Raffaele Di Giacomo, Chiara Daraio, and Bruno Maresca

Significance

We present a bionic material made of plant cells and carbon nanotubes (CNTs) that exhibits record high temperature sensitivity. This material outperforms by ∼2 orders of magnitude the best man-made materials. The basic mechanism governing this response is the ionic conductivity in the egg-box structure of the pectin backbone, which interconnects cellulose microfibrils in the plant cell wall. The use of CNTs allows the bioelectrical property found in living plants to persist after cell death and stabilizes the response of the dried cells at high temperatures.

Abstract

Conventional approaches to create biomaterials rely on reverse engineering of biological structures, on biomimicking, and on bioinspiration. Plant nanobionics is a recent approach to engineer new materials combining plant organelles with synthetic nanoparticles to enhance, for example, photosynthesis. Biological structures often outperform man-made materials. For example, higher plants sense temperature changes with high responsivity. However, these properties do not persist after cell death. Here, we permanently stabilize the temperature response of isolated plant cells adding carbon nanotubes (CNTs). Interconnecting cells, we create materials with an effective temperature coefficient of electrical resistance (TCR) of −1,730% K⁻¹, ∼2 orders of magnitude higher than the best available sensors. This extreme temperature response is due to metal ions contained in the egg-box structure of the pectin backbone, lodged between cellulose microfibrils. The presence of a network of CNTs stabilizes the response of cells at high temperatures without decreasing the activation energy of the material. CNTs also increase the background conductivity, making these materials suitable elements for thermal and distance sensors.

See: http://www.pnas.org/content/112/15/4541.abstract.html?etoc

PNAS April 14, 2015 vol. 112 no. 15 4541-4545

Fig. 1.

Schematic diagrams and scanning electron microscopy (SEM) images of cyberwood. (A) Representation of the material (black) with sputtered coplanar gold electrodes (yellow) and current measurement setup. (B) Optical image of a sample. (C) Diagram of BY-2 cells (gold) with MWCNTs (black lines). The cell walls are emphasized in gray. (D) SEM picture of tobacco cell (dark gray) with MWCNTs inside the cell wall (brighter lines). (E) Schematic diagram of the pectin backbone structure (blue lines) interconnecting cellulose microfibrils (gray bars), and the encapsulated metal ions (green circles) in the egg-box structure. Micropores between cellulose microfibrils are shown filled with water (light blue circles) and/or MWCNTs. (F) SEM image showing MWCNTs penetrating the cell wall of a BY-2 cell. Orange arrows indicate the edge of the cell wall and green arrows indicate the MWCNTs.