Microphase separation. Microcrystallization is the process of polymer chains forming submicrometer-sized crystallites, representing a microphase-separated domain surrounded by amorphous polymer chains (Figure ). PVA is an attractive candidate for the formation of hydrogels via crystallization. During the process of freeze-thawing, the polymer solution is frozen at degrees below 0 °C to grow the crystallites followed by thawing at room temperature. With incremental freeze-thaw cycles, the hydrogel's mechanical properties are enhanced together with the overall crystallinity (saturating towards 35%). It is hence unsurprising that in various cases PVA has been exploited for the synthesis of tough hydrogels, especially given its nontoxic and biocompatible nature (Figure ). In a recent work, the method of directional freezethawing was proposed to create tough hydrogels with anisotropic morphology. As opposed to freezing the hydrogel precursor solution at once, Zhang et al. prepared a mold that was lowered at a constant velocity into a bath of liquid nitrogen (much like a dip-coating approach), causing the freezing process to occur in a directional manner. A PVA crystallinity of 55% was achieved through this procedure with SEM micrographs showing the oriented pore morphology of the hydrogels. Despite the hydrogel's anisotropy, selective parallel and orthogonal mechanical testing revealed only mild differences (E ∥/⊥ = 70/90 kPa, Γ ∥/⊥ = 0.27/0.43 kJ/m 2 ). In a work by Hao et al, PVA was combined with hyaluronic acid (HA) and collagen to form a semi-interpenetrating network (SIPN, since only the PVA is cross-linked). The resulting gels showed a greatly improved stiffness (E = 25.4 MPa) at a high water content of 80%, yet a fracture energy of Γ = 1.27 kJ/m 2 was measured. The microcrystallization of PVA successfully produced a number of double networks, such as PVA/PAAm, PVA/Alg-Ca 2+ , PVA/Pectin-Ca 2+ , or PVA/PAAc-Fe 3+ (Figure ). The last mentioned report by Wang and co-workers used a second network of PAAc in combination with Fe 3+ ions, which promoted metal-ligand interactions in both the PAAc and the PVA network interconnecting the two. This enabled excellent mechanical and fracture properties (E = 7.1 MPa, Γ = 101 kJ/m 2 ), at the same time maintaining excellent strength and ductility of the hydrogel (σ b = 20.7 MPa, ε b = 1080%). Moreover, when the sample was colddrawn to orient the polymer chains, an elastic modulus of up to E = 100 MPa was measured with the fracture energy sadly not being reported for this sample. Likely, both the Fe 3+ as cross-linker and the reduced water content of 36% contribute to the mechanical properties and it remains difficult to decouple the two.