On Sep. 1, Kailue Chen, a PhD student in the School of Chemistry and Chemical Engineering of Beijing Institute of Technology, published a paper entitled "An Edible and Nutritive Zinc-Ion Micro-supercapacitor in the Stomach with Ultrahigh Energy Density" as the first author in the journalACS Nano, with BIT as the sole correspondent, and Special Researcher Yang Zhao from the School of Chemistry and Chemical Engineering of BIT as the corresponding author. Prof. Liangti Qu from Tsinghua University and Prof. Yu Ma from Lanzhou University gave important suggestions and guidance to this work. This research was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China.

As effective clinical tools, orally ingestible biomedical electronic devices, which not only record pH, pressure, temperature, and gas release but also perform capsule endoscopy and drug delivery in the gastrointestinal (GI) tract, have been developed for therapy to treat, monitor, or diagnose health conditions in humans. To drive these sophisticated temporary implantable electronics, standard button batteries (e.g., RENATA 337) with available high energy density and long life are currently employed as power supplies of commercial capsule equipment. However, these button batteries usually contain toxic electrode materials and organic electrolytes. Once the protective shell breaks, accidental battery intake may lead to the risk of serious electrochemical burns, making it potentially life-threatening. When implantable batteries fulfill their mission, they may need to be removed, increasing the risk of surgery and financial losses. Although some advanced energy storage or energy capture technologies have been made over the past few years, including biological energy sources that capture heat or muscle movements and wireless charging energy storage microdevices, the terms of security and practical use are still limited. Therefore, these circumstances have created a great need for a new energy device that can drive biomedical electronic equipment and is biocompatible and biodegradable in the field of biomedical engineering.

Edible energy storage devices such as batteries and supercapacitors that can be absorbed or degraded in vivo are the hot research topics in recent years. In terms of batteries, they have high specific energy densities but limited power density due to the slow ion diffusion rates in solid electrode materials. Current reported edible batteries are still limited by the narrow voltage window (usually less than 1 V) and unsatisfactory electrochemical performance due to the limitation of choices for electrodes and electrolytes. Nadeau et al. fabricated a Zn–Cu primary cell using gastric or intestinal fluids of the surrounding environment as an electrolyte in vivo to power a sensor for detecting the temperature of the GI tract of pigs. This primary cell with a working voltage of only 0.1–0.2 V needs to install an additional boost converter for use by the circuit, and the used electrode has a large size (30 mm × 3 mm), which is out of the size range to swallow. On the contrary, supercapacitors with high power density and fast charge/discharge rate may solve the power problem of batteries and thus have attracted extensive research. However, their fatal disadvantages lie in low energy density and short single service life, leading to limited long-term energy output. Meanwhile, their intrinsic characteristics of a high self-discharge rate will cause damage to gastric mucosa. These factors have led to the fact that these devices with capacitive behavior have not yet been available in vivo for electronics demonstrations.

These challenges motivate researchers to continuously explore and develop fully bioabsorbable and safe edible devices. Considering these aspects, a hybrid micro-supercapacitor consisting of a capacitor electrode and a battery-type electrode can achieve high energy density while retaining most of the power density, which offers an alternative option to power these devices. Compared to other aqueous systems based on monovalent or multivalent cations, Zn-ion-based hybrid micro-supercapacitors (ZMSCs) are an ideal choice due to extremely high theoretical capacity (823 mAh g-1，5855 mAh cm-3), low redox potential of 0.76 V vs the standard hydrogen electrode (SHE), relatively fast ion transport kinetics, and high safety. In addition, zinc is also one of the essential trace elements needed for human health. The maximum daily intake of zinc recommended by the U.S. Nutrition Committee can reach up to 40 mg when zinc is ingested into the body. Not only that, the zinc sulfate (ZnSO4) electrolyte can be used as a nutrient to promote growth and development and improve tissue energy metabolism and bactericidal function of phagocytes. To date, various ZMSCs have been developed using different cathode materials, such as commercial activated carbon (AC), MXenes and carbon nanotubes. However, because of the strict biocompatibility requirements of electrode materials for edible devices, most of the currently developed cathode materials cannot be directly applied into living organisms. For instance, commercial ACs are preferably used as the electrode candidates in Zn-ion-based energy storage devices due to their excellent conductivity, high specific surface area, and developed pore structure, but they are usually obtained from pyrolysis and activation of carbon materials such as coal and petroleum tar, which may contain some unknown contents of heavy metals and their hardness and ash are also relatively high. Therefore, in this work, we used edible carbon that is produced by thermal cracking of wood and husk with special treatment as the positive electrode material and Zn as the negative electrode material to prepare a new edible ZMSC by "Pre-drawn" patterning and template method (Figure 2). With features including mechanical flexibility and light weight, it avoids the damage of rigid materials to organs, potentially mitigating GI obstruction.

