applied sciences Review Towards Bio-Hybrid Energy Harvesting in the Real-World: Pushing the Boundaries of Technologies and Strategies Using Bio-Electrochemical and Bio-Mechanical Processes Abanti Shama Afroz 1 , Donato Romano 1,2,* , Francesco Inglese 1 and Cesare Stefanini 1,2,3 1 The BioRobotics Institute, Scuola Superiore Sant’Anna, Viale Rinaldo Piaggio 34, 56025 Pontedera, Italy; [email protected] (A.S.A.); [email protected] (F.I.); [email protected] (C.S.) 2 Department of Excellence in Robotics & AI, Scuola Superiore Sant’Anna, Piazza Martiri della Libertà 33, 56127 Pisa, Italy 3 Healthcare Engineering Innovation Center (HEIC), Khalifa University, Abu Dhabi 127788, United Arab Emirates * Correspondence: [email protected] Abstract: Sustainable, green energy harvesting has gained a considerable amount of attention over the last few decades and within its vast field of resources, bio-energy harvesters have become promising. These bio-energy harvesters appear in a wide variety and function either by directly generating energy with mechanisms similar to living organisms or indirectly by extracting energy from living organisms. Presently this new generation of energy harvesters is fueling various low-power electronic devices while being extensively researched for large-scale applications. In this review we concentrate on Citation: Afroz, A.S.; Romano, D.; recent progresses of the three promising bio-energy harvesters: microbial fuel cells, enzyme-based Inglese, F.; Stefanini, C. Towards fuel cells and biomechanical energy harvesters. All three of these technologies are already extensively Bio-Hybrid Energy Harvesting in the Real-World: Pushing the Boundaries being used in small-scale applications. While microbial fuel cells hold immense potential in industrial- of Technologies and Strategies Using scale energy production, both enzyme-based fuel cells and biomechanical energy harvesters show Bio-Electrochemical and promises of becoming independent and natural power sources for wearable and implantable devices Bio-Mechanical Processes. Appl. Sci. for many living organisms including humans. Herein, we summarize the basic principles of these 2021, 11, 2220. https://doi.org/ bio-energy harvesting technologies, outline their recent advancements and estimate the near future 10.3390/app11052220 research trends. Academic Editor: Keywords: bio-hybrid systems; bio-energy sources; energy; bioengineering; microbial fuel cells; bionics Borja Velazquez-Marti Received: 7 January 2021 Accepted: 24 February 2021 1. Introduction Published: 3 March 2021 Development of sustainable and zero-carbon-emission energy resources is considered Publisher’s Note: MDPI stays neutral one of the most demanding goals for the current world [1]. Within the vast field of green with regard to jurisdictional claims in energy harvesting, an important one is bio-energy harvesting [2,3]. Although such possibil- published maps and institutional affil- ities were proposed much earlier [4], this started to gain prominence in the early 2000s [5–7]. iations. This relatively new research field is opening up opportunities for producing energy from the wide range of living creatures, including microorganisms [6] and macro-organisms [8], as well as from bio-hybrid organisms [9]. The application fields of these bio-energy har- vesters are equally elaborated, ranging from industrial-scale energy production [10] to environmental monitoring [11,12] and biomedical applications [13,14]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. The first bio-energy harvesters reported in this review are known as microbial fuel This article is an open access article cells (MFCs) which offer the possibility to become one of the next big industrial energy distributed under the terms and solutions [15,16]. MFCs utilize catabolic metabolism of different microorganisms on a wide conditions of the Creative Commons range of organic substrates [17] and turn them into micro bio-reactors generating electrical Attribution (CC BY) license (https:// energy [18]. Extensive works have been performed on summarizing its various aspects creativecommons.org/licenses/by/ like development in electrode configurations [19–22], dedicated power management sys- 4.0/). tems [23,24], and cell separators [25–27]. With the prospects of becoming a next-generation Appl. Sci. 2021, 11, 2220. https://doi.org/10.3390/app11052220 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 47 Appl. Sci. 2021, 11, 2220 electrical energy [18]. Extensive works have been performed on summarizing its various2 of 44 aspects like development in electrode configurations [19–22], dedicated power man- agement systems [23,24], and cell separators [25–27]. With the prospects of