Ensuring the provision of sustainable and secure electrical power for ingestible/implantable medical devices (IMDs) is crucial for facilitating the multifaceted capabilities of these IMDs and preventing the need for recurrent battery replacements. Using photovoltaic (PV) energy harvesting in conjunction with an external light source can be advantageous for an optical wireless power transfer (OWPT) system to enable energy self-sufficiency in IMDs. This study investigates the performance of OWPT using commercial monocrystalline silicon PV cells exposed to an 810 nm Near-infrared (NIR) LED light. The ethical concerns are addressed by utilizing porcine samples (ex vivo approach), eliminating the need for live animal experimentation. The experimental setup employs porcine meat samples with several compositions, e.g., pure fat, pure muscle, and different layers of fat-muscle. The primary goal of this initial study is to analyze the open-circuit voltage output (VOC) of the PV against received optical power in the presence of biological tissue. Our study demonstrates that PV cells can generate voltage even when exposed to light passing through porcine samples with a thickness of up to 30 mm. Furthermore, the VOC values of PV cells attained in this study meet the required voltage input level for supplying current IMDs, typically ranging from 2V to 3V. The findings of this study provide valuable insights into OWPT systems in the future, where monocrystalline silicon PV cells can be employed as energy harvester devices to supply various IMDs utilizing NIR light.
1 Introduction
IMDs offer significant advantages in real-time health monitoring and targeted treatments within the human body [1‐3]. So far, a massive amount of research has been conducted to enhance wireless implantable medical devices (IMDs) dedicated to improving patients’ well-being, for instance, in [4‐10]. Various types of IMDs have been developed, each with unique functions and designs [11‐13]. A crucial aspect in making these resource-limited IMDs more practical is providing sustainable electrical power [14], eliminating the need for frequent surgical procedures to replace batteries, typically occurring every 3 – 7 years [15, 16].
Emerging technologies for delivering sustainable and reliable electrical power to IMDs encompass piezoelectric or triboelectric generators, biofuel cells, inductive radio frequency (RF), photovoltaics (PV), and other approaches. Inductive RF as an OWPT system stands out due to its ability to provide relatively higher power levels [17, 18]. However, efficiency may be compromised when the transceiver is reduced in size or needs to be correctly aligned [14]. The PV energy harvesting also offers a viable solution by utilizing ambient or external light sources to generate sufficient electrical power for IMDs. Nevertheless, to the best of our understanding, most of the studies in the literature were conducted using visible light spectrum (e.g., sunlight) as a light source to power PV cell, for instance, as done by [19, 20]; as a result, the light beam can not penetrate deeply to the human body (limited to human skin layer) as visible light does not propagate efficiently across biological tissue [21‐23]. This is due to the fact that the skin absorbs the majority of the visible light spectrum; particularly light with a wavelength of 1,300 nm, as it is almost absorbed by the water content in the skin layer [24]. Near-infrared (NIR) light which is a part of the light spectrums has the ability to penetrate deeper into biological tissue [4]. Unlike other wavelengths, NIR radiation is less affected by the absorption and scattering properties of biological tissues. Studies have confirmed that NIR wavelengths >700 nm, referred to as long waves, do not pose any harm to the human body and can effectively penetrate tissues [24]. To this end, there needs to be a more comprehensive investigation regarding the electrical performance of commercial PV cells when subjected to NIR through biological tissue.
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Up to the authors’ knowledge, there is still few research on OWPT using NIR. Authors in [25] studied efficient enhancement strategies for OWPT systems using NIR and PV cells; showing that up to 48% of OWPT efficiency can be reached. However, this research is conducted on a free space [25]. In [26], evaluation of OWPT employing a 750 nm 5 mW laser is investigated; it has been shown that the proposed system could recharge a 150 mAh battery even when situated beneath a skin tissue and regulated the power provided to a low power IMDs, which is less than 10 mW. Nevertheless, the efficacy of LED power level usage diminishes when applied to thicker biological tissue due to the substantial loss (caused by absorption, scattering, and reflectance factors) of optical power during the propagation of light through tissue [26].
This study analyzes the electrical performance attributes of a commercial PV that can be implanted in the human body for the OWPT system employing NIR (810 nm 375 mW), focusing on electrical voltage. We considered NIR light as it can propagate relatively well through biological tissue. Our experimental trials involved testing a PV placed under a porcine sample with the surface emitted by an NIR light source in an aligned setting. We varied the transmitted optical power, and afterward, we measured the received power density and the output PV cells in an open circuit (VOC). The findings of our study proved that harvesting energy using NIR and PV cells across the biological tissue is promising. On the other hand, our study offers unique insights into the design and implementation of OWPT using NIR for IMDs by studying the received power density and VOC of commercial PV cells.
