Biomechanics of Scar Tissue and Uninjured Skin

Abstract

Significance: Skin exhibits direction-dependent biomechanical behavior, influenced by the structural orientation of its collagen-rich fibrous network and its viscous ground-substance matrix. Injury can affect the skin's structure and composition, thereby greatly influencing the biomechanics and directionality of the resulting scar tissue.

Recent advances: A combination of stress-relaxation and tensile failure testing identifies both the tissue's physiologically relevant viscoelastic behavior and resistance to rupture. When studied in mutually orthogonal directions in the plane of the tissue, these measures give insight into the directional properties of healthy tissue, and how they change with injury. By controlling the biomechanics of the wound environment, a new force-modulating dressing has demonstrated the ability to improve healing and reduce scar formation.

Critical issues: Skin and scar biomechanics are typically characterized by using tensile failure, which identifies the tissue's resistance to rupture but offers limited insight into its normal daily function. Characterizing physiologically relevant biomechanics of skin, and how they change with injury, is critical to understand the tissue's ability to resist elongation, bear load, and dissipate energy via viscous means.

Future directions: Compared with uninjured skin, scar tissue demonstrates similar high-load stiffness, greatly reduced resistance to failure, reduced low-load compliance, and altered material directionality. These findings, identified through combined stress relaxation and failure testing, suggest morphological changes with injury that are consistent with the viscoelastic and directional changes observed biomechanically. A more complete understanding of the directional, physiologically relevant skin biomechanics can guide the design and critical functional assessment of wound treatments, scaffolds, and tissue-engineered skin replacements.

David T. Corr, PhD

Figure 1.

(a–c) Schematic of biomechanical protocol. (a) Mechanical test specimens are obtained by using a punch in both uninjured skin and scar, in two orthogonal directions: axial (cranial–caudal) and transverse (dorsal–ventral). (b) A stress-relaxation protocol (e.g., a 10% elongation, held for 4 min [first 10 s shown]) is used to characterize the low-load viscoelastic response, nondestructively. (c) Finally, the specimen is stretched to failure to obtain measures of the toe-in region (approximated by arrow), high-load stiffness (gray area of line), and failure properties. (d–f) Skin and scar data in a representative animal. (d) Mechanical test specimens obtained in scar (left) and uninjured skin (right). (e) Relaxation tests reveal that scar relaxes faster and is stiffer at lower loads. (f) Failure testing shows, despite similar high-load linear stiffness values, that scars exhibit stiffer low-load behavior, and greatly reduced resistance to rupture. This combined protocol identifies the physiologic and failure properties, as well as their planar directionality, and how these change with injury (adapted from Ref.).

Figure 2.

Polarized light microscopy of skin (left) and scar (right), showing collagen network organization with regard to axial and transverse axes. Skin displays a collagen network with principal direction biased toward the transverse direction. Scars healed with considerable organization, in a fiber orientation with similar components in both directions, and exhibited much thinner fiber bundles than uninjured skin. These morphologic differences between skin and scar support the observed viscoelastic and failure biomechanics (modified from Ref.).