Anisotropic Thermal Conductivity in Topologically Networked Protein-MXene Composites
Governing thermal transport in engineered materials creates opportunities to redirect and recover the excess heat generated in electronic and energy-conversion devices. Materials that pair low cross-plane thermal conductivity with high in-plane thermal conductivity are particularly valuable because they confine heat and channel it away from sensitive regions, preventing localized device failure. Two-dimensional crystals are efficient building blocks for such anisotropic thermal conductors, but they are brittle, and the polymer composites used to toughen them usually forfeit much of the intrinsic anisotropy: in conventional percolation-based design, filler fraction is the only handle available, and it governs both in-plane and cross-plane conduction. Here we report a composite of Ti3C2Tx (MXene) nanosheets and squid ring teeth (SRT) inspired recombinant tandem-repeat (TR) proteins in which the protein serves as a molecular template and bridge, setting the spacing between nanosheets with angstrom-level precision through the number of tandem-repeat units and independently of the filler fraction. This structural handle provides a second, independent design parameter. At a fixed MXene loading, the number of repeats tunes the cross-plane conductivity (0.30 to 0.93 W/mK) and, with it, the thermal anisotropy ratio over a wide range (from about 70 down to 17), while the in-plane conductivity stays high (16 to 21 W/mK). We rationalize these trends with a Gaussian Network Model (GNM) of the protein embedded in a two-phase layered medium, which reproduces the measured directional conductivities from a single structural parameter and identifies the protein gallery as the cross-plane bottleneck. Extending the model to a mechanically loaded five-period stack, we find that the anisotropy is robust to reversible compression and twist, changing by only a few percent, so the number of tandem repeats, not the applied strain, is the dominant design handle. Because anisotropy is tuned structurally rather than volumetrically, these protein-MXene composites decouple thermal anisotropy from filler content, pointing toward flexible thermal materials that are not bound by the rules of mixture and percolation.