Are GPC molecular weights for PLGA absolute values?

GPC molecular weights for PLGA aren't absolute when the column is calibrated with polystyrene standards, as in a 2026 study of in situ forming implants, so compare them only within one method. Absolute molar mass takes a light-scattering detector, which a 2019 depot study used with dn/dc values measured for its PLGA.

Can depot degradation be followed without destroying the sample?

Depot degradation can be followed without destroying the sample by weighing it wet or imaging it, though neither reads chain length. Published in situ implant studies have used blotted wet mass, CT volume and diffusion-weighted MRI. MRI also works in vivo, and one study ran it in mice over five days.

Does molecular weight fall before mass in every degradable polymer?

Molecular weight falling before mass is shown here for PLGA and shouldn't be assumed for other polymers. A 2002 model predicts that polyanhydrides surface erode down to about 0.1 mm, so their behavior can differ from that of a bulk-eroding polyester.

Polymer degradation testing in depots

Polymer degradation in a depot takes more than one readout, because each sees a different stage. Molecular weight tracks chains breaking. Dry mass tracks material leaving. Wet mass tracks water and polymer together. Mechanics tracks what the depot can still hold. In one 52 kDa PLGA in situ depot, molecular weight had fallen to about 36% of its day-1 value before polymer mass loss began.

What’s the difference between degradation and erosion?

Degradation is chains breaking, and erosion is material leaving. Göpferich’s 1996 review draws that line: degradation is scission of the polymer backbone, and it leads in the end to erosion, the loss of material from the bulk.

The two can sit days or weeks apart, and drug can leave during that gap. A depot releases drug by diffusion, swelling and degradation, often at once. So the readout you pick decides which of those you can see.

What does each readout answer?

Use this table to match a readout to the question you have. The readout detail page covers how to run them.

Readout	How it’s read	What it answers	What it can’t show
Molecular weight	Freeze-dry the depot, dissolve it, run GPC	Whether chains are breaking, and how fast	Whether anything has left the depot
Dry mass	Pull the sample, freeze-dry it, weigh it	How much material has left, which is erosion	Chains that are cut but still too long to leave
Wet mass	Drain or blot the sample, weigh it	What the hydrated depot is doing, on the same sample over time	How much of a change is water and how much is polymer
Structure imaging	Microscopy, CT or MRI	Where the depot is opening up, and from which side	Chain length, or how much material has gone
Mechanics	Rheology or compression, on separate samples	Whether the depot still holds its shape or load	Why it softened

Structure imaging. A depot can change where a balance can’t see it. Hopkins and colleagues mapped diffusivity in 52 kDa PLGA in situ implants by MRI. After the first few hours the shell averaged about 0.0012 mm²/s, while the interior climbed from about 0.0006 mm²/s at day 1 to 0.0019 mm²/s at day 14, matching the surrounding bath. Zolnik and Burgess describe PLGA microspheres at pH 7.4 degrading from the inside out.

Mechanics. Mechanics doesn’t always move one way. Deng and Uhrich saw PLGA’s dynamic mechanical properties fall steadily over six weeks in buffer, and they report that absorbed fluid changed storage modulus, so softening can be water as well as chain loss. Agrawal and colleagues saw the elastic modulus of porous scaffolds rise first and then fall.

Why does molecular weight fall before mass in PLGA?

Chains have to get short enough to dissolve and leave, and until then they stay in the depot. Lu, Garcia and Mikos saw this in PLGA films in PBS at 37 °C. Molecular weight fell as soon as the films went into buffer. Weight held for the first several weeks, then dropped sharply.

Hopkins and colleagues measured the same order in a depot: 50:50 PLGA in NMP with fluorescein as a mock drug, injected into PBS at 37 °C on a shaker.

Starting polymer	Mn at day 7	Mn at day 14	Erosion began	Degradation-facilitated release began
15 kDa	46%	35%	Between days 7 and 10	Day 7
52 kDa	64%	36%	Between days 14 and 17	Day 10

Mn is number-average molecular weight, shown as a percentage of the day-1 value.

In the 52 kDa row, release was already picking up at day 10, and the mass didn’t start falling until days later. A mass curve alone wouldn’t explain that change, and a release curve alone wouldn’t name the cause. Zolnik and Burgess found a linear relationship between post-burst release and the log of molecular weight in two PLGA microsphere formulations.

