## 11 Bioplastic Foams for Electromagnetic Interference Shielding  
The replacement of conventional fossil fuel-derived synthetic polymeric foams with bio-sourced, biodegradable and sustainable alternatives will be crucial for promoting a circular economy. Synthetic foams such as polystyrene (PS) and polyolefin foams end up as landfill after their lifecycle and are estimated to remain in soil for generations and wreak havoc on the environment. Recently, much attention has been focused on PLA, which is a linear aliphatic polyester sourced from renewable feedstocks such as corn, potato, sugar and other agricultural sources. In general, there are three types of PLA, viz. poly(D-lactic acid) (PDLA), poly(L-lactic acid) (PLLA) and a racemic blend of D,L-PLA (PDLA). Nowadays, a majority of sustainable research is inclined towards poly(lactic acid) (PLA) foams and they are  
Figure 11: (**A**) Stepwise fabrication process of PU/graphene foams, including dip-coating GO sheets onto PU foams followed by hydrothermal reduction using hydrazine vapor. (**B**) A piece of foam showing uniform coating after being torn from the middle portion. (**C**) A block of foam on the leaf of an asparagus fern. (**D**,**E**) Optical and SEM images of pristine PU sponge and PU/graphene foam. (**F**) Compressing and releasing these foams depicting excellent compressibility and recovery. Reproduced with permission from [113]. Copyright 2016, American Chemical Society. being widely explored as promising biodegradable substitutes for synthetic foams, which currently own a major stake in the industry. This is primarily due to the ease of processing, good barrier and mechanical properties of PLA, accompanied by their environmentally friendly nature. Besides the packaging industry, there are huge prospects for PLA foams for use in biomedical, energy, sensors and EMI shielding applications. In this context, [PERSON] et al. [114] fabricated a facile and eco-friendly route for the large-scale production of biodegradable and ultra-low-threshold PLA/MWCNT porous nanocomposites for superior thermal insulation and EMI shielding. Herein, PLA beads were foamed using a bead-foaming process to obtain expanded PLA (EPLA) beads, followed by mechanical mixing with (0.25-2.5 wt.%) MWCNT dispersion. This resulted in a uniform coating of MWCNT onto EPLA beads, which were then sintered together by the steam-chest molding process to fabricate bulk PLA/MWCNT porous nanocomposites with 3D segregated MWCNT networks distributed between the EPLA pellets, as shown in Figure 12A. It also worth mentioning that porous nanocomposites fabricated in this fashion exhibited an ultra-low percolation threshold of 0.00094 vol.% and a density of 0.045 g/cm\({}^{3}\), as seen in Figure 12B. A maximum EMI SE value of ~50 dB was recorded at 8 GHz, and a specific EMI SE value of 1010 dB-cm\({}^{3}\cdot\)g\({}^{-1}\)[114]. Moreover, the shielding mechanism in these porous structures was primarily governed by absorption of incident EM waves. However, PLA exhibits relatively less heat resistance and can hardly be used in extremely hot conditions above 100 \({}^{\circ}\)C. Therefore, to enhance the thermal stability of PLA foams, another strategy that was adopted by [PERSON] and coworkers is creating stereo complex crystallites (sc) between enantiomeric poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) via stereo complexation crystallization. Initially, CNT was homogeneously dispersed in the PLLA/PDLA/dichloromethane (DCM) solution by ultrasonic dispersion and mechanical mixing. Hexane was then added dropwise to promote the formation of stereo complex crystallites, which will then form a 3D network-like structure due to gelation. The porous  
Figure 12: (**A**) Schematic representation of the fabrication technique used to design segregated MWCNT networks in porous PLA/MWCNT foams. (**B**) Variation of the electrical conductivity of these foams with respect to MWCNT content. The inset plot shows \(\sigma\) vs. (\(\varphi-\varphi_{c}\)).