Device allows needle-free injections into smallest vessels

By Dan Keller

WASHINGTON, DC - A new low- cost and needle-free liquid microjet device could prove useful for the delivery of thrombolytic drugs to treat small-vessel occlusions in the eye, according to a report at a Research to Prevent Blindness Foundation seminar.
The device, called a pulsed liquid microjet, delivers picolitre to microlitre volumes. A high voltage wire electrode and surrounding grounded metal sheath sit inside a tapered capillary tube micronozzle.

When short duration voltages are applied, current flows from the central electrode to the sheath and heat causes the explosive growth of a vapour bubble, ejecting liquid. The velocity, volume, penetration depth and diameter of the expelled jet depend on the energy of the electric discharge and the diameter of the exit hole in the nozzle.

In experiment studies, the device produced a jet that penetrated a vessel 60 micrometers in diameter but left the opposite vessel wall intact. This could prove useful for localised injection in ocular vessels, Daniel Palanker PhD said.

Using 0.7% agarose gel as a test medium, Dr Palanker documented penetration depths with varying discharge energies. The micronozzle was loaded with microscopic polystyrene beads suspended in isotonic saline and positioned in contact with the agarose surface immersed in saline.

A discharge energy of 0.16 mJ caused penetration to a depth of 80 micrometers while a 1.56 mJ discharge ejected a jet to a depth of 500 micrometers.
In vitro experiments on excised chorioallantoic membrane of a chicken embryo showed that the microjet could penetrate the proximal wall of a 60 micrometer diameter vessel and inject fluorescent dye.

Fluorescence microscopy showed that with repeated pulses from the microjet the dye was confined to the vessel and moved in the direction of blood flow.
Dr Palanker noted that to be clinically useful, a surgeon must be able to hold and position the probe against a pulsating vessel. Experiments in vivo on chicken chorioallantoic membrane demonstrated that an operator could inject a 100 micrometer diameter artery under physiological conditions with a handheld probe.

Microscopic examination of histological sections of the artery showed tissue damage was confined to a zone approximately 20 micrometers in diameter and only the proximal wall was penetrated. Bleeding lasted only a few seconds. The endothelium of the distal wall was not damaged.

"This device will permit procedures currently impossible to perform including the direct injection of pharmaceuticals into very small vessels such as the retinal arterioles and venules in an atraumatic fashion compared with direct cannulation," Mark Blumenkranz MD remarked.

The pulsed liquid microjet may allow intradermal or transdermal injections as well as injections into the brain, heart or other organs. It may also permit injection of recombinant material or cells into organs or cells themselves with less pain and trauma than conventional needles and cannulas.

The device may enter clinical practice in two or three years depending on the pace of clinical trials, Dr Blumenkranz predicted.
Pulsed electrical discharges also form the basis of a new device that can provide tractionless cutting of tissues ranging from soft membranes to bone, with independent control over the width and depth of the cut.

Dr Palanker foresees early applications in vitreoretinal surgery and capsulotomy. Currently, traction accompanying mechanical segmentation, peeling or delamination of vitreoretinal membranes can damage retina and cause tears and bleeding.
Various types of lasers for such applications have had their own problems, including collateral damage of surrounding tissues, harmful effects of radiation, high costs and large size, he noted.

The microjet device generates pulsed waveforms that cause the formation of a thin layer of pulsed plasma around an elongated microelectrode in a conductive fluid.
The elongated shape results in dissection with the edges of the electrode not only with its apex and therefore no connections remain between the two edges of the cut surface.

Animal tests have shown the device efficiently dissects retina, lens capsule, sclera and iris. It may be applicable to vitreoretinal, cataract and glaucoma surgery.
Dr Palanker showed a video of smooth dissection of a chicken embryo membrane. Microscopic examination showed the cuts had very clean edges.

"It will be possible to do sclerotomies without sutures due to the small diameter of the instrument which can be up to 0.25 mm (30 Gauge needle). We basically remove the instrument and tissue seals by itself.
"It also allows a blind capsulotomy where you can cut under the iris without actually seeing the edge," he said

He added that the new tool will also be useful in cases of mature cataract where the surgeon cannot see the capsule. The microjet device is described by Dr Palanker and colleagues in an article in the September 2002 issue of the Archives of Ophthalmology.

Top