PURASORB PL 18 Medical Device Poly(L-lactide) is a semicrystalline aliphatic polyester synthesized by ring-opening polymerization of L-lactide. It is supplied as a medical-grade homopolymer in pellet form for downstream conversion into absorbable implants and finished medical devices, not as a sterile implant or terminal device. The nominal intrinsic viscosity of this grade is 1.8 dL/g when measured in chloroform at 25 °C and a polymer concentration of 0.1 g/dL using dilute-solution capillary viscometry per ISO 1628-1:2021. As an L-lactide homopolymer, PL 18 can crystallize when annealed above its glass transition, typically at 110–120 °C under nitrogen; this semicrystalline character differentiates it from amorphous copolymers such as PDLLA and PLGA and contributes to longer hydrolytic stability under physiological test conditions. The actual resorption time is not intrinsic to the polymer alone and must be established by device-specific testing according to ISO 13781:2017 or ASTM F1635-16.

How Does the Nominal 1.8 dL/g Intrinsic Viscosity Affect Melt Processing?

In dilute solution, the intrinsic viscosity of PL 18 is used as an indirect measure of molecular weight. At equivalent processing temperature, PL 18 exhibits lower melt viscosity than higher-IV PLLA grades such as PL 24 or PL 32, which reduces filling pressure in thin-wall injection molds but also narrows the molecular weight reserve that remains after melt processing. Melt viscosity measurements for PLLA are preferably performed by capillary rheometry at 180–200 °C and shear rates from 10 s⁻¹ to 10,000 s⁻¹; single-point melt mass-flow rate values per ISO 1133-1:2022 are not recommended as a lot-release criterion for this medical grade because the test conditions do not sufficiently control moisture and degradative chain scission. Molders using PL 18 for small orthopedic components maintain barrel zones between 175 °C and 200 °C, hold the nozzle below 200 °C, and keep screw back pressure below 2 MPa to limit shear heating.

Capillary rheometry data for PLLA within this intrinsic-viscosity range show pronounced shear thinning; apparent viscosity at 100 s⁻¹ can be approximately one order of magnitude higher than at 1,000 s⁻¹ at 190 °C. For thin-wall parts, gate sizing should maintain shear rates below 50,000 s⁻¹ to limit shear heating and molecular weight loss. Single-stage screws with L/D 24:1 to 40:1 and compression ratios from 2.5:1 to 3:1 are used for plastication; the feed zone is kept at 170–180 °C, the compression zone at 180–190 °C, and the metering zone at 190–200 °C. A melt temperature above 220 °C is tolerated only with short residence time because ester pyrolysis and lactide monomer reformation accelerate sharply.

Before melt processing, pelletized PL 18 is dried in a desiccant dryer with a dew point below -40 °C and bed air temperature of 80–100 °C for 4–6 h. The target residual moisture is below 250 ppm. At barrel temperatures above 180 °C, moisture hydrolyzes the ester linkages and reduces molecular weight; a single pass through an extruder with moist feed can reduce intrinsic viscosity by more than 0.3 dL/g and create surface splay on molded parts. A twin-screw extruder with L/D 40:1, modular screw elements, and vacuum venting below -0.08 MPa is commonly used for compounding or reactive extrusion control; screw configurations with compression ratios from 2.5:1 to 3:1 and low-shear mixing zones are selected to avoid excessive torque and thermal degradation. The melt residence time is kept below 5 min, and regrind content is limited to 20 % unless lot-specific data demonstrate equivalent molecular weight and color stability.

When Gamma Irradiation Is Selected for Terminal Sterilization

Gamma irradiation at a nominal dose of 25 kGy in air produces radical-mediated chain scission in PL 18, reducing molecular weight and broadening molecular weight distribution. The effect is dose-rate and atmosphere dependent; irradiation in vacuum or nitrogen reduces oxidative chain scission but does not eliminate it. Devices sterilized by gamma according to ISO 11137-1 should therefore be processed with initial intrinsic viscosity at the upper end of the release interval and tested after terminal sterilization to ISO 527-2:2012 or ISO 178:2019. Ethylene oxide sterilization imposes less molecular weight loss in dry heat-sensitive polyesters, but residual gas limits and aeration must comply with ISO 10993-7:2008. Steam sterilization is not appropriate for PL 18 because moisture and temperature above the glass transition initiate bulk hydrolysis and distort molded geometry.

