PURASORB PL 65 Medical Device Poly(L-lactide) is a semi-crystalline poly(L-lactide) homopolymer supplied as white cylindrical granules. The grade is differentiated from lower viscosity PURASORB PL 38 and PURASORB PL 49 by a nominal inherent viscosity of 0.65 dL/g, measured in chloroform at 25 °C and 0.1 g/dL polymer concentration according to ISO 1628-1:2021. Supplier certificate-of-analysis data list residual lactide at ≤0.5 wt% and tin catalyst residue at ≤150 ppm. Thermal analysis by ASTM D3418 places the glass transition at 55–60 °C and the melting endotherm at 170–180 °C; solid-state density determined by ISO 1183-1:2019 is approximately 1.24 g/cm³. The stereoregular backbone develops measurable crystallinity after annealing, which distinguishes the homopolymer from amorphous poly(D,L-lactide) grades such as PURASORB PDL 02A. PURASORB PL 65 is a raw material for further melt processing and is not a finished medical device; biological evaluation of the final packaged component remains necessary under ISO 10993-1:2018.

The grade designation follows the supplier convention in which “PL” denotes poly(L-lactide) homopolymer and the numeric suffix corresponds to the nominal inherent viscosity midpoint expressed as 0.65 dL/g. The medical device designation indicates that the resin is produced under quality controls appropriate for implantable applications; however, the designation does not substitute for final-device validation. Reported applications for high-inherent-viscosity poly(L-lactide) include resorbable bone fixation screws, suture anchors, craniofacial plates, and porous scaffolds. In such applications, the higher molecular weight of PL 65 is selected when the device must retain mechanical integrity beyond the initial tissue-healing period. However, the final resorption time is geometry-dependent and cannot be assigned from the resin specification alone. Processors selecting PL 65 over PL 38 or PL 49 typically accept a narrower processing window in exchange for higher melt strength and slower hydrolytic chain scission.

What Processing Conditions Preserve Molecular Weight in Extrusion and Injection Molding?

Pre-drying in a vacuum dryer at 80 °C for 4–6 h to a residual moisture below 250 ppm is mandatory because the polyester backbone undergoes hydrolysis in the melt. For single-screw extrusion of rod, plate, or filament, barrel profiles from 180 °C at the feed throat to 210 °C at the die are used, with a melt temperature not exceeding 230 °C. A barrier screw with 24:1 L/D and a compression ratio of 2.5:1 provides acceptable melt homogeneity at screw speeds of 30–80 min^-1. The polymer is pseudoplastic; capillary rheometry rather than melt flow index is preferred because long residence times at 190 °C cause progressive chain scission.

Injection molding of thin-walled fixation pins or anchors uses a reciprocating screw with check ring, clamp force from 600–1200 kN, injection pressure of 80–120 MPa, and mold temperature held at 15–25 °C for rapid solidification. Rapid cooling creates an amorphous skin with low crystallinity; subsequent annealing at 110–120 °C for 2–4 h raises the crystalline fraction and improves dimensional stability. Excessive residence time above 230 °C produces lactide monomer, yellowing, and measurable loss of inherent viscosity. On production lines without adequate venting, trapped moisture causes bubble formation, surface splay, and an inhomogeneous molecular weight distribution. The supplier certificate of analysis provides batch-specific inherent viscosity and residual monomer values so that barrel temperatures and back pressure can be adjusted within the 180–210 °C envelope.

Across ethylene oxide, gamma irradiation, and electron-beam sterilization cycles, the chain-scission response of PURASORB PL 65 is dose-, moisture-, and temperature-dependent. Ethylene oxide processing in accordance with ISO 11135:2014 does not inherently reduce molecular weight, but residual water and elevated aeration temperatures must be controlled because hydrolytic degradation can be accelerated. Gamma sterilization at a typical terminal dose of 25 kGy under ISO 11137-1:2006 produces measurable chain scission; published studies on high-molecular-weight poly(L-lactide) report inherent viscosity retention of 70–85% at 25 kGy, while a 50 kGy dose may reduce inherent viscosity by 30–40% and increase brittleness. Electron-beam irradiation at equivalent doses produces a similar radical yield with lower thermal load. If terminal radiation doses above 25 kGy are required, retained molecular weight must be measured on fabricated parts, not on raw resin. Published data for the response of PURASORB PL 65 in specific implant geometries is limited.

