Engineering Hard-Soft Interfaces: Insights from the Root-Soil Interaction for Bioinspired Design

Abstract

A hard-soft interface refers to the boundary between two materials or regions with significantly different mechanical properties, where one is rigid or hard and the other is flexible or soft. These interfaces are common in both engineered and natural systems and are characterized by the contrast in how each material deforms, transfers loads, or responds to environmental stress. Engineered hard-soft interfaces often experience failure due to high interfacial stresses, poor adhesion, and localized stress concentrations caused by mismatched mechanical properties. An example of engineered hard-soft interfaces can be found in tissue engineering, where they are used to replicate natural transitions between tissues, such as those between bone and cartilage. In contrast, hard-soft interfaces in nature, such as the root-soil system, demonstrate remarkable strength and adaptability, efficiently distributing loads and reducing stress concentrations despite differences in material properties.
The pull-out force was chosen to assess the strength of the root-soil interface, capturing the mechanical interactions of roots with their environment. This study focused on barley and mung bean seeds, chosen for their distinct root structures, barley with a fibrous system and mung bean featuring a taproot system. Over a 15-day growth period, various root characteristics such as length, diameter, tortuosity, and branching patterns were analyzed across soil and hydrogel substrates, each with distinct material properties and stiffness. The methodology included measuring growth in terms of days and stem height, along with 2D root trait extraction to analyze characteristics such as length, diameter and number of branches. Additionally, 3D computed tomography (CT) scanning was used to visualize root architecture, while pull-out tests provided key data on resistance and force-displacement curves, and finite element method (FEM) simulations enabled sensitivity analyses of various root structure configurations in a non-destructive manner. Lastly, experiments with hydrogel tested its viability for root growth, involving detailed protocols for hydrogel composition and seed preparation.
Plant growth measurements revealed a consistent increase in stem height over time, effectively captured by the logistic growth model. Laboratory pullout tests and root extraction demonstrated that increases in root characteristics such as length, diameter, and branching significantly improve pullout force in both barley and mung bean seeds. Moreover, pullout test results showed that barley roots have greater mechanical resistance and higher maximum forces than mung bean roots, although with greater variability in the data. FEM simulations indicated that a 45° vertical branching angle yielded the highest pullout force for barley in soil (5.09 N), while an 80° angle was most effective in hydrogel (4.98 N). In contrast, radial branching angles had negligible effects in both substrates. Tortuous root configurations significantly increased pullout force in soil, nearly doubling it from 5.09 N for straight roots to 9.80 N, but only slightly improved it in hydrogel, from 3.20 N to 3.61 N. The addition of branches in mung beans significantly increased pullout forces in both substrates due to the greater surface area, which enhanced root-substrate interaction. The FEM simulations showed that pullout forces were generally higher in soil due to its rigidity, which leads to a rapid increase in pullout force until root failure. Hydrogel, with its elastic properties, allowed roots to stretch more under load, providing uniform and gradual resistance. The FEM model was also validated through energy history output results and mesh convergence analysis. Lastly, initial experiments growing roots in hydrogel show promise for this substrate as a soil alternative, however further research is required to optimize its properties for plant growth.
Overall, this study provides a better understanding of the factors optimizing root anchorage and interface strength, offering design strategies for bioinspired engineered hard-soft interfaces. It also emphasizes the need to tailor natural design principles to the specific material properties of substrates in engineered contexts.