a flexible and wearable battery-free backscatter wireless communication system tailored for colour imaging. The system incorporates a hybrid RF-solar EH array that enhances EH from ambient RF signals and visible lights, overcoming the limitations of conventional approaches. The hybrid EH array significantly boosts energy harvesting, facilitating ultra-low power backscatter communication and effective power management for both outdoor and indoor usage. Additionally, flexibility is achieved through the use of flexible conductors and substrates, allowing the system to operate after conforming to the human body, thus ensuring comfort, user-friendliness, and safety due to minimized specific absorption rates (SAR). The system also exhibits stable off-body communication performance, a benefit stemming from its radar cross-section (RCS) being minimally affected by modulation states. Furthermore, a compact form is achieved through the FS-SWIPT technique and a transparent stacked hybrid RF-solar EH array. The FS-SWIPT approach facilitates the concurrent functioning of RF EH and backscatter communication through a shared-aperture antenna array, effectively reducing interference between these operations. And the optically transparent stacked structure not only maintains both RF and solar EH efficiency but also ensures that the performance of RF components remains unaffected by the presence of solar cells. The innovative strategies employed address critical challenges in power generation, system flexibility, and compactness, demonstrating a sustainable and practical solution for the next generation of wearable and power-demanding devices.

Results and Discussion

Flexible battery-free communication system

Figure 1 illustrates the compact, flexible, and battery-free backscatter communication system designed for wearable wireless colour image capture. The system, covering an area of 400 cm2, comprises a transparent FS-SWIPT device, a hybrid RF-solar EH array, a flexible printed circuit (FPC) with a camera (CAM), a PMU, and a microcontroller (MCU). The transparent FS-SWIPT device, placed on top of the solar cells and FPC, uses a shared-aperture-type design to obtain RF signals for backscatter modulation and rectification at 3.5 GHz and 2.4 GHz, respectively. The patch-type FS-SWIPT device is chosen to mitigate the impact of human body influences, while the transparent conductor in FS-SWIPT device shields it from solar cells and the FPC. To ensure a stable connection, the RF switch and rectifier in the FS-SWIPT device are connected with transparent microstrip lines via a combination of conductive ink and solder paste. The solar cells are connected in series with the rectenna array for large-area hybrid RF-solar EH, with the solar cells covering 80% of the entire area. The PMU is responsible for energy storage and output stability. Once sufficient energy is generated, colour images are captured and coded by CAM and MCU. Then the modulator in the FS-SWIPT device is controlled by the MCU for backscatter communication. Figure 2c demonstrates the system’s flexibility, showcasing its ability to conform to a cylindrical shape.

Fig. 1: Highly compact flexible and battery-free backscatter communication system for wearable wireless imaging.
figure 1

The system consists of a transparent SWIPT device, solar cells, a PMU, an MCU, and a CAM. When equipped on the chest and next to an access point (AP), it captures and transmits the image back to the AP for display after harvesting sufficient RF and light energy.

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Fig. 2: Proposed flexible and battery-free system for wearable wireless imaging.
figure 2

a On-body communication prototype indoors and outdoors. b SAR simulation at 2.4 GHz (left) and 3.5 GHz (right) in backscatter communication. c Photograph of the flexible system bent on a cylinder with 15 cm radius. d Spectrum in backscatter communication. e Performances of backscatter communication, RF and solar EH.

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Figure 2a demonstrates the system’s indoor and outdoor communication prototypes, highlighting its compactness and flexibility when mounted on clothing, with an USRP LW-N310 configured for 3.5 GHz backscatter communication and a 2.4 GHz RF source. During indoor testing, after being fully charged by light and RF energy, the system captures one color image at a resolution of 100 × 100 pixels via the CAM. The captured image is then transmitted to the USRP for display and storage through backscatter communication (Supplementary Movie 1). In outdoor scenarios, the hybrid RF-solar EH array consistently powers the power under direct sunlight. The tests confirm a communication distance of 25 cm and a data rate of 500 kbps (detailed in Supplementary Note 4). Figure 2b illustrates SAR simulations for backscatter communication, with a 38 dBm equivalent isotropic radiation power (EIRP) RF source positioned 25 cm from the system. The maximum SAR values are 0.50 W kg−1 and 0.66 W kg−1 at 2.4 GHz and 3.5 GHz, respectively, which are lower than the standard value of 1.6 W kg−1 regulated by the Federal Communications Commission (FCC)29. Figure 2d and Supplementary Figure 8c-e illustrate the backscatter communication performance using binary phase-shift keying (BPSK) modulation at 3.5 GHz. The communication achieves a carrier-to-noise ratio (CNR) of 62 dB. Additionally, the backscatter signals, 500 kHz from the carrier, exhibit a signal-to-noise ratio (SNR) of 32 dB. The modulator power consumption is below 6.6 mW30, whereas the operating CAM and MCU consume approximately 100 mW, as tested with the RIGOL programmable DC power supply DP831. Additional experimental data (Supplementary Fig. 8f) with a 27 dBm EIRP RF source investigate the relationship between communication distance and data rate. Data rate is set at levels above 100 kbps, considering the increased noise levels in the USRP within 100 kHz bandwidth, as depicted in Fig. 2d. It reveals that at a data rate of 125 kbps, the system achieves its maximum communication distance of 250 cm. When the data rate is lower to 100 kbps, the distance is reduced, which is attributed to the higher noise levels in the USRP.

