Researchers at Harvard Medical School have developed a mechanical syringe that can automatically guide a needle into specific, hard-to-access regions of tissue.

Drugs often have to be injected into highly precise locations in order to have the greatest effect.  To deliver drugs into the back of the eye, for instance, one new method involves injecting directly into the suprachoroidal space, a very thin region between the white outer surface of the eye (sclera) and the layer below that includes the retina. The needle must be positioned with extreme precision to ensure that the drug reaches the right place, and to avoid causing damage to the eye. This is made even harder because the sclera is around 10 times stiffer than the tissue behind it, requiring great skill to place the needle in the correct location without overshooting.

The new device — the intelligent injector for tissue targeting (I2T2) — is similar to a normal syringe but has a needle that can move independently of the barrel when pressure is applied to the plunger. The tip of the needle is inserted into the tissue and is advanced by the clinician pushing the syringe’s plunger. The force created when moving through the stiffer outer tissue pushes the needle in. However, when it reaches the soft target tissue, the needle stops advancing and the plunger instead forces the drug out of the tip.

“The device was designed with simplicity in mind,” says Girish Chitnis, lead author of the paper describing the new injection system (Nature Biomed. Eng. 10.1038/s41551-019-0350-2).

An alternative approach

The gold standard for performing injections in difficult-to-access locations is to use real-time imaging such as ultrasound, which can track the tip of the needle as it passes through different regions of tissue.

“I don’t think the i2T2 will replace the image-guided technique altogether,” says Chitnis. “However, based on feedback from clinicians, we have learnt that image-guided techniques are not very convenient and are time consuming.” As a result, the majority of injections performed today are “blind” and rely on the physician’s skill and experience.

“The device is best suited to fill the gap where such image-guided techniques are not suitable, due to low resolution, availability, imaging time, probe size or emergent situations,” Chitnis adds. “Additionally, the imaging systems can only provide feedback about the location of the needle tip, while the i2T2 has an active mechanism that stops needle motion after reaching the target site, and effectively reduces the reliance on the physician to perform the injection.”

One downside in the current design is that it makes it less obvious to the clinician exactly how much drug has been injected, as only some of the distance moved by the plunger actually results in fluid being expelled. However, there are ways to mitigate this.

The device has not yet been trialled on patients, but preliminary tests on animal tissues showed that it is effective in controlled settings. The team successfully delivered a dye to the suprachoroidal space in cow, pig and rabbit eyes, showing good distribution within the space, with limited spread into other regions, and without adverse effects such as bleeding or movement of the retina.

The researchers are confident that the device could easily be used in other difficult injection sites. They have tested it at several other sites in animal tissues, including the spinal canal, the middle layers of the skin and the space between internal organs in the abdomen, achieving good results with each.

“If all goes well, this could be ready for human testing in one to two years”, says team leader Jeffrey Karp.