Contribution.
This paper is the first time to exploit the feasibility of employing commercial monocrystalline silicon PV cells for OWPT through biological tissue under NIR LED under 810 nm, focusing on analyzing the VOC characteristics. We considered porcine samples with different compositions, including pure fat tissue, pure muscle tissue, and porcine samples with thicknesses up to 30 mm. Moreover, we also considered much deeper provision of optical energy that in previous literature that focused under-skin cases.
2 Methodology
Figure 1 depicts the experimental setup used in this study; it consists of a LED driver (DC2200, Thorlabs), NIR LED (M810L4, Thorlabs), optical power meter (PM100D, Thorlabs) to measure received optical power (in mW) and power density (in mW/cm2 unit), and multimeter (MM400, Klein tools) to measure VOC of commercial PV cells. Figure 2 shows the commercial monocrystalline silicon PV cells used in this study. Three different PV cells were used: SM710K12L R3.0, SM111K04L R3.0, SM141K08LV R3.0 (Ixolar, ANYSOLAR), then denoted as #PV1, #PV2, #PV3, respectively. These PV cells are suitable for both outdoor and indoor applications. Table 1 summarizes the specifications of PV cells used. The number of cells of PV#1, #PV2, and #PV3 are 12, 4, and 8, respectively.
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In this study, we considered only VOC, that is open circuit performance. Therefore, we did not connect the output of PV cell to any load. Figure 3 shows the porcine samples used in this study, we used four samples: #1 (15 mm of thickness, fat layer), #2 (15 mm of thickness, muscle layer), #3 (25 mm of thickness), and #4 (30 mm of thickness). In the experimental procedure, all fresh meat samples were heated to a temperature of 37 ℃ using an air heater within a chamber, aiming to align with the typical body temperature of humans.
The dimensions of each sample are approximately 5 cm × 5 cm, and the LED is positioned in alignment with the receiver through the sample without any free space between them. Despite the small area of the used samples, the optical signal does not pass through the sides of the sample. The LED as a transmitter source typically has a limited field-of-view (FOV), leading to a very narrow optical beam. The propagation of optical light is more directional compared to radio waves. In the case of our study, the NIR LED has a confined viewing angle of 80° maximum, resulting in a narrow beam and very short optical distance. A similar study conducted by [14] also employed a small tissue area.
Fig. 1.
Experimental setup.
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We first measure the received power in the air propagation medium (free space). In the experimental setup, the NIR LED is positioned in direct line-of-sight (aligned) to the optical sensor (S121C, Thorlabs) and exposed towards 4 cm of free space optical distance. The optical sensor is connected to an optical power meter with settings as follows: attenuation = 0 dB, input aperture = Ø9500 nm, and wavelength = 810 nm. By adjusting the LED current through the LED, the subsequent endeavor entails measuring the received optical power and power density.
The results of measurements conducted in a 4 cm free space using LED driver settings of 500 mA, 400 mA, 300 mA, 200 mA, and 100 mA yielded power outputs of 47.3 mW, 38.5 mW, 29.1 mW, 19.4 mW, and 9.37 mW, respectively (Fig. 4a). Correspondingly, the power densities were calculated to be 66.7 mW/cm2, 54.2 mW/cm2, 41.1 mW/cm2, 27.4 mW/cm2, and 13.21 mW/cm2, when subjected to 500 mA, 400 mA, 300 mA, 200 mA, and 100 mA of LED current, respectively (Fig. 4b). It was observed that an increase in LED current resulted in higher light intensity emitted by LED and consequently, higher power received by the receiver. The relationship between light intensity and received power was found to be linear, confirming previous findings by [27, 28].
Characteristics of NIR LED at 4 cm of optical distance (free space): (a) received power in mW; (b) power density in mW/cm2.
Table 1.