To determine bending effects on the performance in energy harvesting and backscatter communication, we conducted experiments with the flexible system attached to flat surface and cylinders with radii of 25 cm (Supplementary Fig. 11b) and 15 cm (Supplementary Fig. 11c). In Fig. 2e, the blue and red columns represent the experimental SNR in backscatter communication (at a data rate of 500 kbps) and the output voltage of RF EH, respectively. These were measured using a 27 dBm EIRP RF source, placed 25 cm from the system. The results indicate a slight decrease in both backscatter communication and RF EH performance as the degree of bending increases, attributed to the antenna array’s radiation deterioration. The green column shows the solar EH output voltage under approximately 8000 lux the light intensity on the solar cell array, demonstrating stable performance despite varying degrees of bending.

Transparent FS-SWIPT device

Figure 3a depicts a schematic of the FS-SWIPT device, The solid line represents the forward link channel at frequency fB and RF energy at fR, emitting from the transmitting antenna to the terminal antenna. The dotted line denotes the backscatter link channel at fB, reflected from the terminal antenna to the receiving antenna. The duplexer splits the forward link channel and RF energy for backscatter modulation and RF EH, respectively. The forward and backscatter link budgets are defined as follows31:

$${P}_{{\rm{M}}}=\frac{{P}_{{\rm{T}}}{G}_{{\rm{T}}}{G}_{{\rm{M}}}{\lambda }^{2}{X}_{{\rm{f}}}}{{\left(4\pi {r}_{{\rm{f}}}\right)}^{2}\varTheta {B}_{{\rm{f}}}{F}_{{\rm{f}}}}$$
(1)
$${P}_{{\rm{R}}}=\frac{{P}_{{\rm{T}}}{G}_{{\rm{T}}}{G}_{{\rm{R}}}{G}_{{\rm{M}}}^{2}{\lambda }^{4}{X}_{{\rm{f}}}{X}_{{\rm{b}}}M}{{\left(4\pi \right)}^{4}{r}_{{\rm{f}}}^{2}{r}_{{\rm{b}}}^{2}{\varTheta }^{2}{B}_{{\rm{f}}}{{B}_{{\rm{b}}}F}_{{\rm{b}}}}$$
(2)

where λ is the wavelength; Θ is the on-object antenna gain penalty; PT, PM, and PR are the transceiver transmitting power, modulator power and transceiver receiving power, respectively; GT, GM, and GR are the gains of the transmitting antenna, modulator antenna, and receiving antenna, respectively; rf and rb are the distances of the forward and backscatter links, respectively; Xf and Xb are the forward and backscatter polarization mismatches, respectively; Bf and Bb are the forward and backscatter path-blockage losses, respectively; Ff and Fb are the forward and backscatter fade margins, respectively. The modulation factor M is determined by the modulator reflection coefficients \({\varGamma }_{{\rm{A}}}\) and \({\varGamma }_{{\rm{B}}}\) in different states32:

$$M=\frac{1}{4}{{\rm{|}}{\varGamma }_{{\rm{A}}}-{\varGamma }_{{\rm{B}}}{\rm{|}}}^{2}$$
(3)
$${\varGamma }_{{\rm{A}}}=\frac{{Z}_{{\rm{L}}}^{{\rm{A}}}-{Z}_{{\rm{A}}}^{* }}{{Z}_{{\rm{L}}}^{{\rm{A}}}+{Z}_{{\rm{A}}}}$$
(4)
$${\varGamma }_{{\rm{B}}}=\frac{{Z}_{{\rm{L}}}^{{\rm{B}}}-{Z}_{{\rm{A}}}^{* }}{{Z}_{{\rm{L}}}^{{\rm{B}}}+{Z}_{{\rm{A}}}}$$
(5)

where ZA is the modulator antenna impedance, and \({Z}_{{\rm{L}}}^{{\rm{A}}}\) and \({Z}_{{\rm{L}}}^{{\rm{B}}}\) are the load impedances in different states.

