The Kroenleinia grusonii spine as inspiration for materials

Fibres arranged along the spine

The spine of Kroenleinia grusonii (Echinocactus grusonii in the publications discussed) is a material composed of highly ordered fibres and a sclerotised, or hardened, epidermis. Huang and Guo combined optical and electron microscopy with X-ray microanalysis, nanoindentation and tensile tests. They described fibres 5–15 µm in diameter and hundreds of micrometres long. These were arranged along the spine axis, and their components showed a high degree of crystalline order. The authors related this organisation to the strength of the entire structure. [Huang and Guo 2013, abstract]

Mechanical properties depended on measurement direction. This dependence is called anisotropy. It means that pressure along the axis and from the side can reveal different properties of the same material. For dry spines, the authors reported a tensile strength of about 140 MPa. Fresh spines showed some fracture resistance associated with the absorption of deformation energy. They became brittle when dried. The result thus relates mechanical resistance both to fibre structure and to the state of the material during measurement. [Huang and Guo 2013, abstract]

Cellulose combined with other wall components

In a subsequent paper, Huang, Qiu and Guo studied individual fibre cells. These were 0,32–0,57 mm long and 4,6–6,0 µm wide. X-ray diffraction and infrared spectroscopy indicated a predominance of cellulose I and a crystallinity index reaching about 76%. This number describes the degree of order in the material studied and does not represent the percentage of cellulose in the entire spine. The cells' length and narrow width form a fibrous structure whose elements can bear loads together. [Huang et al. 2014, abstract]

The effect of removing hemicelluloses and lignin was particularly important. In nanoindentation, the authors reported a modulus of about 17 GPa for the natural spine and about 0,487 GPa after these components were removed from the fibre cells. The modulus describes the material's response to deformation in the measurement used. Such a large change indicates that substances connecting cell-wall components contribute to wall stiffness. The materials conclusion concerns the interaction between ordered fibres and their surroundings. The authors proposed using this principle to design composites containing parallel nanofibres. [Huang et al. 2014, abstract]

A slender structure under pressure

In 2025, Pier Nicola Sergi described the spine as a model inspiring the design of an intraneural interface inserted into a nerve by its own shaft. The key issue was buckling, the lateral bending of a slender structure under a compressive force. The model assumed a straight beam and a circular cross-section. The buckling threshold increased in proportion to the material modulus and the fourth power of the radius, and decreased with the square of length. Thickening of the basal section was analysed with radius and modulus equal to 4/3 of the values in the distal section. When reinforcement covered 2/3 of the length, the calculated increase in threshold was 39% for an end sliding along the surface and 59,5% for an end laterally constrained after penetrating it. These are results of the adopted structural model. [Sergi 2025, Sections 2, 3.5 and 4.2]

From spine to interface design

Biomimetics transfers a structural principle from such a description. Plant material provides an example of fibre integration, while spine shape allows the effects of slenderness and local thickening to be studied. An intraneural interface has its own material and biological requirements. In Sergi's paper, resemblance to a spine was used to identify possible changes in geometry and device insertion. The work was theoretical and involved no implantation experiment. Its results describe predicted mechanical stability during insertion. Further testing of such a design requires mechanical measurements combined with assessment of its interaction with tissue. [Sergi 2025, abstract]