3D printed microneedles that dissolve in the skin

Nobody likes getting injections, but research from the University of Texas at Dallas, US, might be able to offer a painless alternative. The team report a method for 3D printing microneedle arrays, able to deliver drugs through a simple patch applied to the skin.

Microneedles have been attracting attention in the medical community for some time, because they offer a way for unskilled carers to deliver drugs safely, with less risk of infection than traditional transdermal injection, and better shelf-life than drugs stored as injectable liquids. 

They consist of arrays of polymer needles, ideally around 100 micrometres wide, which can be coated with a drug. Alternatively, the drug can be incorporated into the polymer so that the needles break off in the skin and dissolve to release their dose. 

Manufacturing microneedles with the right dimensions, however, is not a simple task.  They are typically made from a reusable template, which is difficult to produce. Expensive photolithography equipment is required to complete the process.

The team of researchers at Dallas, led by Jeremiah Gassensmith, looked to use 3D printing techniques to make microneedles from polylactide, a non-toxic, biodegradable and renewable polymer which is approved for use in dissolvable stiches.  

Unfortunately, the printing techniques that are capable of producing features smaller than 100 micrometres are not compatible with the biodegradable polymer required. Gassensmith’s team instead used fused deposition modelling (FDM), a 3D printing technique that works well with polylactide, but has lower resolution.   

Using FDM printing, the team produced polylactide pillars with a width of 400-600 micrometres, but could not achieve the tapered needle shape that they needed. To accomplish this, they turned to chemical etching to complete the process.

Polylactide dissolves in water at mildly acidic or alkaline pH. In fact, the slight acidity of skin is what allows the microneedles to dissolve once they have broken off in the patient’s skin. Gassensmith’s team used an alkaline solution of potassium hydroxide to etch away at the pillars, producing barbed needle-like structures with tip sizes between one and 55 micrometres.

These microneedles were then tested on pig skin, showing that they could deliver a dose of a dye molecule beneath the skin, mimicking how a drug would be delivered.  

The researchers also found that applying a mild sideways force to the microarray led to 84% of the needles breaking off in the skin, which could be used to slowly deliver drugs as the needles dissolve. 

Most importantly, the microneedles could be quickly and cheaply produced, which allows researchers to evaluate different needle sizes and designs, hopefully leading to the optimal microarray for pain free drug delivery.

This work has been published in ChemRxiv, the chemical preprint server.