Validation of gamma sterilization for PL 18 requires post-sterilization intrinsic viscosity and mechanical testing. Electron beam sterilization at equivalent dose produces a narrower dose distribution in uniform devices but still generates chain scission. Absorbed dose mapping is performed according to ISO 11137-2; the maximum observed dose, not the nominal dose, controls molecular weight loss. Packaging for irradiation should exclude oxygen and, when moisture-sensitive, include desiccant. Irradiation in air increases hydroperoxide formation and subsequent hydrolytic degradation at an accelerated rate; therefore, long-term packaging stability studies should include post-sterilization aging at 40 °C and 75 % RH or equivalent accelerated conditions referenced to real-time data.

Annealing, Crystallinity Development, and Dimensional Stabilization

Injection molding of thin-wall fracture fixation pins, interference screws, and craniofacial plates from PL 18 uses melt temperatures of 180–200 °C and mold temperatures below 40 °C when an amorphous, transparent preform is desired. The part is then annealed at 110–120 °C for 2–4 h under nitrogen to develop crystallinity, increase modulus, and stabilize dimensions. Mold temperature above the PLLA cold-crystallization onset can be selected for in-mold crystallization, but ejection and handling require longer cooling and controlled tools. Part design avoids sharp wall-thickness transitions because differential crystallization shrinkage generates internal stress and reduces impact load capacity. Tensile and flexural property measurements to ISO 527-2:2012 and ISO 178:2019 are required after annealing because crystallinity, not intrinsic viscosity alone, controls short-term mechanical performance.

Annealing of PL 18 also changes solvent uptake and degradation behavior. Specimens cooled rapidly below the glass transition retain amorphous regions that hydrolyze first, while annealed specimens develop crystalline domains that resist water ingress. The result is a difference in in vitro mass-loss profiles for chemically identical material; therefore, processing history must be fixed before degradation testing or regulatory test article preparation. Differential scanning calorimetry according to ISO 11357-2:2020 and ISO 11357-3:2018 is used to document glass transition, cold crystallization, and melting endotherm before lot acceptance.

PLLA Homopolymer Versus PLGA Copolymer Performance Boundaries

PL 18 differs from PURASORB PDLG and PDLLA grades primarily by its L-lactide homopolymer structure and the absence of glycolide or D-lactide units. Glycolide-containing copolymers are amorphous and absorb water more readily; their ester bonds are more exposed to hydrolytic attack, so mass loss and strength loss occur on shorter timescales under the same test protocols. In contrast, PL 18 can crystallize, which restricts water uptake and chain mobility in the crystalline domains. Degradation of semicrystalline PLLA is therefore heterogeneous: amorphous regions hydrolyze first, while crystalline domains persist and release acidic oligomers more slowly. This difference means PL 18 is selected when a device must retain mechanical integrity for periods longer than amorphous PLGA or PDLLA devices at the same implant site. Direct product comparison requires identical test specimens, pH, temperature, enzyme load, and dynamic loading history; ASTM F1635-16 provides an in vitro degradation method but does not predict clinical behavior.

Relative to higher-IV PLLA homopolymers, PL 18 has a lower molecular weight reservoir. The processing advantage is lower melt viscosity and easier filling of micro-scale features; the trade-off is that hydrolytic chain scission reaches a critical molecular weight for loss of tensile strength earlier if all other factors remain constant. Strength retention comparisons among PLLA grades therefore require molded specimens of equal crystallinity, sterilized by the same method, and conditioned according to ISO 291:2008 at 23 °C and 50 % RH before testing.

In vitro degradation studies on PL 18 are typically conducted in phosphate-buffered saline at 37 °C and pH 7.4 per ASTM F1635-16 or ISO 13781:2017. Specimen geometry, pH refresh rate, and load condition must be controlled because oligomer accumulation autocatalyzes hydrolysis. During early stages, molecular weight loss occurs before mass loss; mechanical strength loss follows as chain scission reduces tie-chain density in the amorphous phase. The crystalline phase persists longer and can maintain mass until the surrounding solution becomes sufficiently acidic. Published data for the specific device geometry of PL 18 may be limited; therefore, implant manufacturers generate degradation curves for each wall thickness, sterilization dose, and annealing history.

A certificate of analysis for lot release of PL 18 typically reports intrinsic viscosity, appearance, and residual monomer or solvent content; the specific release interval is defined by the manufacturer, and users verify that the lot values remain inside their validated process window. The polymer is manufactured under a quality management system conforming to ISO 13485:2016. Biocompatibility of the finished device is evaluated according to ISO 10993-1:2018; chemical characterization per ISO 10993-18:2020 is required when changes in processing or packaging are made. Material-level screening to USP <88> Class VI is commonly reported but is not a substitute for device-level biological evaluation under ISO 10993. Storage of PL 18 should be in closed, dry containers at low temperature; condensation is avoided when opening chilled packaging. Lot-to-lot variation in molecular weight and residual moisture is tracked using ISO 1628-1:2021 and Karl Fischer titration, respectively.