Process Conflicts in Solvent Evaporation for Depot Microsphere Manufacturing

Acid-terminated 50:50 DL-PLG (B6013-1) is processed into long-acting injectable microspheres by o/w single-emulsion or w/o/w double-emulsion solvent evaporation, and the terminal carboxyl groups change hydration and degradation kinetics relative to ester-capped grades. In a typical o/w line, the polymer is dissolved in dichloromethane at 10–20% w/w or in ethyl acetate at 8–15% w/w, while the continuous phase contains 0.5–2.0% poly(vinyl alcohol) with a hydrolysis degree of 87–89 mol%; drug:polymer ratios for low-dose peptide salts are commonly 1:10–1:20, and high-load formulations may approach 1:5 only when the dispersed-phase viscosity is raised with a co-solvent such as benzyl alcohol or dimethyl sulfoxide at <2% of the dispersed phase. A rotor-stator high-shear mixer, typically a Silverson L5T fitted with a fine emulsor screen, is operated at tip speeds of 5–15 m/s to achieve target mean particle diameters of 20–80 µm; droplet breakup is governed by the balance between viscous stress and interfacial tension, and excessive shear beyond 15 m/s may induce polymer precipitation at the droplet boundary, creating surface pores that later raise the initial release burst above 20% of total payload. Solvent removal is performed in a jacketed vessel with a temperature ramp from 4 °C to 35 °C over 4–8 h, followed by vacuum stripping at 250–400 mbar; the acid-terminated grade requires tighter control of quench temperature than ester-terminated analogues because rapid water uptake into the polymer-rich shell can lower the local glass transition temperature and cause droplets to coalesce before hardening. Residual dichloromethane is assessed by headspace gas chromatography under USP <467> or ICH Q3C(R8), and the limit for dichloromethane is 600 ppm in many parenteral products; ethyl acetate, when used, has a permitted daily exposure of 5,000 ppm but slower extraction rates and higher aqueous solubility, which can lower encapsulation efficiency for hydrophobic actives. After the hardened microspheres are washed with water for injection and collected by filtration, the batch is lyophilized using a staged cycle with primary drying at −20 °C and secondary drying below the polymer glass transition temperature, a conservative set point being 35–40 °C for acid-terminated 50:50 material; residual moisture should remain below <1.0% to avoid premature hydrolytic chain scission during storage. Terminal products in this segment include leuprolide acetate, triptorelin pamoate, exenatide, risperidone, and naltrexone depot formulations; the acid-terminated polymer is preferred where faster release after hydration is desired, but the same attribute narrows the processing window during solvent evaporation and lyophilization.

Nanoprecipitation and microfluidic antisolvent precipitation offer a downstream route for B6013-1 when the required injectable particle population lies below the microsphere range. The acid-terminated polymer is dissolved in a water-miscible solvent such as acetone at 5–15 mg/mL, while an aqueous antisolvent phase contains 0.1–0.5% poloxamer 188 or polyvinyl alcohol; mixing is performed in a confined impinging jet mixer or a microfluidic chip with a channel width of 100–500 µm and a total flow rate of 1–10 mL/min, producing particles with mean diameters of 80–180 nm and polydispersity indices below 0.15 when the acetone:water ratio is maintained at 1:5–1:10. The acid terminal groups lower the interfacial tension of the precipitating polymer phase and can yield more negative zeta potential values, typically −20 to −35 mV, which improves colloidal stability without high concentrations of surfactants; however, zeta potential alone is not a release specification, and batch acceptance relies on dynamic light scattering with cumulant analysis and transmission electron microscopy for morphology. Solvent removal is carried out by rotary evaporation at 25–30 °C under reduced pressure, and residual acetone is measured by headspace gas chromatography against ICH Q3C(R8), where acetone is a Class 3 solvent with a permitted daily exposure of 5,000 ppm. Freeze drying is performed with a cryoprotectant such as trehalose or mannitol at 1:0.5–1:2 nanoparticle:cryoprotectant mass ratios; primary drying at −25 °C and secondary drying at 30 °C are adjusted to keep the product temperature below the collapse temperature of the amorphous excipient matrix. Terminal applications include intravenous taxane and anthracycline analogue formulations as well as ligand-targeted particles for solid tumours; release from acid-terminated 50:50 PLGA nanoparticles typically begins with a hydration lag of <24 h followed by a diffusion phase and an erosion phase, and the carboxylic acid end groups can shorten the erosion phase by 7–14 days relative to ester-capped analogues when particle size and molecular weight are matched. For sterile filtration, a population with mean diameter below 180 nm and D90 below 220 nm may pass through a 0.2 µm sterilizing-grade membrane, but membrane adsorption often causes yield loss, and aseptic processing may be required if terminal gamma irradiation reduces molecular weight beyond release specifications.

