All posts by Marta Scali

Ovipositor Needle II – Self-Propelling & Steering through Tissue

Developed in 2016, diameter 1.2 mm (tip) & 0.75 mm (body).

A wasp ovipositor is a needle-like structure composed out of three elements, called valves. A female wasp uses this structure to drill into wood or fruit and deposit eggs inside a living host. The propagation of the ovipositor through the substrate is achieved by a push-pull mechanism, in which one of the valves is pushed while the other two are pulled.

Inspired by the ovipositor of parasitoid wasps, we developed a new Ovipositor Needle with a diameter of 1.2 mm at the tip and 0.75 mm along the body. The needle consists of six superelastic Nickel Titanium (NiTi) wires (Ø 0.25 mm, length 160 mm) concentrically arranged around a seventh NiTi wire. The seven wires are interconnected at the tip with a flower-shaped ring (Ø 1.2 mm, length 2.0 mm), manufactured for minimal resistance during propulsion. The ring has a central hole to which the central wire is glued and six holes through which the six other wires can slide back and forth.

Each proximal end of the six movable wires is connected to a stepper motor, in which a leadscrew-slider mechanism converts rotational motion into linear motion. During an experiment, the needle was inserted in a stationary tissue-mimicking phantom, placed on a cart with low-friction wheels. The wires were sequentially moved back and forth inside the phantom, generating a net pulling motion of the phantom towards the actuation unit, and resulting in the needle moving forward inside the phantom. Different sequences of wire actuation were used to achieve both straight, curved and S-shaped trajectories.

In a follow-up prototype we changed the shape of the interlocking ring from cylindrical to conical to investigate the effect of pre-curved wires. We found out that pre-curved wires facilitate steering, however, at the drawback of a slightly larger tip diameter due to the use of a conical flower-ring.

Ovipoistor Needle II is, to our knowledge, world’s thinnest self-propelled-steerable needle. Our novel bio-inspired steering and propulsion mechanism allows for the design of extremely long and thin needles that can be used to reach deep targets inside the body without a risk of buckling and with the possibility to correct the trajectory.

Ovipositor Needle II is part of the WASP project that focuses on the development of steerable needles for localized therapeutic drug delivery or tissue sample removal (biopsy). We are currently  working on further miniaturization to diameters <0.5 mm.

(Picture at the top adapted from “Braconid Wasp Ovipositing” by Katja Schulz is licensed under CC BY 2.0.)


3D printed wasp-ovipositor replica: reverse engineering approach

In nature, several species of parasitoid wasps have a thin and flexible needle-like structure, called ovipositor, which is used to deposit eggs in a host (e.g., a larva) hidden into tree trunks or fruits. The wasp ovipositor consists of three segments, called valves, longitudinally connected that can slide along each other.  The animals can drill in different substrates by actuating the valves in a reciprocal motion and steer by changing the relative positions of the valves during probing (i.e. protracting and retracting of the valves).

We are currently developing a novel steerable needle for minimally invasive interventions inspired by the wasp-ovipositor. However, the steering mechanisms used by the animal is not yet fully understood.

The project will focus on understanding how the steering mechanism works and which characteristics of the ovipositor play a relevant role.

The student will use detailed 3D images of different ovipositors to design several replicas of the wasp ovipositor in larger scale with 3D printed techniques. The prototypes will be tested with an experimental facility where motion pattern and speed can be controlled. The ovipositors will be inserted in gelatine of different concentration to study the design parameters effecting the steering mechanism.

Contact: Marta Scali,

Picture adapted from “Braconid Wasp Ovipositing” by Katja Schulz is licensed under CC BY 2.0.

Design of an endoluminal ovipositor-device

During colonoscopy procedures an endoscopic device is inserted into the patient and pushed through the colon with consequential discomfort to the patient.  Self-propelling devices that are able of moving through a lumen without the need of external push could be beneficial for these applications. Research in this topic is ongoing, but no successful solutions have yet been discovered.

At TU Delft a former master student (Perry Posthoorn) developed a self-propelled device inspired by the mechanism of the ovipositor of the wasp. The ovipositor is a needle-like structure which consists of three elements that can slide along each other. By means of a reciprocal movement of the elements the wasp is able to insert the ovipositor through a substrate. The reciprocal sliding mechanism of multiple elements has inspired the design of our ovipositor-device.

Preliminary tests have shown that the device is able to move through an ex-vivo porcine colon, although at extremely slow speed due to a sub-optimal internal construction of the device.

The aim of this graduation project  is to develop a strongly improved endoluminal device aiming at maximizing propulsion speed at minimal internal complexity with the final aim to make a revolutionary new system suited for disposable use.

For more information contact Marta Scali (

Ultra-Thin Steerable Needle for Solid-Organ Interventions – WASP

This research project is part of the iMIT program and funded by the Netherlands Organization for Scientific Research NWO. The iMIT Program, executed by a community of Dutch Universities, university medical centers, and companies, aims to develop instruments for minimally invasive interventions. The program will result in the development of interactive Multi-Interventional Tools (iMIT)  that can adapt to their environment and integrate diagnostic and therapeutic functionalities, thus permitting effective single-procedure interventions.

The WASP project focuses on medical needles – common devices used in minimally invasive percutaneous procedures, such as localized therapeutic drug delivery or tissue sample removal (biopsy). Reaching the target with high accuracy and precision is necessary for the success of these procedures and becomes a challenge when the target is located deep inside the body. The surgeon needs a steerable flexible needle that can follow complex curved trajectories while avoiding sensitive structures, such as blood vessels, located along the trajectory between the insertion point and the target site. Looking in nature we find an interesting behavior in wasps which can be used as source of inspiration for facing this challenge. The wasp has a thin and flexible needle-like structure, called ovipositor, used for laying of eggs inside wood or larves. It is composed of three longitudinal segments, called valves, that can be actuated individually and independently of each other with musculature located in the abdomen of the insect. In this way the wasp steers the ovipositor along curved trajectories inside different substrates without a need for rotatory motion or axial push.

Inspired by the anatomy and the steering mechanism of this needle-like structure we aim to develop an ultra-thin steerable needle that can follow curved paths through complex solid organs while avoiding obstacles.