Fig. 3: Transparent SWIPT device.
figure 3

a Schematic of the FS-SWIPT device. b, Exploded diagram of the flexible type of device. c, Dual-band shared-aperture antenna array. d Duplexer. e Modulator. f Rectifier, where C1 = 10 pF, C2 = 100 pF, and RL = 1 kΩ. g Measured sheet resistances of the TMMF in different widths over a duration of one year.

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The transparent FS-SWIPT device, detailed in Fig. 3b–f, incorporates a dual-band shared-aperture antenna array with a duplexer, modulator, and rectifier. Constructed using transparent micromeshed metal film (TMMF) and polydimethylsiloxane (PDMS) for the conductor and substrate, respectively, the device’s detailed parameters are provided in Supplementary Figure 10. Comparisons between substrates of varying thicknesses are explored in Supplementary Note 3 and Supplementary Fig. 5. Stability tests of the TMMF’s sheet resistances over a year, depicted in Fig. 3g, confirm their consistency, underlining the system’s reliability for long-term use.

The electromagnetic (EM) waves in the dual-band shared-aperture antenna array (Figs. 3c, 4c), operating at 2.4 GHz and 3.5 GHz, are divided by a 1:4 power divider. At the endpoints (red points in Fig. 3c), the lengths of the phase delay lines (blue and green lines in Fig. 3c) are adjusted to minimize interference between adjacent antennas at different frequencies (detailed in Supplementary Note 3 and Supplementary Fig. 4). The far field of the antenna array is measured in an anechoic chamber (Fig. 4a), and the simulated and measured results of the antenna array are presented in Fig. 4b, c. The results show that the reflected wave is in-phase with the input wave at the endpoint of the antenna unit (Fig. 4c) when the length of the phase delay lines is adjusted to 7.5 mm and 14 mm at 2.4 GHz and 3.5 GHz, respectively. This confirms that the proposed antenna array is well-matched at both frequencies. The gains are 7.7 dBi at 2.4 GHz and 8.4 dBi at 3.5 GHz (Fig. 4b and Supplementary Fig. 4), but the sidelobe is high at 3.5 GHz due to the same distance p between adjacent antennas at different frequencies (0.648 of the wavelength at 2.4 GHz, 0.945 of the wavelength at 3.5 GHz). The SAR of the antenna array is discussed in Supplementary Note 3 and Supplementary Fig. 4.

Fig. 4: Simulated and measured results for the transparent FS-SWIPT device.
figure 4

a Photograph of the far-field experimental setup. b, c Results of the dual-band shared-aperture antenna array: measured radiation patterns at 2.4 GHz and 3.5 GHz (b), and S11 (c), where the scale bar is 30 mm. d Output voltage, and rectification efficiency of the rectifier, where the scale bar is 20 mm. e S-parameters of the modulator in different states, where the scale bar is 20 mm. f Relationship between the distance of the USRP device and the system’s energy harvesting and backscatter communication. g SAR simulation with and without the FS-SWIPT device at 2.4 GHz. h SAR simulation with and without the FS-SWIPT device at 3.5 GHz.

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Upon reception by the antenna array, the EM waves are directed to the duplexer (Fig. 3d and Supplementary Fig. 5d), which separates them into the rectifier and modulator. The duplexer is composed of two 1st-order coupled-line bandpass filters, and the lengths of the phase delay lines are adjusted to minimize interference between the filters. The simulated and measured results of the duplexer are presented in Supplementary Fig. 5. At 2.4 GHz and 3.5 GHz, the duplexer insertion loss measures 2.4 dB and 2.7 dB, respectively, while the isolation between the two outputs is approximately 25 dB.

