PURASORB PL 38 Medical Device Poly(L-lactide) is a semi-crystalline resorbable homopolymer of L-lactide supplied as a granular resin for melt processing into implantable device components. The grade designation denotes a nominal inherent viscosity of 3.8 dL/g measured in chloroform at 25 °C using an Ubbelohde capillary viscometer according to ISO 1628-1. Lot release documentation typically reports inherent viscosity in a controlled band around that nominal value; the certificate of analysis should be consulted for the lot-specific interval, because small shifts in molecular weight alter both melt rheology and hydrolytic degradation kinetics. The polymer exhibits a glass transition temperature of 60–65 °C and a melting endotherm at 170–185 °C when measured by differential scanning calorimetry according to ISO 11357-3. Density at 23 °C is ordinarily 1.24–1.26 g/cm³ by ISO 1183-1. Residual lactide monomer is controlled below 0.5 wt% by gas chromatography–flame ionisation detection, and moisture content is reduced below 0.5 wt% by coulometric Karl Fischer titration using ISO 15512 methodology. Tin catalyst residues are managed through the manufacturer’s medical-device release protocol; published data for this exact grade is limited, but high-purity PLLA resins of this class are commonly released with tin below 100 ppm by inductively coupled plasma–mass spectrometry after microwave digestion.

Inherent viscosity as a lot-to-lot processing control parameter

The inherent viscosity value is not merely a raw-material specification; it functions as a processing-control surrogate for molecular weight distribution and melt strength. During twin-screw extrusion or injection moulding, PL 38 exhibits higher apparent melt viscosity than lower-IV PURASORB PLLA grades such as PL 18 or PL 24. Capillary rheometry on high-IV medical-grade PLLA at 190 °C and an apparent shear rate of 100 s^-1 typically places apparent viscosity between 2,000 and 4,500 Pa·s, although direct lot-specific measurement is required because residual monomer and absorbed moisture shift the flow curve. On production-scale equipment, excessive back pressure and torque rise are observed when the resin is not adequately dried or when barrel residence time is extended. Injection moulding lines using general-purpose screws with L/D ratios of 20:1 to 25:1 can process PL 38, but high-inherent-viscosity lots may require an increase in barrel temperature of 5–15 °C relative to PL 32 to maintain equivalent fill. The relationship between inherent viscosity and injection pressure is non-linear; a lot at the upper end of the release band can increase peak injection pressure by 10–25% in thin-wall fixation pins, depending on gate geometry and runner dimensions. Such lot-to-lot rheological variation should be captured in process capability studies and monitored by in-mould pressure transducers for critical load-bearing components.

What drying and injection moulding boundaries are required for high-IV PLLA?

Poly(L-lactide) undergoes hydrolytic chain scission when melt-processed in the presence of moisture. PL 38 should be dried under vacuum at 70–80 °C for 4–16 h until residual moisture is below 100 ppm; on production lines, desiccant dryers with a dew point of -40 °C or lower are preferred. Melt processing is commonly conducted between 180 °C and 210 °C, with total residence time above 200 °C limited to approximately 15 min to minimise thermo-oxidative and random chain-scission events. Injection mould temperatures are typically maintained at 15–30 °C for rapid solidification, but mould temperatures above 60 °C can be used when elevated crystallinity or improved dimensional stability is desired. Barrel temperatures above 220 °C are not recommended for extended periods because lactide reformation and discolouration may occur. Post-moulding annealing at 105–120 °C for 2–4 h increases crystalline content and reduces warpage in semi-crystalline fixation devices, though flexibility decreases. Processing under humid ambient conditions above 60% RH requires closed hopper feeding and dry-air purge to prevent rapid moisture regain. High-shear dispersion of fillers is possible on a corotating twin-screw extruder with L/D ratio of 40:1, but intensive screw configurations should be evaluated for local temperature overshoot above the degradation threshold.

