Auxetic structures used in kinesiology tapes can improve form-fitting and personalization

Abstract

Each year 65% of young athletes and 25% of physically active adults suffer from at least one musculoskeletal injury that prevents them from continuing with physical activity, negatively influencing their physical and mental well-being. The treatment of musculoskeletal injuries with the adhesive elastic kinesiology tape (KT) decreases the recovery time. Patients can thus recommence physical exercise earlier. Here, a novel KT based on auxetic structures is proposed to simplify the application procedure and allow personalization. This novel KT exploits the form-fitting property of auxetics as well as their ability to simultaneously expand in two perpendicular directions when stretched. The auxetic contribution is tuned by optimizing the structure design using analytical equations and experimental measurements. A reentrant honeycomb topology is selected to demonstrate the validity of the proposed approach. Prototypes of auxetic KT to treat general elbow pains and muscle tenseness in the forearm are developed.

Figure 1

Existing KT (a) The compression on the skin, fascia, and muscle due to an injury. Image inspired by^–. (b) The positive decompressive effect generated by the KT application. A one-dimensional wave-like pattern is generated on the skin. Image inspired by^–. (c) The wave-like pattern of the acrylic adhesive at the bottom of a KT. (d) The one-dimensional wave-like pattern visible to the bare eye when a KT is applied to a leather sample used to mimic the skin. (e, f, g) Examples of KT applications on the knee (e) and on the calf (f). In (g) the application of a fan-shaped tape on the forearm for lymphatic drainage.

Figure 2

Novel auxetic KT (a) The positive decompressive effect generated by the auxetic KT application. A two-dimensional wave-like pattern is generated on the skin. Image inspired by^–. (b) The two-dimensional wave-like pattern visible to the bare eye when a KT is applied on a leather sample used to mimic the skin.

Figure 3

The reentrant honeycomb design and its main geometrical parameters.

Figure 4

Scatterplots used to analyze the effect of the geometrical parameters $\theta$, $h,$ and $l$ on the Poisson's ratio $\nu$, based on the analytical equation [Eq. (1), considering $K_{{\text{hg}}_{\text{global}}}$ as hinging force constant], for fixed values of the $t$ and $b$ parameters, i.e., $t=0.8\text{mm}$ and $b=1.5\text{mm}$ (a), $t=1.2\text{mm}$ and $b=1.5\text{mm}$ (b), $t=1.6\text{mm}$ and $b=1.5\text{mm}$ (c), and for fixed values of $\theta$, $t$ and $b$ parameters (d, $\theta ={20}^{\circ };t=0.8\text{mm};b=1.5\text{mm}$).

Figure 5

Experimental results (a) The five different geometries of reentrant honeycombs investigated. (b) Experimental results of the Poisson's ratio $\nu$ over the engineering strain $\epsilon$ for the five geometries. In this chart also the Poisson’s value obtained from the analytical equations (global hinging) are mapped as lines (they do not depend on the engineering strain) (c) The four structures tested to measure the Poisson's ratio, involving Structure 1 as a standard KT, Structure 2 realized as standard KT with auxetic perforations, Structure 3 as a 3D-printed auxetic structure, and Structure 4 as a combination of Structures 2 and 3 glued to each other. Structures 1 and 2 include v-shaped marks as relevant points for measuring the Poisson’s ratio (d) Results of the Poisson's ratio $\nu$ over the engineering strain $\epsilon$ for the four analyzed structures. In this chart, for Structure 3, also the Poisson’s value obtained from the analytical equations (global hinging) is mapped as a line.

Figure 6

Comparison between the application procedure of the traditional KT and the novel auxetic KT for two cases. (a) The traditional KT application procedure for general elbow pain divided into 4 steps. (b) The auxetic KT for elbow pain. (c) The traditional KT application procedure for forearm muscle tenseness divided into 4 steps. (d) The auxetic KT for forearm muscle tenseness.

Figure 7

Fabrication steps of the auxetic KT. (a) The auxetic structure is laser cut into the KT. (b) The flexible material (TPU) is 3D-printed onto the tape. (c) Fabricated auxetic KT prototype. (d) Supporting rectangles and horizontal bars are added to the auxetic structures used for the mechanical characterization. (e) v-shaped marks indicated on the only tape samples (Structure 1 in Fig. 5c) as relevant points for measuring the Poisson’s ratio. (f) The sample used for Structure 2 (Fig. 5c) and a detail of the v-shaped marks.

Figure 8

Mechanical characterization and data analysis. (a) Tensile test setup for the mechanical characterization. (b) Data analysis: the Poisson’s ratio is determined for the highlighted central unit cell. The coordinates of relevant points (labeled as A_i and B_i, i = 1, 2, 3, 4) are computed in pixels, from which the coordinates of the middle point of each vertical beam (labeled simply using numbers from 1 to 4) are calculated. (c) Undeformed and deformed auxetic structure in the tensile test measurement device: $X_0$, $Y_0$, $X_{\text{D}}$ and $Y_{\text{D}}$ are used to calculate the Poisson’s ratio at the given strain, determined using the $L_0$ and $L_{\text{D}}$ measurements. (d) Undeformed and deformed auxetic KT. The points analyzed for this second experiment are also highlighted.