When N-Methyl-2-pyrrolidone Enters the Injectable In Situ Implant

In situ forming implants based on acid-terminated 50:50 DL-PLG use N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide, or triacetin as a water-miscible solvent to dissolve the polymer at 30–50 wt%, and the polymer solution is injected into subcutaneous or intramuscular tissue where solvent exchange precipitates the polymer into a solid depot. NMP is the most widely used solvent for this mechanism, but its strong hydrogen-bonding capacity and high water affinity create a burst release window during the first 2–24 h that is partly controlled by polymer concentration, solvent type, and acid end-group content. At polymer loadings below 30 wt%, the depot remains soft and porous, and the initial release can exceed 15–25% of the payload; increasing the concentration to 40–50 wt% raises solution viscosity to 500–5,000 mPa·s at 25 °C and slows phase inversion, but syringeability through a 20–23 G needle becomes a limiting variable. A positive-displacement syringe pump or a pre-filled cartridge with a short needle is used to control injection force; the acid-terminated polymer has lower solution viscosity than an ester-capped grade of equal molecular weight, which can be an advantage in cold-chain filling but may reduce the mechanical integrity of the depot. The phase inversion process includes rapid solvent/non-solvent exchange, vitrification of the polymer-rich shell, and internal pore formation; residual NMP in the depot is limited by ICH Q3C(R8), where NMP is a Class 2 solvent with a permitted daily exposure of 5.3 mg/day, and batch release testing uses headspace gas chromatography after extraction from the dried formulation. Published data for this specific B6013-1 configuration with NMP is limited; the ranges above are drawn from acid-terminated 50:50 PLGA literature rather than a vendor-specific dataset. Terminal products in this category include leuprolide acetate, buprenorphine, and doxycycline depot formulations, with the polymer serving as the rate-controlling matrix rather than a drug-loaded microsphere. Sterility is achieved by aseptic processing or terminal filtration of the polymer solution before filling; gamma irradiation is generally avoided because acid-terminated 50:50 PLGA undergoes chain scission at doses as low as 5–10 kGy, shifting the degradation profile and increasing the content of low-molecular-weight oligomers.

What Limits Injection Moulding of Amorphous 50:50 DL-PLG Fixation Pins?

Melt processing of acid-terminated 50:50 DL-PLG into orthopaedic pins, screws, and plates is constrained by the amorphous character of the 50:50 copolymer, the low glass transition temperature, and the greater thermal sensitivity of acid end groups. The resin is first vacuum-dried at 25–35 °C for 24–72 h until moisture is below 0.05% by Karl Fischer titration, because residual water at 0.1% can hydrolyse the ester backbone during extrusion and reduce molecular weight. Compounding is performed in a co-rotating twin-screw extruder with an L/D ratio of 25–40:1 and a temperature profile of 120–155 °C; screw speed is held at 100–200 rpm, and torque is monitored as an indicator of chain scission because torque decreases as molecular weight falls. The melt is then injection-moulded with a barrel temperature not exceeding 160 °C and a mould temperature of 20–35 °C; clamp force is calculated from the projected cavity area and a conservative melt pressure of 400–800 bar. Because acid-terminated 50:50 PLGA is amorphous, it does not exhibit a sharp melting transition, and crystallization cannot be used to strengthen the part; the tensile modulus of compression-moulded or injection-moulded specimens is typically in the range of 1.5–3.0 GPa when measured by ASTM D638-14, but this value falls below 1 GPa after 2–4 weeks in phosphate-buffered saline at 37 °C. The acid-terminated grade degrades faster than ester-capped 50:50 PLGA of equivalent inherent viscosity, so fixation devices require careful assessment under ASTM F1635-16 to track mass loss, molecular weight retention, and pH drop in buffer. Terminal applications are limited to non-load-bearing or lightly loaded paediatric and maxillofacial fixation where resorption is preferred over long-term metal hardware; for load-bearing orthopaedic applications, the acid-terminated 50:50 material is generally not used alone because the loss of mechanical properties occurs before bone union in cortical bone. Sterilization by ethylene oxide is preferred over gamma irradiation, with residual ethylene oxide limits specified in ISO 10993-7:2008; if gamma is unavoidable, doses should be kept below 10 kGy and post-irradiation molecular weight change should be quantified.

