The tunability of the lattice spacing of hydrogel colloidal crystals has been achieved by simply varying the water content of the microgel pellet before annealing and crystallization. The manipulation of the lattice constant of the crystals and therefore of the wavelength of the resultant Bragg peak, i.e., the color of the crystal, leads to a myriad of photonic applications.
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Debord et al. (2002) studied this question.