Compression-moulded plates and injection-moulded tensile bars fabricated from PL 38 have been evaluated under ASTM D638-14 and ISO 527-2. Published data for high-molecular-weight PLLA homopolymers place tensile strength between 60 and 75 MPa, tensile modulus between 3.0 and 3.8 GPa, and elongation at break between 2 and 5%. These values support use in orthopaedic interference screws, suture anchors, and fracture fixation pins where several months of load-bearing retention are required before hydrolytic mass loss becomes measurable. The resin is not intended for elastomeric components, rapidly resorbable soft-tissue meshes, or applications requiring more than 5% strain at break. In such cases, PURASORB PLGA copolymers with lower lactide content or higher comonomer ratios are typically selected because the amorphous structure lowers modulus and accelerates water uptake. PL 38 may also be considered for drug-device combination products requiring slow matrix erosion, provided the active pharmaceutical ingredient is stable during melt processing above 180 °C and is not susceptible to acid-catalysed degradation during extended residence in the implant.

When PL 38 replaces PL 32 or PL 49 in load-bearing fixation devices

Within the PURASORB PLLA homopolymer series, PL 38 occupies an intermediate molecular-weight position between PL 32 with a nominal inherent viscosity of 3.2 dL/g and PL 49 with a nominal inherent viscosity of 4.9 dL/g. Compared with PL 32, PL 38 exhibits higher melt viscosity, higher tensile strength retention, and longer hydrolytic stability under in vitro conditions. The processing penalty is increased injection pressure and greater sensitivity to moisture-induced viscosity loss. Compared with PL 49, PL 38 has lower melt viscosity and can fill thin-wall mould cavities at lower barrel temperatures, but PL 49 retains molecular weight longer during degradation and may better sustain mechanical integrity in thicker load-bearing constructs. Under ISO 13781:2017 in vitro degradation conditions in phosphate-buffered saline at 37 °C, the higher-molecular-weight PL 49 grade typically shows a longer lag phase before measurable mass loss than PL 38, while PL 32 enters the mass-loss phase earlier. The choice among these three grades is therefore governed by the required load-bearing duration, part wall thickness, and acceptable injection-moulding pressure. Unlike PURASORB PLGA 85/15 or 75/25 copolymers, PL 38 contains no glycolide repeat units; the absence of glycolic acid segments reduces the early acid-burst autocatalysis observed in high-glycolide PLGA matrices and gives PL 38 a semicrystalline morphology with a melting endotherm near 175 °C. This distinction is relevant for orthopaedic devices in which dimensional stability and slower water uptake are required during the first six months after implantation.

Comparative compliance and release metrics

The medical-device designation differentiates PL 38 from technical-grade PLLA of equivalent molecular weight by requiring lot-level documentation, change control, and biological evaluation according to ISO 10993. This does not alter the intrinsic polymer physics but constrains raw-material sourcing and lot-release testing for implantable applications.

Hydrolytic degradation of PL 38 proceeds by random chain scission of ester bonds, with water uptake concentrated initially in the amorphous regions before crystalline lamellae are eroded. Under ISO 13781:2017 in vitro conditions, the material typically shows a lag phase of 6–12 months before measurable mass loss, with total resorption of high-inherent-viscosity PLLA extending beyond 24–36 months depending on part geometry, residual crystallinity, and local pH. Annealing at 110 °C can extend this lag phase by increasing crystalline order, but may leave crystalline debris after matrix collapse at sites where macrophage-mediated clearance is limited. Processing-induced orientation in fibres or injection-moulded rods also slows water penetration because oriented crystalline domains resist hydrolysis. PL 38 should not be steam-sterilised above 45 °C in humid conditions because moisture ingress at elevated temperature reduces molecular weight before implantation. Gamma irradiation at 25 kGy can reduce inherent viscosity by 10–20% in high-IV PLLA; therefore post-sterilisation inherent viscosity should be included in the release specification when sterilisation is performed after final packaging. Ethylene oxide is an alternative for heat-sensitive formats, but residual ethylene oxide and ethylene chlorohydrin levels must be validated and monitored. In absorbable fixation components, the design should assume that mechanical strength decays before mass loss becomes detectable, and the implant must therefore carry load only during the early healing phase while surrounding tissue assumes increasing mechanical responsibility.