Electrospinning of B6013-1 into nonwoven scaffolds for guided tissue regeneration and wound dressing depends on solution conductivity, polymer concentration, and the solvent volatility of fluorinated or chlorinated solvent systems. A solution of acid-terminated 50:50 PLGA is prepared in 1,1,1,3,3,3-hexafluoroisopropanol at 6–10% w/v or in a mixed chloroform/dimethylformamide system at 80:20 v/v with a polymer concentration of 12–18% w/v; the solution is fed through a 0.5–1.0 mm blunt-tip needle at 0.5–2.0 mL/h, while the collector is positioned 10–20 cm from the tip and charged to 15–25 kV relative to the grounded collector. The resulting fibre diameter is typically 300–1,000 nm, and the acid end groups increase surface wettability and provide anchoring points for subsequent peptide or extracellular-matrix coating; however, the same carboxyl functionality can bind cationic antimicrobials or growth factors too strongly, altering release kinetics. Fibre mats are dried for 24–48 h at 25–30 °C under vacuum to remove residual solvent, and residual hexafluoroisopropanol is quantified by headspace gas chromatography; because hexafluoroisopropanol is not listed as a Class 1 or Class 2 solvent in ICH Q3C(R8), batch acceptance is usually set through supplier-specific validated limits and should not exceed the general toxicological limit for unknown solvents unless justified. Mechanical properties are evaluated on a tensile tester using ASTM D882-18 for thin plastic sheeting, with acellular mats exhibiting tensile strengths of 1–5 MPa and elongations at break of 10–60%; after immersion in phosphate-buffered saline at 37 °C, the mats lose tensile strength rapidly due to fibre swelling and acid-catalysed hydrolysis, often reaching 50% strength loss within 14–21 days. Terminal products include guided tissue regeneration membranes for periodontal defects, dural substitutes, and wound-contact layers where the scaffold resorbs after new tissue formation. Biocompatibility is confirmed by ISO 10993-5:2009 for cytotoxicity, ISO 10993-10:2010 for sensitization and irritation, and ISO 10993-6:2016 for local effects after implantation.

Solvent-Cast Barrier Films and Low-Temperature Drying Constraints

Solvent casting of acid-terminated 50:50 DL-PLG into thin barrier films and drug-loaded patches is a lower-shear alternative to melt processing for heat-sensitive peptides and nucleic acids. The polymer is dissolved in dichloromethane at 5–10% w/v or in ethyl acetate at 3–7% w/v, cast onto a release liner using a knife coater with a wet thickness of 100–500 µm, and dried at 25–35 °C for 12–24 h under a solvent recovery hood. Because the acid-terminated copolymer plasticizes in the presence of residual solvent, films dried too rapidly develop skinning and internal bubbles that become defects in a drug-eluting coating; a staged drying profile with a low initial dew point below −20 °C reduces moisture uptake and yields optically clear films. Residual solvent is tested by headspace gas chromatography under USP <467>, and films intended for mucosal or subdermal use require residual dichloromethane below the limit set in ICH Q3C(R8), compared with an ethyl acetate limit of 5,000 ppm. Tensile properties of solvent-cast films are measured with ASTM D882-18, and the acid-terminated grade usually exhibits tensile strengths of 2–10 MPa with elongations of 20–150% depending on plasticizer and residual solvent content; plasticizers such as triethyl citrate or polyethylene glycol 400 are incorporated at 5–15 wt% to improve handling flexibility but increase water uptake and accelerate hydrolysis. Terminal products include adhesion barriers, drug-eluting coatings on mesh, and transdermal or implantable delivery patches for proteins; these films are usually sterilized by ethylene oxide because their high surface area and low thickness make gamma irradiation more damaging to molecular weight. In vitro degradation is tracked under ASTM F1635-16, and the acid-terminal group contributes to autocatalytic degradation at later stages, with internal pH dropping more rapidly in thick films than in porous electrospun mats.