This is one mock drug over 21 days, so don’t carry the timing over to another formulation. PLGA degradation and release covers what shifts it, and the release curve and the depot behind it covers reading the two curves together.

Can wet mass rise while a polymer degrades?

Yes, because water can enter faster than material leaves. In Deng and Uhrich’s PLGA specimens, fluid uptake and specimen dimensions both grew over six weeks while molecular weight fell.

Fisher and colleagues weighed an in situ system two ways in PBS at 37 °C: blotted wet, and freeze-dried. Their material was a composite, 50:50 PLGA in NMP with hydroxyapatite and polymer microparticles, with no PLGA-only control, so the pattern carries further than the rates do.

Period	Wet mass (blotted, non-destructive, n = 5)	Dry mass (freeze-dried, destructive, n = 3)
First day	Down 12%	Down 50% in the first hour, about 55% by day’s end
To about day 15	Up about 1.4% a day	Flat, near 45% of injected mass
After day 15	Samples diverge, then fall to about 20% by day 55	Falls to 25 to 29% by day 50, then holds

The authors read the early drop as water replacing NMP, the rise as the matrix swelling, and the flat dry stretch as chains still too long to clear. They put that clearing cutoff near 1.5 kDa, citing von Burkersroda and Kenley. Hydroxyapatite was 30% of the original mass, so most of that flat 45% is mineral, and the 25 to 29% floor is mineral once the PLGA has gone. Wet mass fell later than dry because water uptake offset small polymer losses. They put the lower wet floor down to hydroxyapatite lost with the supernatant at each weighing.

A rising wet curve: water uptake is outrunning material loss for now. It doesn’t show that the polymer is intact.
A falling early curve: in a solvent-based depot, this can be solvent leaving rather than polymer.
A dry mass baseline: a percentage of the injected mass includes solvent and any filler. Hopkins’s implants were 39% polymer by mass, so the authors count erosion only once residual mass drops below 39%. The 52 kDa implants sat at 39.1% at day 14. State the baseline next to the curve.

Swelling timing moves with chemistry too. Lin and colleagues saw ester-capped PLGA implants reach peak size at 5 to 6 days and acid-capped ones at 11 to 13 days. The hydrogel swelling ratio page covers the wet and dry calculation.

Where does ASTM F1635 fit?

ASTM F1635 is a reference method for in vitro hydrolytic degradation, and it weighs dry mass at set timepoints. Its public scope covers polymer resins and fabricated forms for surgical implants, and it suggests techniques for tracking changes in mass, molar mass and mechanical properties. It says the method is intended to help assess degradation rates, meaning the mass loss rate. ISO 13781 is the counterpart for polylactide-based polymers, including copolymers with glycolide.

Both name surgical implants, so a depot team borrows them. Neither public scope mentions a depot that forms in the body and loses solvent, so report that step separately. Scaffold and implant degradation testing is where the method sits most directly.

A testing lab’s summary of the 2011 edition says the tests shall be mass loss, molar mass and mechanical testing, so a dry mass curve alone doesn’t complete the method. It lists weight loss measured to 0.1% of total sample weight, samples dried to constant weight, at least three samples per time period, and molar mass by solution viscosity or size exclusion chromatography.

The scope also states its own limits:

Mechanism: it doesn’t apply to polymers that degrade mainly by something other than hydrolysis. Hydrolytic versus enzymatic degradation covers that choice.
Loading: hydrolytic aging alone isn’t enough when the device’s use includes loading.
Sterilization: it should be expected to change molar mass or structure, so the form you test matters.

What changes the rate in a depot test?

Eight test choices move the result, so test something close to what you’ll inject.

