LACTEL® DL-PL (B6014-2) Biomedical Acid-Terminated PLA

LACTEL® DL-PL (B6014-2) is an amorphous poly(DL-lactide) homopolymer supplied with terminal carboxylic acid functionality. The grade is produced by ring-opening polymerization of racemic DL-lactide, followed by end-group modification and purification steps that reduce residual lactide, residual tin, and process solvents. The backbone repeat unit contains one hydrolytically labile ester bond per lactic acid residue; the racemic stereochemistry suppresses crystallization and yields a single glass transition rather than a melting endotherm. B6014-2 is part of the LACTEL® DL-PL series and is distinguished from ester-capped DL-PL grades of comparable molecular weight by the presence of free terminal acid groups. These terminal groups raise the bulk polarity, increase moisture affinity, and provide a reactive site for carbodiimide-mediated conjugation or drug-polymer interaction. The material is supplied as a low-molecular-weight solid for solvent-based microsphere, nanoparticle, implant coating, and drug-eluting system applications. Lot-specific release data are governed by the certificate of analysis; exact inherent viscosity, molecular weight, acid value, and residual monomer values are lot-controlled and should not be inferred from similar DL-PL grades.

Long-term storage of the dry powder is normally specified at −20 °C in sealed vacuum pouches under an inert gas atmosphere. Before opening, the pouch should be equilibrated to ambient temperature to prevent condensation on the cold surface. The polymer is hygroscopic, and exposure to ambient humidity during weighing and handling should be minimized because adsorbed water accelerates hydrolytic chain scission even before the material enters the formulation process.

Which Release Parameters Control B6014-2 Performance in Drug-Eluting Devices?

Release control for B6014-2 depends on dilute-solution viscosity, acid end-group concentration, residual DL-lactide monomer, residual tin, and moisture. Inherent viscosity is measured in chloroform at 30 °C and a polymer concentration of 0.1 g/dL according to ISO 1628-1. The viscosity value correlates with viscosity-average molecular weight and therefore with solution viscosity during microsphere hardening and with the polymer contribution to diffusion resistance in the final matrix. Acid value is determined by potentiometric titration against 0.1 N potassium hydroxide in ethanol and is expressed as mg KOH/g. A higher acid value indicates a higher number of carboxylic acid end groups per unit mass, which increases hydrophilicity and shortens the initial lag phase during hydrolytic degradation. Residual DL-lactide monomer is measured by gas chromatography with flame ionization detection using an internal standard. Elevated residual lactide acts as a plasticizer, depresses the glass transition temperature, and can produce an early release burst if not controlled. Residual tin from stannous octoate catalysis is determined by inductively coupled plasma optical emission spectroscopy after acid digestion according to ISO 11885. Moisture is measured by Karl Fischer coulometry under ISO 15512; this is a critical release parameter because acid-terminated low-molecular-weight PLAs sort moisture rapidly and undergo chain scission during processing if the water content is not controlled. Glass transition temperature is measured by differential scanning calorimetry on the second heating scan at 10 °C/min according to ISO 11357-2. The absence of a melting endotherm confirms the amorphous DL-lactide architecture.

For biomedical use, supporting documentation may include residual solvent data, elemental impurity data, and a risk-based biocompatibility evaluation matrix. Residual solvent is generally assessed according to USP <467>; elemental impurities are controlled according to USP <232> and <233> or ICH Q3D. The finished device sponsor is responsible for selecting the appropriate endpoints from ISO 10993-1, typically including ISO 10993-5 cytotoxicity, ISO 10993-6 implantation, and ISO 10993-10 sensitization when manufacturing changes alter surface properties. ISO 10993-13 is used to identify and quantify degradation products from the polymeric device under accelerated hydrolysis. The polymer itself is an excipient or device component, not a drug substance; therefore, the applicable regulatory route depends on the finished product classification.

When B6014-2 Replaces Ester-Capped DL-PL in Solvent-Based Fabrication

Replacement of an ester-capped DL-PL with B6014-2 is not a drop-in substitution when the process involves oil-in-water emulsification, solvent casting, or in situ precipitation. The terminal carboxylic acid increases the polarity of the polymer and changes the partitioning of drug molecules between dichloromethane, ethyl acetate, and the continuous aqueous phase. With basic drug salts, the acid-terminated backbone can form ionic pairs or promote drug adsorption at the particle surface; this may reduce initial burst in some formulations or alter encapsulation efficiency in others. The effect is greater in low-molecular-weight grades because the number of acid end groups per gram is higher. Particle size, residual solvent removal, drug loading, and release profile should therefore be re-qualified after substitution. Comparative formulation studies of acid- and ester-terminated PLAs in poly(DL-lactide) microspheres have shown faster water uptake and shorter degradation lag times for the acid-terminated series when tested in phosphate-buffered saline at 37 °C under ASTM F1635.

