Dynamic mechanical interaction between injection liquid and human tissue simulant induced by needle-free injection of a highly focused microjet

Abstract

This study investigated the fluid-tissue interaction of needle-free injection by evaluating the dynamics of the cavity induced in body-tissue simulant and the resulting unsteady mechanical stress field. Temporal evolution of cavity shape, stress intensity field, and stress vector field during the injection of a conventional injection needle, a proposed highly focused microjet (tip diameter much smaller than capillary nozzle), and a typical non-focused microjet in gelatin were measured using a state-of-the-art high-speed polarization camera, at a frame rate up to 25,000 f.p.s. During the needle injection performed by an experienced nurse, high stress intensity lasted for an order of seconds (from beginning of needle penetration until end of withdrawal), which is much longer than the order of milliseconds during needle-free injections, causing more damage to the body tissue. The cavity induced by focused microjet resembled a funnel which had a narrow tip that penetrated deep into tissue simulant, exerting shear stress in low intensity which diffused through shear stress wave. Whereas the cavity induced by non-focused microjet rebounded elastically (quickly expanded into a sphere and shrank into a small cavity which remained), exerting compressive stress on tissue simulant in high stress intensity. By comparing the distribution of stress intensity, tip shape of the focused microjet contributed to a better performance than non-focused microjet with its ability to penetrate deep while only inducing stress at lower intensity. Dynamic mechanical interaction revealed in this research uncovered the importance of the jet shape for the development of minimally invasive medical devices.

Figure 1

(a) Schematic diagram of polarization measurement with high-speed polarization camera. (b) (i) Schematic diagram of experimental setup for the photoelastic measurement of stress field induced by (ii) focused microjet, (iii) non-focused microjet, and (iv) needle penetrations. (c) Ejection shapes of (i) focused microjet and (ii) non-focused microjet.

Figure 2

Image sequence of stress fields induced by the penetration of (a) the focused microjet, (b) the non-focused microjet, and (c) the needle. Note that the relative intensity distribution of a phase difference field is proportional to the intensity distribution of a stress field.

Figure 3

(a) Illustration of principal stress vector and normal vector (moving direction of cavity wall). (b) Stress vector fields induced by the penetration of (i) the focused microjet, (ii) the non-focused microjet and (iii) the needle. The black arrows indicate the direction between the principal stress vector and the normal vector of the cavity wall. The blue or pink arrows show the principal stresses which are dominated by either compressive stress or shear stress, respectively. (c) Conjecture of flow direction and stress vector induced by the focused microjet. The yellow vector indicates the flow direction of the liquid. The dominance of different types of stress vectors is related to the flow direction of the liquid.

Figure 4

(a) Histogram of area percentage N of different ranges of phase difference $\Delta$ in the whole image at the time of highest spatial averaged phase difference. The red, blue, and green bars show the result of the focused jet, non-focused jet, and needle respectively. (b–c) Spatial averaged stress versus time t for comparison between (b) the focused and non-focused microjets, (c) the microjets and the needle penetration. The red, blue, and green plots show the spatial averaged stress of the focused microjet, the non-focused microjet, and needle injection, respectively.