Resorbable medical printing materials

A resorbable implant is designed to disappear. Getting the disappearance to happen on the right schedule, and to be mechanically graceful, is harder than making the material degrade at all.

Bulk erosion versus surface erosion#

Two limiting behaviours are possible, and which one occurs depends on a race between two rates: how fast water penetrates the material, and how fast the polymer’s bonds hydrolyse.

Surface erosion happens when hydrolysis is faster than water penetration. The material degrades only at its outer boundary, like a dissolving tablet — it thins predictably while the core stays intact and load-bearing. Polyanhydrides behave this way.

Bulk erosion happens when water penetrates faster than the polymer degrades — the case for the polyesters used clinically. Water saturates the whole part within hours to days, and hydrolysis then proceeds everywhere at once. The part keeps its shape and dimensions for a long time while its molecular weight falls throughout, and mass loss only begins once chains are short enough to dissolve.

The clinical consequence follows directly: an implant can look unchanged and be near the end of its strength. Mechanical failure precedes visible dissolution, because strength depends on molecular weight and entanglement, and those are lost long before mass is.

Why the failure is abrupt#

Ester hydrolysis produces a carboxylic acid end group on each chain it cuts, and ester hydrolysis is acid-catalysed.

In a bulk-eroding part, those acidic products are generated throughout the interior and cannot diffuse out faster than they are made. The interior pH therefore falls, which accelerates hydrolysis there, which produces more acid. The degradation is autocatalytic and centred on the inside of the part, so the core degrades faster than the surface — the reverse of intuition.

This produces two effects that matter clinically. Strength is lost suddenly once the hollowing interior can no longer carry load, rather than tapering. And the accumulated acid creates a locally acidic microenvironment on release, which is the accepted explanation for the sterile inflammatory reactions occasionally seen with resorbable fixation devices.

Thicker sections are worse, because diffusion distance is longer — so degradation rate is a function of part geometry and not only of material.

Tuning the lifetime#

PLGA’s lactide-to-glycolide ratio is the main handle. Glycolide units are less hydrophobic and less sterically hindered, so glycolide-rich copolymers take up more water and hydrolyse faster. Copolymers also disrupt crystallinity, and amorphous regions degrade first, so a 50:50 copolymer degrades fastest — faster than either homopolymer. Lifetimes span weeks to a year.

Polycaprolactone sits at the far end: hydrophobic, highly crystalline, and degrading over two to three years, which suits long-term scaffolds and makes it unsuitable where timely resorption is wanted.

Sterilisation attacks the same bond#

A resorbable device must be sterile, and every standard method interferes with the chemistry that makes it resorbable.

Steam autoclaving is hydrolysis by definition — hot water — and also exceeds the glass transition of PLA and PLGA, so parts deform. Gamma irradiation causes chain scission in aliphatic polyesters, reducing molecular weight before the device is ever implanted and shortening its effective life; the dose is a specification, not a detail. Ethylene oxide is the usual compromise, being low-temperature, but it requires long aeration to clear residuals and can plasticise the polymer.

There is a further point specific to printing. The printed object is not the certified resin. Melt processing itself cuts molecular weight through thermal and hydrolytic scission, so a printed device’s degradation profile must be characterised as printed, and a resin certificate does not transfer to it.