Fabricating damage tolerant porous ceramics with efficient energy absorption and impact-resistant capability has been a challenge because of the brittle nature of ceramic materials. In nature, mineralized tissues or organisms such as cuttlebones and diatoms have evolved with hierarchical porous structures to overcome this difficulty. A bioinspired design of ceramic lattice structure with pores at multiple length scales, ranging from few nanometers to hundreds of micrometers, is proposed in the present work. These ceramic lattices with hierarchical porous structures were successfully fabricated via 3D cryogenic printing. Under quasi-static compressions, the printed ceramic lattices showed unprecedented long plateau strain (~60%) and a specific energy absorption of ~10 kJ·kg^‒1 with a porosity of ~90%. The resulting energy absorption capability was comparable with most composites and metals, thus overcoming the brittle nature of traditional porous ceramics. This was attributed to the delayed destruction of the lattice structure, as well as the gradual collapse of pores at multiple length scales. Similar trends have also been observed under split Hopkinson pressure bar (SHPB) tests, indicating excellent energy absorption under high strain-rate impacts. The proposed 3D printing technique that produces hierarchical pores was also demonstrated to apply to other functional materials, such as silicon carbide, barium titanate, hydroxyapatite, and even titanium alloy, thus opening up new possibilities for fabricating bioinspired hierarchical porous structures.

In tissue engineering (TE), tissue-inducing scaffolds are a promising solution for organ and tissue repair owing to their ability to attract stem cells in vivo, thereby inducing endogenous tissue regeneration through topological cues. An ideal TE scaffold should possess biomimetic cross-scale structures, similar to that of natural extracellular matrices, at the nano- to macro-scale level. Although freeform fabrication of TE scaffolds can be achieved through 3D printing, this method is limited in simultaneously building multiscale structures. To address this challenge, low-temperature fields were adopted in the traditional fabrication processes, such as casting and 3D printing. Ice crystals grow during scaffold fabrication and act as a template to control the nano- and micro-structures. These microstructures can be optimized by adjusting various parameters, such as the direction and magnitude of the low-temperature field. By preserving the macro-features fabricated using traditional methods, additional micro-structures with smaller scales can be incorporated simultaneously, realizing cross-scale structures that provide a better mimic of natural organs and tissues. In this paper, we present a state-of-the-art review of three low-temperature-field-assisted fabrication methods—freeze casting, cryogenic 3D printing, and freeze spinning. Fundamental working principles, fabrication setups, processes, and examples of biomedical applications are introduced. The challenges and outlook for low-temperature-assisted fabrication are also discussed.

Biomass dielectric polymers hold promise in developing renewable and biodegradable capacitive energy storage devices. However, their typical discharged energy density remains relatively low (<20 J/cm³) compared to other existing synthetic polymers derived from petroleum sources. Here a greatly enhanced discharged energy density is reported in diluted cyanoethyl cellulose (CEC) nanocomposites with inclusion of ultralow loadings (0.3%, in volume) of 30 nm sized TiO₂ nanoparticles. Owing to the interfacial polarization introduced by interface, the composite of 0.3% exhibits a large dielectric constant of 29.2 at 1 kHz, which can be described by interphase dielectric model. Meanwhile, the introduction of nanofillers facilitate the formation of deeper traps impeding electrical conduction in CEC, which results in an ultrahigh breakdown strength of 732 MV/m. As a result, a remarkable discharged energy density of 12.7 J/cm³ with a charge-discharge efficiency above 90% is achieved, exceeding current ferroelectric-based and biomass-based nanocomposites. Our work opens a novel route for scalable biomass-based dielectrics with high energy storage properties.