nomic losses, and a reduction in natural fossil resources. With the current recycling infrastructure, only 14% of current plastic-packaging waste is collected, and even with this recycling, 4% is lost during the process and 8% is redirected for the fabrication of lower-value applications. [5] The long degradation half-lives of synthetic plastics, which break down in the environment over decades or centuries, is the central problem of sustainability. [4,6] Therefore, seeking sustainable and biodegradable alternatives to synthetic plastics have become a pressing task. Developing biodegradable products from renewable resources (e.g., natural biopolymers) represents the best option by fitting material design, production, use, and disposal into a circular materials lifecycle approach (Figure 1b). [7] Instead of losing fossil fuel resources into landfills and causing environmental burdens, natural biopolymers offer options for composting/degradation, thus, recycling the carbon back to environmental cycles for the regeneration of feedstocks. Besides being sustainable and biodegradable, fibrous structural biopolymers from plants (e.g., cellulose) and animals (e.g., silk, chitin) have unique hierarchical structures, with structural integrity, flexibility, and toughness. Therefore, fibrous structural biopolymers represent a class of material that could help to supplement or replace some conventional synthetic plastics. Additionally, the inherent biocompatibility of most natural biopolymers makes them important candidates for added-value applications, such as in regenerative medicine, drug delivery, and tissueimplantable devices. Among these natural biopolymers, cellulose, chitin, and silk fibroin represent the most abundant and investigated biopolymers in the categories of polysaccharides and proteins (Table 1).The first step in this progression toward sustainability is to process fibrous biopolymers into material formats suitable for targeted applications. The sophisticated hierarchical structure of fibrous biopolymers, which consists of well-organized structural features from molecular to nano-to macroscopic length scales, employ extensive hydrogen bonding networks embedded within semicrystalline structures across all structural levels. This structural stability endows fibrous biopolymers with exceptional mechanical properties, but also generates challenges with the direct processing of the fibrous materials into useful material formats via traditional thermal processing methods that are widely used for synthetic polymer processing Some of the most abundant biomass on earth is sequestered in fibrous biopolymers like cellulose, chitin, and silk. These types of natural materials offer unique and striking mechanical and functional features that have driven strong interest in their utility for a range of applications, while also matching environmental sustainability needs. However, these material systems are challenging to process in cost-competitive ways to compete with synthetic plastics due to the limited options for ther...
