# Towards Bio-Hybrid Energy Harvesting in the Real-World:
## 6 Promising Bio-Energy Solutions, Lessons Learned
### MFCs  
MFC technology is the most promising bio-reactor with the possibilities of becoming the next green industrial alternative to conventional high carbon-footprint solutions [15; 16]. However, the bottleneck remains at the point of finding suitable technologies for bringing the MFCs from the lab environment to the chemically harsh external world. For the application in wastewater treatments, the anodes must be robust for withstanding the impacts of diverse organic and inorganic components [60]. Presently, carbon based anodes are preferred for their bio-compatibility, porosity, corrosion resistance and conductive features. We predict that this research trend will continue for a few more years until an optimal anodic configuration is achieved. For cathodes, two major concerns were prevention of bio-fouling [65] and ensuring continuous supply of electron acceptors [31] at cathodic chamber. In the field of MFC membranes, new porous, ceramic membranes have emerged as a promising solution and our prediction indicates that we shall find more economically viable and mechanically robust solutions from this line of research. Moreover, some research groups consider MFC as the most suitable technology for providing energy autonomy to robots since the process can convert organic waste directly into electrical energy and this concept was proven by the 'Ecobot's [382; 383] and 'Slugbot' [381]. [PERSON] et al. [425] and [PERSON] et al. [385] focused on this concept in their reviews. Thus, an interesting field is emerging where MFC will be utilized as a robotic gut to power next-generation, bio-inspired robots. Early generation of such robots were limited by their substrate extraction mechanism. At this point, the challenges are twofold: to design small-scale energy efficient MFC cells and to develop robotic systems for engulfing organic waste from their surrounding [384; 426]. These examples indicate a next generation of MFC-powered energy autonomous robots, capable of sustaining by themselves without requiring additional intervention for power maintenance. In order to bring this output to an applicable range, MFC-based fuel cells require efficient and dedicated power management systems. The reviews by [PERSON] et al. [200] and [PERSON] et al. [211] give a quick overview of the recent progress in the dedicated power management systems for MFCs where [PERSON] et al. [200] elaborately summarizes MFC energy harvesting systems, methodologies and components [PERSON] et al. [211] focuses more on the newly emerging maximum power point tracking (MPPT) systems. The major challenges in energy harvesting from MFCs for making them an alternative energy solution can be outlined as:  
1. MFCs provide much lower volumetric power densities for bigger applications compared to smaller ones [147]. 2. The power generation mechanism of MFC systems is not inherently self starting and usually require additional jumpstarting technology [200]. 3. They do not offer the flexibility of stacking cells for increasing voltage and current ratings since a slight voltage mismatch creates local voltage reversal circuits and reduce the total output [205]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Biomechanical Energy Harvesting Focus Topic** & **Sub-Topic(s)** & **Studies** \\ \hline Review articles & data & [40; 286; 408; 409] \\ Harvesting mechanism & Pezoelectrinc & [1; 290; 316; 409; 410; 411; 412] \\  & TENG & [1; 36; 298; 300; 302; 307; 316; 319; 320; 408; 413; 414; 415] \\ Harvesting from non humans & Insets & [42; 322] \\  & Animals of higher order & [289; 292; 294; 301; 211; 416] \\ From human wearables & Skin patches & [36; 318; 320; 414; 415; 417] \\  & Smart textile & [316; 317; 413; 418; 419] \\  & Shoes & [306; 411; 412; 415; 420; 421; 422; 423; 424] \\  & Backpacks & [41; 303; 304] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of reviewed works on biomechanical energy harvesters.