In many industrial applications, titanium dioxide (TiO₂) is the pigment of choice for a bright white color impression and its production accounts for over 90 % of the global titanium mineral consumption [1].  However, sustainability [2] and health concerns [3] on TiO₂ led to a continuous controversial debate of its usage and the EU recently introduced first bans on TiO₂ as food additive.
In nature, a bright white color impression frequently originates from transparent bio-materials e.g. chitin, cellulose or keratin equipped with sophisticated nanostructures such as porous networks [4], ellipsoidal beads [5] or air voids [6]. The latter can be mimicked by foaming polymers with non-toxic, inert, supercritical CO₂ resulting in light scattering air voids in the nanometer range and thus a white color impression.
As an effect caused by the porous structure alone, the white color is achieved without additional pigments. The respective foamed polymer remains a mono-fraction, which simplifies recycling, the polymer becomes transparent again by heating.
The supercritical CO₂ foaming process is suitable for polymers with a high CO₂ solubility and can be adapted for foaming foils, thin films [7] and particles [8]. It is also applicable to many bio-based and/or bio-degradable polymers such as polylactide, cellulose acetate and polyhydroxy butyrate to name a few, which enable further application possibilities in cosmetics or paper industry.

References
[1] J. Winkler, Titandioxid; Vincentz Network GmbH & C, 2013
[2] M. J. A. Ruszala et al., Int. J. Chem. Eng. Appl., 2015, Vol. 6, No. 5, pp. 331-340
[3] R.Baan et al. Lancet Oncol., 2006, Vol, 7, No. 4, pp. 295-296
[4] P. Vukusic et al., Science, 2007, Vol. 315,   No. 5810 p. 348-348
[5] D. G. Stavenga et al., Proc. R. Soc. Lond. B., 2004, Vol 271, pp. 1577–1584
[6] A. J. Parnell et al. ,Sci. Rep. , 2015, Vol. 5, No. 1, p. 18317
[7] J. Syurik et al., Sci. Rep., 2017, Vol 7, 46637
[8] L.M. Borgmann et al., Bioinspir Biomim . 2023 , Vol. 18, No. 2, 026011