Electrical characteristics of commercial PV cells used in the study
Cell Parameter Typical Ratings
#PV1
#PV2
#PV3
Open circuit voltage (VOC)
8.29V
2.76V
5.53V
Short circuit current (ISC)
29.2 mA
46.7 mA
58.6 mA
Voltage at max. Power point (VMPP)
6.70V
2.23V
4.46V
Current at max. Power point (IMPP)
27.4 mA
43.9 mA
55.1 mA
×
×
×
3 Results and Analysis
3.1 Received Optical Power
In this section, the optical power received after the NIR optical signal passes through each sample is measured. The experimental scenario refers to Fig. 1, where the NIR beam is directed towards the sample, while a sensor positioned behind the sample captures the optical power. The measurement of LED power reception is crucial as it determines the level of intensity received by the PV cells. The PV cell is capable of converting light into electricity as long as the given light can penetrate the skin’s thickness, with the conversion process being directly proportional to the intensity of the light. The experimental findings, as presented in Fig. 5(a) and (b) reveal that the power density and optical power received by each sample are much lower compared to free space conditions, indicating a reduction in power due to the optical properties of the tissues. The percentages of power density and optical power received in relation to free space are 13%, 9%, 2%, and 1% for sample #1, sample #2, sample #3, and sample #4 respectively. These results suggest that fat tissue (sample #1) is a suitable signal propagation medium compared to muscle tissue (sample #2), in line with previous research on RF wave cases [10, 29]. We found that the results in optical waves coincide with those in RF waves on biological tissue. It is observed that thicker tissues lead to a decrease in optical power and power density, which is consistent with the results found in [30].
Fig. 5.
Measurement of received power on various samples: (a) in mW; (b) power density in mW/cm2.
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3.2 VOC Measurement of Each PV Cells
As mentioned in the introduction, the main objective of this research was more highlighted on measuring the electrical voltage generated by PV cells under the OWPT cases. The VOC of a single PV cell is a reliable indicator for assessing the depth of light penetration into the human body and the amount of external light collected by the PV cell implanted within the body; this is because the PV cell exhibits high sensitivity to variations in low-light environments [31]. Figures 6(a), (b), (c) show the results of PV#1, PV#2, and PV#3, respectively. The measurement results of #PV1 on a sample #1 show the VOC for LED currents 500 mA, 400 mA, 300 mA, 200 mA, and 100 mA are 5.24 V, 5.04 V, 4.77 V, 4.40 V, and 3.75 V, respectively. Upon comparison with #PV1, it is observed that the VOC values for #PV2 and #PV3 are lower, with average reductions of 32% and 71% respectively. This average reduction is determined based on the comparison between sample #1 and sample #4. The typical VOC of #PV1 based on the datasheet (Table 1) is higher than #PV2 and #PV3, the voltage rating remains higher even when NIR is utilized through biological tissue. It can be summarized that the commercial PV cells are capable of operating when exposed to NIR light, particularly in applications involving biological tissue. The VOC of #PV1 and #PV2 meets the IMD’s voltage input requirements. The typical voltage demand for IMDs (e.g., pacemakers) is generally 2 V–3 V to operate [32]. These mentioned voltages are the ideal threshold of the voltage received in the PV cells that indicate a satisfactory for OWPT purposes. However, the average VOC of #PV2 does not meet this standard, which is below 2 V. Possible solutions that can be addressed further include connecting multiple PV cells either in series or parallel to attain adequate VOC level, or keep using single PV cells but incorporating storage (e.g., supercapacitors or rechargeable batteries) to store the generated energy – a similar approach has been suggested by [33].
Fig. 6.
Measurement result of VOC of each PV cells on different samples: (a) #PV1; (b) #PV2; (c) #PV3.
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Table 2 provides a summary of the voltage (VOC) generated by PV cells. Several research studies have been conducted to design PV cells specifically for IMDs. However, it is important to note that these custom-designed PV cells as references [14, 34‐36] are not currently available in the market. Instead, our study investigates the commercially available PV cells for IMDs. The VOC level can be enhanced by configuring PV cells in parallel as proved in Table 2. The results show that the maximum attainable VOC is 5.25 V (at an LED current of 500 mA on #PV1 and sample #1), while the lowest recorded VOC is 0.67 V (at an LED current of 100 mA on #PV2 and sample #3). Using single PV cells, the VOC attained in this study is relatively higher compared to the provided literature as in Table 2.
Table 2.
Profile of VOC of PV cell for IMDs on similar works
This section focuses on observing the influence of tissue thickness on the VOC of each PV cell. Performing measurements on ex-vivo meat samples offer a more convenient option compared to measurements on anaesthetized animals, as it eliminates the requirement for a clinical setting as typically conducted in hospital environments. Furthermore, the tissue dielectric characteristics of adult pigs closely mimic those of humans, rendering a commonly employed substitute for simulating the human body in medical studies [10]. In addition, adjusting the meat to temperatures closer to the average human body temperature, specifically 37 ℃, is important as it will yield more realistic results, as concerned by [9, 38].
