Fig 1.
Visualisation of the motion sequence of the ovipositor of a parasitic wasp.
(A) The ovipositor consists of three parallel valves that can move reciprocally (based on Cerkvenik et al. [20]). (B) Schematic illustration of ovipositor-inspired needle insertion into tissue with one advancing needle segment (yellow) and two retracting needle segments (grey). Ffric,i is the friction force along the advancing needle segment, Fcut,i is the cutting force on the tip of the advancing needle segment, and Ffric,j is the friction of the retracting needle segments, which works in the opposite direction as the friction force of the advancing needle segments.
Fig 2.
Visualisation of the motion sequence of the needle segments.
(A) During the motion, one needle segment moves forward over the stroke distance while the other needle segments move slowly backwards over one-fifth of the stroke distance in a consecutive manner. (B) Manual translation of a translation ring (red) drives the actuation system. The actuation system converts the reciprocating motion of the translation ring into a sequential translation of the six needle segments.
Fig 3.
Click-pen mechanism of a ballpoint pen.
Illustration of one of the first patented click-pen mechanisms (Parker Pen Co Ltd) [31].
Fig 4.
Schematic representation of the selector motion mechanism in 2D, including the selector (green), housing (grey), cam (orange), and needle segment holders (yellow).
The columns show the subsequent steps in the motion cycle. The rows show the different layers of the selector.
Fig 5.
Schematic motion sequence of the selector in 3D, including the selector (green), housing (grey), cam (orange), and needle segment holders (yellow).
Fig 6.
Exploded view (A) and cross-section (B) of the actuation unit of the Ovipositor MRI-Needle, consisting of a housing bottom (1), cam bottom (2), cam top (3), translation ring (4), selector (5), needle segment holder (6), housing top (7), lock ring (8), inner double cone (9), and outer double cone (10).
Fig 7.
Ovipositor MRI-Needle (A) Close-up of the needle tip consisting of six sharpened Nitinol rods held together by a shrinking tube (Vention Medical) glued to one of the six rods. (B) Assembled prototype. The grey parts, i.e., the housing components, were produced using fused deposition technology (FDM) in polylactic acid (PLA) on an Ultimaker 3 printer. The orange parts, i.e., the actuation unit’s internal components and the inner and outer double cones, were produced using stereolithography (SLA) technology in Dental Model resin V2 (Formlabs) on a Formlabs Form 3B printer. A transparent polymethyl methacrylate (PMMA) support structure supports the prototype, and a white polylactic acid (PLA) guide tube supports the needle in the support structure.
Fig 8.
Experimental setup of the ex vivo prostate tissue experiment.
(A) The instrument was placed in a half-round tube with a support structure in between. The half-round tube was slid into the MRI bore. (B) Close-up of the radiofrequency (RF) coil with the tissue box on the RF base plate, guided on rails. (C) Close-up of the proximal side of the tissue box containing seventeen insertion holes. (D) Close-up of the ex vivo prostate tissue embedded in solidified 2.5%wt agar, with the needle inserted through the agar in front of the tissue.
Fig 9.
MR images of the needle inside the agar and ex vivo prostate tissue.
Each row represents one measurement. The first column shows the initial frame where the tip is positioned inside the agar in front of the prostate tissue. The second column shows the frame after actuation over five cycles. The third column shows the frame after the second actuation over five cycles. The yellow, red, and green contours show the needle, the prostate tissue, and the tissue box sides, respectively. The arrow marks the needle tip. The orange crosshair shows the reference point on the side of the box that indicates a 40-mm insertion depth. The number in the upper right corner in black shows the measured travelled distance of the tissue box, dm, in mm, with respect to the previous MR image.
Table 1.
Results of the ex vivo evaluation.