It all starts with bacterial cellulose (C6H10O5) – a gelatinous, absorbent compound that is produced naturally by certain types of bacteria. In contrast to cellulose from plants, this material can also be made with high chemical purity at a large scale in the lab. To transform it into a carbon aerogel, researchers first wash freeze-dried pellicles of bacterial cellulose in hot sodium hydroxide (NaOH) to remove the bacteria, and then carbonize it. This process generates wastewater that requires further treatment before disposal. In addition, the mechanical properties of the resultant aerogels (named tCBC) leave a lot to be desired – they can be easily deformed under compression.
To tackle both of these issues, the Jiaotong team skipped a step. Rather than soak the cellulose in NaOH, they rinsed it with water, preserving the bacteria in situ. The bacteria-laden pellicle was then carbonized as before (and named uCBC). The two discs (one tCBC and the other uCBC) could then be put through their paces, and the results of this study appear in a recent issue of Carbon [DOI: 10.1016/j.carbon.2021.07.021].
Visually there were differences between the discs, most notably in size. Nominally 20 mm in diameter and 10 mm in height, the bacteria-free aerogel tCBC was notably smaller than the bacteria-laden uCBC after carbonization. In mechanical tests, the uCBC sample exhibited extraordinary supercompressibility and elasticity, recovering its original shape even under cyclical tests at 70 % strain. The tCBC sample experienced permanent deformation under the same load. In static creep and fatigue tests, uCBC again outperformed its competitor, remaining unchanged even at high pressures and strains. The electrical conductivity of uCBC was also found to be strain-dependent, which suggests that it could be useful as a pressure sensor. The authors attribute this mechanical performance to the presence of carbonized bacteria in uCBC, which they say “serve as buffers to store energy when the external stress is applied, and convert the stored energy to mechanical energy upon the release of the external stress.”
The samples also differed greatly in their response to water. The tCBC aerogel was seen to be superhydrophobic, with a contact angle ~139° over the duration of the sessile drop test. In contrast, uCBC was hydrophilic – it exhibited an initial contact angle of ~40° which then decreased to 0° (or full wetting) within 15 seconds. This observation was attributed to the presence of nitrogen in the bacteria-laden aerogel. The authors say that this property could be beneficial for tissue engineering applications, potentially enhancing cell adhesion to the aerogel.
Though still at a very early stage of development, this methodology shows a great deal of promise. Further studies are ongoing.
This story was written for Materials Today: https://www.materialstoday.com/carbon/news/nanofiber-aerogel-made-from-bacteria/
Research paper: Jie Wang, Yizao Wan, Xiaowei Xun, Liyun Zheng, Quanchao Zhang, Zhaohui Zhang, Yu-Xin Xie, Honglin Luo, Zhiwei Yang. “Engineering bacteria for high-performance three-dimensional carbon nanofiber aerogel”, Carbon, Volume 183 (2021) 267-276 DOI: 10.1016/j.carbon.2021.07.021