The RF switch (Figs. 3e, 4e) is responsible for modulating RF signals at 3.5 GHz using BPSK modulation. To prevent RF signals from reaching the control port, a bandstop stub is included (detailed in Supplementary Note 3). Both conductive ink and solder paste are used to connect the RF switch and microstrip lines (Fig. 6b and Supplementary Fig. 3). The simulated and measured results of the modulator are presented in Fig. 4e and Supplementary Fig. 6a, b. The modulator displays a phase difference of approximately 160° and has a 3 dB return loss in the on/off states at 3.5 GHz. The modulation factor M is calculated as 0.45 as per Eq. (3).

The rectifier (Figs. 3f, 4d) is responsible for converting RF energy at 2.4 GHz to DC energy and storing it in the PMU. The soldering process for the Schottky diodes and capacitors is the same as that used for the RF switch above. The simulated and measured results of the proposed rectifier are displayed in Fig. 4d and Supplementary Fig. 6c. The rectifier output voltage evidently ranges from 0.17 V to 1.7 V when the input power increases from -10 dBm to +10 dBm. However, power management chips often have cold-start voltages that cannot be achieved with weak RF energy alone. Therefore, hybrid RF-solar EH, which combines RF energy with light energy, is currently a high-priority research topic.

To study the performance of the FS-SWIPT device in wearable battery-free communication, a linearly polarized wave is radiated on the FS-SWIPT device to observe the induced currents and scattered patterns, as shown in Supplementary Fig. 7. The magnitudes of the induced currents and normalized radar cross-section (RCS) patterns are relatively insensitive to the state of the modulator. The induced currents flow into the modulator and rectifier at frequencies of 3.5 GHz and 2.4 GHz, respectively (Supplementary Fig. 7a, b). Additionally, the sidelobe levels (SLL) in RCS patterns are observed to be above 10 dB in Supplementary Fig. 7c. When the FS-SWIPT device is attached on the human body, the RCS patterns are approximately unchanged (Supplementary Fig. 7c). Meanwhile, the SAR reduces apparently within the range of the FS-SWIPT device, compared with the SAR without the device (Fig. 4g, h). Additionally, we conducted experiments on the relationship between the USRP device distance and the FS-SWIPT device performance, using a 27 dBm EIRP RF source and setting the data rate to 500 kbps. Figure 4f describes a decrease in SNR and RF EH output voltage with increasing distance, due to RF energy dissipation.

Hybrid RF-solar EH array

The hybrid RF-solar EH array is shown in Fig. 5a, which consists of both RF EH and solar EH. To achieve a high energy conversion efficiency, a single stub is used to match the RF-EH output in Fig. 5a to the rectified diodes. The RF energy is then converted to DC energy E1 through full-wave rectification. Capacitors C1 and C2 are used for DC blocking and bypass, respectively. Solar cells are used to convert light energy to DC energy E2, with a maximum output power of 2 W outdoors. Zener diodes D1 and D2 are used to prevent overloaded voltage and reverse current, respectively. Both E1 and E2 are managed and stored in supercapacitor C3 by the PMU to activate the load. The RF-to-DC conversion efficiency ηRF can be calculated using33:

$${\eta }_{{\rm{RF}}}=\frac{{P}_{{\rm{out}}}}{{\left.{P}_{{\rm{in}}}^{{\rm{RF}}}\right|}_{{P}_{{\rm{in}}}^{{\rm{Solar}}}=0}}\times 100 \%$$
(6)
$${P}_{{\rm{out}}}=\frac{{V}_{{\rm{out}}}^{2}}{{R}_{{\rm{L}}}}$$
(7)

where PRFin and PSolarin are the RF and solar input power, respectively, and Vout and Pout are the voltage and power across load RL, respectively. In Fig. 4f, the conversion efficiency ηRF of the rectifier ranges between 27% and 46%.

Fig. 5: Hybrid RF-solar EH.
figure 5

a Schematic. b RF EH test scenario. c Output voltages of the hybrid RF-solar EH array and the PMU under different light and RF intensities. d Energy harvesting capabilities with respect to different directions of light and electromagnetic waves.

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The RF EH test scenario and output voltages of the hybrid RF-solar EH array and PMU under different light and RF intensities are shown in Fig. 5b and c. The results demonstrate that the performance of the hybrid RF-solar EH array is influenced by both light intensity and EIRP. Notably, the PMU maintains a stable output across these varying conditions. It is capable of functioning efficiently at a light intensity as low as 13 lux or with an EIRP input of 27 dBm from a distance of 25 cm.