Figures 7(a) – (d) show the measurement results of samples #1 to #4, respectively. When measuring the VOC of #PV2 in sample #1 (fat tissue), the recorded values were 1.85 V, 1.77 V, 1.67 V, 1.51 V, and 1.29 V, with corresponding LED currents of 500 mA, 400 mA, 300 mA, 200 mA, and 100 mA, respectively. Subsequently, when measuring VOC of #PV2 in samples #2, #3, and #4, the average reductions were 95%, 62%, and 60% respectively. Meanwhile, for #PV1 in samples #2, #3, and #4, the VOC were 98%, 74%, and 68%, respectively. Similarly, the VOC for #PV3 in samples #2, #3, and #4 were 95%, 74%, and 72%, respectively.
It is evident that as tissue thickness increases, the intensity received by PV cells decreases, resulting in smaller VOC values for each PV cell. Notably, sample #4, the thickest, exhibited the most significant reduction of 60%–72%. Conversely, muscle tissue (sample #2), which has the same thickness as fat tissue (15 mm), experienced the most minor reduction, which is 95%–98%. This study did not include repeat measurements; thus, future studies should consider conducting repeated measurements and analyzing them statistically. This is crucial due to the potential deviations between initial and final measurements, as highlighted by [31].
Fig. 7.
Measurement result of VOC of each PV cells on different samples: (a) sample #1; (b) sample #2; (c) sample #3; and (d) sample #4.
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4 Conclusion
The findings presented in this paper provide an understanding of the performance attributes of the electrical power delivery system within biological tissues; this includes scenarios where devices such as NIR LED 810 nm and PV cells. This study considered 375 mW of transmitter power maximum (supplied by 500 mA of LED current) and conducted on ex-vivo experiments using porcine samples. OWPT through biological tissue employing commercial PV cells and single beam NIR LED 810 nm is promising. PV cells can generate voltage despite the light being highly attenuated thick porcine samples up to 30 mm. Fat tissue is a better medium for light propagation than muscle tissue, as it results in higher optical power received by the receiver and, consequently, greater VOC of the PV cell. The experimental measurements in this study furnish essential and foundational data for developing robust OWPT tailored to various types of IMDs. The VOC value obtained in this study is greater than that reported in comparable literature utilizing commercial PV cells. The VOC value obtained in this study is greater than that reported in comparable literature utilizing commercial PV cells. This study only relies on VOC parameter since it is initial form of extensive study, hence, more analyses are required (for instance, electrical current, power, and energy).
In the future, we will consider integrating the presented approach with an energy harvesting circuit to analyze its electrical power and energy charging level against time. Thus, we will able to determine the duration of charging periods using PV cells and storage (e.g., coin rechargeable battery or supercapacitor) and its operating time when connected to IMDs (e.g., pacemakers). When connecting PV cells to a commercial power management integrated circuit (PMIC) development kit, it is crucial to be aware of the VOC value of the PV cells. This is because the kit has a specific input voltage rating, for instance, the E-peas PMIC AEM10330 or AEM10300 has a voltage input rated from 100 mV – 4.5 V; exceeding these maximum limits for voltage input could result in damage. The VOC measured in this study satisfy the minimum requirements for current IMDs devices, which is typically 2V – 3V. The #PV2 and #PV3 can be seamlessly integrated with these mentioned PMIC for further analysis as it falls within safe limits, which is lower than 4.5V. The PMIC with a higher maximum input voltage, like the AEM10941 which has an input voltage range of 50mV to 5V, provide an alternative option for #PV2 and #PV3. However, caution should be exercised before integrating to PMIC when using #PV1 as the measured voltage level for some samples exceeds 4.5 V. The compatible PMIC designated for #PV1 could be BQ25504RGTT, which supports a maximum input voltage of 5.5V [39]. One of the challenges is that PV cells are naturally designed for wide spectrum operation (e.g., outdoor light coming from the sun or solar light, indoor light, and so on), while in our case, we used a narrowband light source, which is NIR light. Employing commercial PV cells for NIR light can be a fascinating issue in future.
Acknowledgments
This study was funded by the University of Oulu’s Infotech (CWC-NS 2406124). The authors would like to acknowledge to the Research Council of Finland (former Academy of Finland) Profi6 funding, 6G-Enabling Sustainable Society (6GESS) the University of Oulu and the Research Council of Finland 6G Flagship Programme (Grant Number: 346208).
Disclosure of Interests
The authors have no competing interests to declare that are relevant to the content of this article.
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