Figure 2. Design, fabrication, and optical images of the e-ZMSC.

The edible ZMSCs (e-ZMSCs) combining the non-faradaic surface reaction on the edible activated carbon (e-AC) microcathode and the faradaic reaction on the Zn microanode render an edible hybrid microdevice delivering a high energy density of 215.1 μWh cm-2, superior to that of the currently reported biocompatible supercapacitors/micro-supercapacitors and even traditional ZMSCs reported previously (Figure 3). It also exhibits an excellent area capacity of 605 mF cm-2, a voltage window of 1.8 V, and high self-discharge resistance. The 50 charge/discharge cycling measurements of the edible ZMSC were subsequently monitored with the help of XRD, SEM and impedance, and the reaction mechanism was explored, showing that reversible electrochemical reactions occur at the positive and negative electrodes during the first 50 cycles and that the electrolyte ion transport rate increases, improving the electrochemical performance of the device (Figure 4). Taking inspiration from the humidity-dependent viscosity feature of a gelatin sheet, a circuit design provides a solution to make up for the shortcomings of large volume and limited performance of edible batteries and supercapacitors at present.

Figure 3. Electrochemical performance of e-ZMSC.

Figure 4. Study of electrodes reaction mechanism and spatial optimization of e-ZMSCs.

Finally, we demonstrated the edibility and safety of the devices by in vitro and in vivo tests, and the bacterial inhibition rate is close to 100% of the bacteria within 20 min by electrical stimulation, demonstrating the antibacterial ability of the devices (Figure 5, Figure 6). In addition, the integrated edible ZMSC was successfully applied to power an electrical device (LED) in vivo. This study provides a new idea for the design and preparation of next-generation edible energy storage devices and lays the foundation for the eventual realization of a real in vivo capsule that can be used for testing.

Figure 5. In vitro characterization and biocompatibility study.

Figure 6. In vivo studies and biocompatibility characteristics.

Link:https://doi.org/10.1021/acsnano.2c06656.

Personal profile:

Kaiyue Chen is a PhD student of the School of Chemistry and Chemical Engineering, Class of 2019, supervised by Special Researcher Yang Zhao and has been pursuing her PhD in this research group since 2019. In this period, she has published one paper inACS Nano,Journal of Power Sources, andNano Materials Sciencejournals respectively as the first author and one paper inChemSusChemjournal as the co-author.

Yang Zhao is a special researcher and doctoral supervisor of the School of Chemistry and Chemical Engineering, Beijing Institute of Technology. She has published more than 50 SCI papers as the first or corresponding author, includingNat. Commun.,SCI. Adv.,JACS,Angew. Chem. Int. Ed.,Adv. Mater.,Energy Environ. SCI.,ACS Nano. In total, more than 90 SCI papers have been published, with more than 10,000 citations, and 4 patents have been authorized. One of the achievements has been industrialized. She has presided over a number of National Natural Science Foundation of China and Beijing Natural Science Foundation projects, and participated in a number of national Important Basic Research and Development (973) program subjects, key research and development programs, etc.