To comprehensively evaluate the hybrid RF-solar EH array, we investigated the directionality effects on its energy harvesting capabilities, with findings presented in Fig. 5d. Using a 27 dBm EIRP horn antenna placed 25 cm from the system, we measured the RF EH output voltage across various incident angles. This data, shown as the red line, correlates with the radiation pattern in Fig. 4b, revealing a minimum RF EH output voltage at a 60° incident angle. Additionally, the solar EH output voltage, depicted by the blue line, was assessed using a light source emitting around 8000 lux at vertical incidence on the surface of the solar cell array. Here, the solar EH output voltage decreases as the incident angle increases, a trend explained by the diminishing light intensity on the system’s surface.

Discussion

We have successfully developed and demonstrated a flexible and wearable battery-free backscatter wireless communication system for colour imaging. The system incorporates several innovative features and achieves significant advancements in performance. One key aspect is the integration of a hybrid RF-solar EH array, which efficiently harvests energy from both ambient RF signal and visible light, ensuring sustainable and reliable energy sources. Experiments enrich the understanding of the EH array’s performance under various environmental conditions. Additionally, the use of flexible materials enhances its wearability and comfort. We have demonstrated through off-body simulations that the system effectively reduces the SAR and maintains a stable RCS. Experiments evaluate the robustness of the system under different bending scenarios. Furthermore, the stacked and shared-aperture design achieves the balance between energy-harvesting power density and device compactness. Finally, the off-body communication prototype demonstrates the battery-free system’s potential in wearable wireless colour imaging.

There are several limitations that offer opportunities for future research. First, the hybrid RF-solar EH, especially under conditions of low light and RF energy, presents a challenge for power-hungry applications. Future versions could incorporate diverse types of EH to bolster energy intake under various environmental conditions. Second, the current data transmission rate and resolution of color imaging, while adequate for certain applications, may not meet the demands of more advanced or high-resolution imaging tasks. Potentially integrating machine learning algorithms, such as NeuriCam19, this system can further reduce power consumption and facilitate color video transmission, expanding the system’s capabilities. Third, the envisioned integration with existing wireless networks and IoT devices requires a more in-depth exploration. Future research should focus on seamless interoperability with current communication protocols and standards, like BackFi34, to ensure easy adaptability of the system within the rapidly evolving tech landscape.

Methods

Flexible and transparent materials

The flexible and transparent FS-SWIPT device is fabricated using PDMS as the substrate (εr = 2.8, tan δ = 0.02, 1.5 mm thick), involving a process of mixing, stirring, confining, expelling, and thermal curing35, as detailed in Supplementary Note 1. The conductor TMMF with a sheet resistance of ≤0.09 Ω sq−1 and a thickness of 150 µm, is adhered to the substrate and shaped using laser cutting36, with further details in Supplementary Note 2.

Modulation

For the RF switch, we employed a BAR64-02V PIN diode from Infineon37, controlled by the Microcontroller Unit (MCU), as shown in Fig. 3e. The PIN diode’s behavior, alternating between inductance with low resistance at a high control voltage (Vctrl = 0.8 V) and inductance with parallel high resistance and capacitance at a low control voltage (Vctrl = 0 V), enables variable RF switch states.

Energy harvesting

The rectifier employs two SMS7630 Schottky diodes from Skyworks for full-wave rectification38 (Fig. 3f). These diodes are sensitive to -60 dBm of RF energy, though the PMU necessitates a higher input power for activation. The system’s flexible solar cell array (Fig. 6a) consists of four panels with an a-Si/a-SiGe/a-SiGe triple-junction structure (Voc = 2.2 V, Vm = 1.6 V, Pm = 0.5 W), as shown in Supplementary Figure 11a39.

Fig. 6: Structure of the flexible solar cell array and circuit board.
figure 6

a Photograph of the connected flexible solar cell array bent on a cylinder with 15 cm radius. b Component connection in the transparent SWIPT device. c Components connection in the FPC. d Exploded diagram of the flexible circuit board with PMU, MCU, and CAM. e Photograph of the flexible circuit board bent on the cylinder.

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Flexible circuits

The design of the circuit board with Flexible Printed Circuit (FPC) techniques (Fig. 6d, e), incorporating PMU, MCU and CAM, achieves flexibility through a layout where rigid chips occupy only a small portion of the board. All components are soldered onto the FPC with solder paste (Fig. 6c), with their schematics and detailed descriptions provided in Supplementary Note 5.