becoming a large-scalenext-generation power large generation-scale power technology, generation they technology, are already they being are used already for being powering used up for remotepowering marine up remote sensors marine [23], long-distance sensors [23], marinelong-distance communications marine communications [28], simultaneous [28], wastewatersimultaneous processing wastewater with processing auxiliary power with supplyauxiliary [29 ],power metal supply recovery [29] processes, metal [recovery30] and inprocesses biosensors [30] [31 and]. in biosensors [31]. TheThe nextnext biochemicalbiochemical energyenergy harvesterharvester thatthat hashas gainedgained aa considerableconsiderable amountamount ofof popularitypopularity are are enzyme-based enzyme-based biofuel biofuel cells cells (EBFCs) (EBFCs) where where synthetically synthetically produced produced enzyme enzyme moleculesmolecules are are immobilized immobilized on on the the electrodes electrodes for performingfor performing glucose glucose oxidation oxidation and inand turn in powerturn power generation gene [ration32]. This [32] particular. This particular type falls type under falls the under bio-hybrid the bio energy-hybrid harvesters energy [har-33] andvesters is considered [33] and is the considered promising the natural promising resource natural for powering resource up for implants powering and up prosthetic implants devicesand prosthetic [34]. Research devices on [34] these. Research implantable on these fuel implantable cells is still fuel at the cells animal is still trial at the level animal [35]. Simultaneously,trial level [35]. Simultaneously, investigations have investigations also been started have also on testing been started the applicability on testing ofthe such ap- cellsplicability from external of such body-fluidscells from external [13]. body-fluids [13]. TheThe thirdthird and and final final bio-energy bio-energy harvester harvester discussed discussed in thisin this review review is mechanical is mechanical in na- in ture.nature. These These biomechanical biomechanical energy energy harvesters harvesters have ahave remarkable a remarkable growing growing market formarket power- for ingpowering smart and smart wearable and wearable devices [ 36devices]. These [36] harvesters. These areharvesters preferred are for preferred their non-invasive for their naturenon-invasive and easy-applicability nature and easy in-applicability monitoring [ 37in], monitoring diagnostic [[37]38], and diagnostic therapeutic [38] [and39] func-ther- tions.apeutic They [39] utilize functions. mechanical They utilize energy mechanical produced fromenergy living produced animals from and living convert animals them into and powerconvert solutions them into [40 power]. While solutions their use [40] for. humanWhile their application use for ishuman eminent application [41], they is have eminent also been[41], usedthey have for powering also been other used living for powering bio-hybrid other animals living [ 42bio].-hybrid animals [42]. InIn this this work,work, we we aim aim to to offer offer a a concise, concise, functional functional summary summary of of these these three three promising promising bioenergybioenergy harvesters harvesters and and their their progress progress in in the the recent recent years. years. 2.2. LiteratureLiterature SearchSearch MethodMethod AA hierarchicalhierarchical surveysurvey waswas performedperformed wherewhere “microbial“microbial fuelfuel cell”, cell”, “enzymatic “enzymatic fuelfuel cell”cell” andand “biomechanical“biomechanical fuelfuel cell”cell” were were utilized utilized as as primary primary keywords. keywords. AtAt the the secondary secondary level keywords “architecture”, “classification” and “applications” were used with all the level keywords “architecture”, “classification” and “applications” were used with all the three primary keywords. “Electrode” keyword was used for both MFCs and EBFCs. three primary keywords. “Electrode” keyword was used for both MFCs and EBFCs. Keywords “exoelectrogens”, “photo-reactors”, “membranes”, “waste management” were Keywords “exoelectrogens”, “photo-reactors”, “membranes”, “waste management” used only for MFCs. Similarly, “enzyme immobilization” was used for EBFCs while were used only for MFCs. Similarly, “enzyme immobilization” was used for EBFCs while “triboelectric nano generator”, “heap”, “ankle”, ”knee”, ”foot” and “upper limb” were “triboelectric nano generator”, “heap”, “ankle”, ”knee”, ”foot” and “upper limb” were used for biomechanical cells. A pictorial description on the use of the keywords’ hierarchy used for biomechanical cells. A pictorial description on the use of the keywords’ hierar- is given in Figure1.