Three-dimensional printing, muscles, and skeleton: mechanical functions of living wood

Abstract

Wood is well defined as an engineering material. However, living wood in the tree is often regarded only as a passive skeleton consisting of a sophisticated pipe system for the ascent of sap and a tree-like structure made of a complex material to resist external forces. There are two other active key roles of living wood in the field of biomechanics: (i) additive manufacturing of the whole structure by cell division and expansion, and (ii) a ‘muscle’ function of living fibres or tracheids generating forces at the sapwood periphery. The living skeleton representing most of the sapwood is a mere accumulation of dead tracheids and libriform fibres after their programmed cell death. It keeps a record of the two active roles of living wood in its structure, chemical composition, and state of residual stresses. Models and field experiments define four biomechanical traits based on stem geometry and parameters of wood properties resulting from additive manufacturing and force generation. Geometric parameters resulting from primary and secondary growth play the larger role. Passive wood properties are only secondary parameters, while dissymmetric force generation is key for movement, posture control, and tree reshaping after accidents.

Plant biomechanics in the 21st century

“…The strategic alternation of rigid and softer tissues in the stems of lianas ensures an architecture that is able to provide this property, as is revealed in a study by Plavcová et al (2019). This study hints at the complexity of the mechanical behavior of woody tissues, a concept that is further elaborated in a review by Thibaut (2019).…”

Tree growth forces and wood properties

“…Since the beginning of the 2000s the biological origin and function of large peripheral residual stress has been thoroughly investigated by experimental work at cell wall level together with biomechanical models of stress generation within wood fibres. From the mechanical viewpoint wood fulfils three basic functions in the tree: achievement of tree architecture (manufacturing), control of axis direction (posture control) and resistance to external forces (resistance) (Alméras et al 2009, Tocquard et al 2014, Thibaut 2019.…”

“…Wood formation, its growth in volume and mass, occurs through a 3D manufacturing process taking place in the thin intermediate living layer. At the cellular level the process requires three steps: cell division, cell expansion until final geometry, cell-wall thickening (maturation) until programmed cell death and full cell-wall lignification (Thibaut 2019). At the plant level it consists in axis lengthening by addition of new growth units and thickening of existing parts by addition of a new peripheral layer thickness.…”

Wood and the Activity of Dead Tissue

“…In some sense, the printing process is comparable to plant growth processes and living wood has recently been considered as additively manufactured by cell division and expansion. [ 91 ] This perception might even better apply for the cell walls where cellulose and the other polymers are immediately after synthesis deposited as extracellular matrix. The fourth dimension arrives from the fact that the cell walls and wood as a composite are able to generate forces, mainly at the periphery of the stem in the form of growth stresses.…”