Size: thick PLGA films degraded faster than thin ones in Lu’s study, which the authors tie to acid autocatalysis, and PLA plates and beads degraded much faster than submillimeter films and particles in Grizzi’s. A small screening depot can understate a larger one, and a larger one uses more material.
Drug: six drugs at 20% loading in the same 50:50 PLGA gave different degradation rates in Siegel’s study. A blank depot is simpler to run, and its rate may not hold once drug is in it.
Flow: 250 µL/min of buffer lowered the degradation rate of porous scaffolds against static buffer in Agrawal’s study. Those were scaffolds, so the direction may differ in a dense depot.
pH: pH 2.4 sped up the second release phase in Zolnik’s microspheres and changed degradation from inside-out to outside-in. Acid saves time and can change the mechanism you’re trying to study.
Copolymer ratio: 50:50 PLGA films degraded faster than 75:25 in Lu’s study, which the authors tie to more hydrophilic glycolic units.
End cap: between days 7 and 15, implants with leuprolide degraded at 2.67% a day with acid-capped PLGA and 1.28% with ester-capped, in Lin’s study.
Temperature: the rate rose with incubation temperature in Dunne’s PLGA particles, with an induction period at the lower temperatures. Zolnik’s microspheres kept first-order molecular weight kinetics from 37 to 70 °C, but pores closed and geometry changed when hot. A hot run saves weeks and can change the depot.
Medium refresh: the 2011 F1635 summary describes PBS at pH 7.4, checked at least weekly, at a ratio as high as practical, with 100 to 1 described. Hopkins replaced the whole bath at each sampling. More frequent refresh uses more medium and shortens how long degradation products sit in the bath, so report the interval.

Which readouts should a depot study start with?

Start with the decision the data has to support, then count samples. These pairings are our own starting points rather than a published rule.

Decision	Start with	Add
Comparing many candidates with little material	Wet mass plus release	Molecular weight on the leads
Why release shifted at a given time	Molecular weight, dry mass and release on one timeline	Imaging, if the shift looks structural
A mass loss rate to compare against a reference	Dry mass, following F1635	Molar mass and mechanics from the same timepoints
Whether the depot holds its shape	Mechanics on separate samples	Wet mass, to read softening against water

Wet mass leads the first row because each depot stays in the run and gives a whole curve. It’s the slower signal: molecular weight moves first, and Fisher’s wet mass fell later than dry. A lead with a quiet wet curve still needs molecular weight.

Destructive readouts set the sample count. Eight timepoints in triplicate use up 24 depots. Hopkins and colleagues ran GPC on freeze-dried implants, so one dried sample can give both dry mass and molecular weight. Mechanics needs its own set.

Does an in vitro curve show what happens at the injection site?

No. Hopkins and colleagues note that implants formed in mice were more planar and heterogeneous than those in a stirred bath, and that water availability differs. Their in vivo diffusivity followed the same trend, over five days. ISO 13781’s scope says in vitro methods can’t definitively predict in vivo behavior unless validated for the device. Release methods have their own guide.

What a GelTech study measures on polymer degradation

Depot mass and drug release go on one timeline. Carousel tracks drained wet mass on the same samples at every timepoint, to 1 mg, six samples at a time, for weeks to months. Integrated instruments such as UV-Vis, HPLC and LC-MS read the drug in sampled medium. Carousel’s mass data carries roughly 3x less data variability than manual work. Samples run for weeks to months at the temperature and pH you set: closer to in vivo conditions, and a stronger basis for IVIVC. That describes the test conditions, and the injection site stays a question for animal studies.

Carousel’s wet mass curve sits beside ASTM F1635, which weighs dry mass at set timepoints, and Carousel doesn’t read molecular weight. For PLGA, pair the mass curve with molecular weight from separate samples, which GelTech runs in its own lab.

Check the sample first. Carousel’s minimum dry sample is 25 mg and it reads to 1 mg. By our arithmetic, 1 mg is 4% of a 25 mg sample, and weighing to 0.1% with a 1 mg readout takes about 1 g of sample, so on a small depot the wet curve shows trend and timing rather than an F1635 mass loss figure. Because size changes the rate, depot size gets agreed before the run.

GelTech doesn’t run animal or clinical studies, or accredited biocompatibility or sterility testing. See drug delivery, tissue engineering, how a pilot study works and what to put in a study brief.

Polymer degradation testing in drug delivery depots

What’s the difference between degradation and erosion?

What does each readout answer?

Why does molecular weight fall before mass in PLGA?

Can wet mass rise while a polymer degrades?

Where does ASTM F1635 fit?

What changes the rate in a depot test?

Which readouts should a depot study start with?

Does an in vitro curve show what happens at the injection site?

What a GelTech study measures on polymer degradation

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