For oil-in-water emulsification, B6014-2 is typically dissolved in dichloromethane or ethyl acetate at 5–15 wt% polymer. The continuous phase is an aqueous solution of polyvinyl alcohol at 0.5–2.0 wt%. Emulsification is performed with a rotor-stator mixer at tip speeds of 10–24 m/s. Higher shear reduces the mean particle diameter, but excessive vortex entrainment increases air inclusions and surface defects. The primary emulsion is transferred into an extraction bath at an aqueous-to-organic volume ratio of 10:1 to 20:1. Solvent removal is continued for 3–6 h with controlled stirring. Hardened microspheres are collected by centrifugation, washed with water for injection, and lyophilized. The terminal carboxyl groups of B6014-2 can increase encapsulation efficiency for drugs with protonatable amine groups; however, an excessively low external phase pH can promote particle aggregation. The dispersion pH is therefore commonly held between pH 4 and pH 7. Release testing after fabrication should follow USP <711> or an equivalent in vitro release method in phosphate-buffered saline at 37 °C.

Melt Processing Boundaries for Acid-Terminated DL-PL

B6014-2 is not typically selected as a load-bearing melt-processed polymer, but melt extrusion and injection molding are used for some implant prototypes. The free carboxylic acid end groups accelerate ester hydrolysis and can catalyze transesterification at elevated temperature. Pre-drying to 250 ppm moisture or below is required before melt processing; this is measured by Karl Fischer coulometry. Vacuum drying is carried out below the glass transition temperature to prevent sintering of the low-molecular-weight powder. A shallow-tray vacuum dryer with a nitrogen bleed is preferred over a hopper dryer because low-molecular-weight amorphous powder can fuse at inlet air temperatures. A co-rotating twin-screw extruder with a screw diameter of 11 mm to 27 mm and an L/D ratio of 25:1 to 40:1 is suitable for laboratory and pilot compounding. Gravimetric feeding is required because the dried powder has low bulk density and variable flow. General poly(DL-lactide) processing evaluations often begin at 130 °C and do not exceed 180 °C; acid-terminated low-IV grades may require the lower end of this range. Published data for this specific configuration is limited, so thermal stability should be confirmed by melt rheometry, thermogravimetric analysis according to ISO 11358, and gel permeation chromatography before scale-up. Melt pressure and torque records are used to detect moisture-induced viscosity loss or chain scission during extrusion.

Relative to a 50:50 DL-lactide-co-glycolide, B6014-2 has no glycolide repeat unit, so the primary hydrolytic degradation product is lactic acid rather than a mixture of lactic and glycolic acid. The methyl side group of each lactic acid residue reduces water uptake compared with glycolide-rich copolymers; therefore, the degradation front matures more slowly and the onset of mass loss is shifted later for equivalent molecular weight and end-group chemistry. Compared with an ester-capped DL-PL of the same molecular weight, B6014-2 has a higher carboxylic acid density, higher moisture affinity, and lower resistance to hydrolysis at the chain terminus. The acid end group can be used for carbodiimide-mediated conjugation to hydroxyl- or amine-bearing molecules; this reactivity is absent in ester-capped grades. Compared with semicrystalline poly(L-lactide), the DL-lactide backbone prevents crystallization and provides a single glass transition without a melting endotherm, as measured by differential scanning calorimetry under ISO 11357-2. The absence of crystallinity eliminates the need to control annealing-induced crystal morphology, but it also reduces tensile modulus and eliminates self-reinforcement by oriented crystallites. Tensile testing, when necessary, should be performed according to ASTM D638-14 on solvent-cast or compression-moulded films; values are lower than PLLA and are strain-rate dependent.

In situ forming implants are prepared by dissolving B6014-2 in N-methyl-2-pyrrolidone or dimethyl sulfoxide at concentrations of 5–40 wt%. Injection into aqueous tissue fluid induces solvent exchange and precipitation of the polymer. The acid-terminated surface hydrates rapidly and may reduce the solvent-exchange lag relative to ester-capped grades, but this also makes the depot more sensitive to early water uptake. Subcutaneous or periodontal pocket administration is more appropriate than orthopaedic load-bearing use because the amorphous DL-PL matrix lacks crystallinity and has limited tensile modulus. The depot is not a terminal barrier device; degradation proceeds by bulk hydrolysis, and the mass-loss profile depends on implant geometry, residual solvent, and local pH. Process validation for such systems should include residual solvent testing, sterility, endotoxin, and degradation product profiling according to ISO 10993-13.