When the Same Device Design Is Converted from PURASORB PL 38 to PL 65

Conversion from a lower-IV grade such as PURASORB PL 38 or PL 49 to PL 65 changes mold-filling pressure, cooling stresses, and in vivo degradation. The higher melt viscosity of PL 65 can require a 5–10 °C increase in melt temperature and a 10–20% increase in injection pressure to maintain shot weight in the same mold. The same device geometry will usually show higher residual molecular orientation and slower mass loss with PL 65 than with PL 49 or PL 38. In comparison with amorphous poly(D,L-lactide) grades such as PURASORB PDL 02A, PL 65 has a melting endotherm at 170–180 °C, higher tensile stiffness, and a substantially slower resorption profile. The semi-crystalline structure also introduces the need for annealing if uncontrolled shrinkage or delayed crystallization is not acceptable in the packed device.

The differences in inherent viscosity are more consequential than the small differences in thermal transitions. PL 65 provides higher melt strength for extrusion of thin-walled tubing or fiber spinning, but extrusion back pressure rises. When a production line is configured for PL 38, a switch to PL 65 should include verification of melt-pressure transducer readings, torque limits, and volumetric throughput because the higher viscosity alters temperature rise from viscous dissipation.

Hydrolytic Degradation and Mass-Loss Profiles in Buffered Media

In phosphate-buffered saline at 37 °C and pH 7.4, performed under ASTM F1635-16, poly(L-lactide) homopolymer undergoes bulk hydrolysis. The initial stage is a decrease in molecular weight at nearly constant mass, driven by random ester-bond cleavage. As oligomers and acidic chain ends accumulate, carboxylic acid end groups accelerate the reaction in thick sections, producing an internal acidic microclimate. PL 65 begins from a higher initial inherent viscosity than PL 49 and PL 38, so the molecular weight threshold for onset of measurable mass loss is reached later. Published in vitro degradation data for high-molecular-weight poly(L-lactide) indicate 50% molecular weight loss at 6–12 months and the onset of mass loss after 12–18 months in specimens of moderate cross section. Complete resorption in vivo generally exceeds 24–36 months depending on part thickness, percent crystallinity, local vascularity, and pH. For PURASORB PL 65 in a specific load-bearing orthopedic configuration, published data is limited; mass-loss timelines should be generated for the exact device geometry and annealing state.

The degradation products are predominantly L-lactic acid and short-chain oligomers, which are metabolized through normal biochemical pathways. Compared with copolymers of L-lactide and glycolide, the homopolymer releases acidic products more slowly and therefore poses a lower local pH burden per unit time. Compared with poly(D,L-lactide), the crystallites in PL 65 act as transient physical crosslinks that reduce water penetration and slow mass loss until the amorphous regions are sufficiently cleaved.

In fused filament fabrication and melt-based additive manufacturing, PL 65 filament requires the same moisture control as molded parts. Nozzle temperatures from 190–210 °C and build-platform temperatures near 55–60 °C reduce warping in small scaffolds; the melt viscosity limits the minimum nozzle diameter to approximately 0.25 mm. Printed struts solidify with low crystallinity unless annealed. Processors report that copolymer grades such as PDL 02A print with lower nozzle pressure but lack the mechanical stiffness required for a temporary load-bearing scaffold. Published data for the mechanical performance of PL 65 in fused filament fabrication is limited.

Under the supplier’s quality system, PURASORB PL 65 is released with a batch certificate of analysis that includes inherent viscosity, residual lactide, residual solvent, tin catalyst content, heavy metals, and appearance. Manufacturing is controlled under EN ISO 13485:2016; final device risk management is governed by ISO 14971:2019. The resin is not sterilized at release and carries no final-device biocompatibility claim. Storage in sealed aluminum-lined bags with desiccant at ≤-15 °C is recommended to minimize atmospheric hydrolysis. After opening, the material should be re-dried before melt processing. The polymer should not be compounded with strong bases, strong nucleophiles, or protic additives that accelerate ester cleavage. Processing with poly(L-lactide-co-glycolide) copolymers is possible but will alter the degradation profile away from that of the homopolymer.