RESOMER R 207 S is a bioresorbable poly(D,L-lactide) medical grade supplied as a white to off-white amorphous powder or pelletized resin with a nominal inherent viscosity of 1.7 dL/g measured at a concentration of 0.1% w/v in chloroform at 25 °C in accordance with DIN EN ISO 1628-1. The polymer chain is ester-terminated; this reduces the concentration of carboxylic acid end groups that would otherwise catalyze bulk hydrolysis during water uptake. The poly(D,L-lactide) backbone is random in D and L monomer distribution, and therefore no crystalline melting endotherm is observed in differential scanning calorimetry under ISO 11357-2; the glass transition temperature is approximately 55 °C. The product is manufactured under a quality management system certified to ISO 13485:2016, and certificates of analysis typically report residual monomer content ≤0.5 wt%, residual solvents ≤0.1 wt%, and heavy metals ≤10 ppm. R 207 S is intended as a feedstock for finished medical devices and parenteral drug-delivery systems; it is not a finished implant, and biological evaluation of the final sterilized configuration is required under ISO 10993-1:2018.

What differentiates ester-terminated R 207 S from acid-terminated PDLLA and from semicrystalline PLLA during hydrolytic degradation?

Ester termination of the chain ends reduces the initial concentration of carboxylic acid species available to catalyze ester hydrolysis. In bulk-eroding poly(D,L-lactide), water absorption into the amorphous matrix follows Fickian uptake; once the local water concentration reaches a critical value, hydrolysis of ester linkages dominates and produces new acid end groups. Acid-terminated grades such as R 207 H accelerate this autocatalysis more rapidly, which shortens the molecular weight decay phase and can shift the onset of mass loss earlier. For R 207 S, the delay in autocatalysis is most pronounced in constructs thicker than 1 mm, where internal oligomers diffuse slowly and the core pH declines. In particles below 50 µm, diffusion distances to the surrounding buffer are short, and the difference between ester-terminated and acid-terminated grades narrows.

Compared with semicrystalline PLLA, R 207 S lacks crystalline lamellae that restrict water ingress and resist hydrolytic attack; therefore, high-molar-mass PLLA typically requires longer resorption periods, whereas R 207 S degrades over shorter timescales but with lower initial modulus. Compared with PLGA 50:50, the absence of glycolide in R 207 S slows chain scission and avoids glycolic acid production; this reduces the magnitude of the initial local pH drop and can be advantageous for acid-labile peptides, although the exact release profile is governed by particle size, drug loading, and porosity.

Standardized in vitro degradation screening may be performed under ASTM F1635-16 in phosphate-buffered saline at 37 °C and pH 7.4; testing should record molar mass by size-exclusion chromatography with multi-angle light scattering, mass loss, water uptake, and pH. Published data for this specific configuration is limited where exact in vivo resorption times are concerned; a single numeric half-life cannot be assigned without specifying geometry, sterilization dose, and implantation site.

Solvent-based microsphere manufacturing with R 207 S typically uses dichloromethane at polymer concentrations of 10–20 wt% emulsified into a continuous phase of 1% w/v poly(vinyl alcohol) and 0.1% w/v sodium chloride in water. A high-shear rotor–stator mixer operated at 10,000 rpm produces an oil-in-water emulsion; the solvent is removed under reduced pressure at 40 °C, and the hardened microspheres are washed with water for injection and lyophilized at −40 °C and 0.1 mbar for 24 h. Residual dichloromethane in the final dosage form is controlled to less than 600 ppm under ICH Q3C Option 1 limits for Class 2 solvents. For melt processing, a twin-screw extruder with a 25:1 L/D ratio and zone temperatures from 140 °C to 180 °C is used; die pressure is commonly in the range of 30–80 bar. Pre-drying at 40 °C under vacuum to less than 0.1 wt% moisture is required before extrusion, and residence time above 5 min should be minimized to limit hydrolytic chain scission. Injection molding of R 207 S should avoid melt temperatures above 200 °C; at higher temperatures, depolymerization to lactide accelerates, and the resulting monomer plasticizes the moulded part and reduces Tg.

Basic additives, primary or secondary amines, and alkaline buffers above pH 10 are incompatible with R 207 S because nucleophilic aminolysis or base-catalyzed ester hydrolysis degrades the polymer during compounding, sterilization, or storage.

Gamma Sterilization Dose Response and Molecular Weight Decay

Terminal sterilization by gamma irradiation at 25 kGy, a common dose selected under ISO 11137-2, induces chain scission in amorphous PDLLA. The resulting reduction in inherent viscosity depends on irradiation temperature, dose rate, initial moisture content, and the presence of antioxidants; reported values range from 10% to 30% loss at 25 kGy for high-molar-mass poly(D,L-lactide). Because the Tg of R 207 S is near 55 °C, ambient irradiation occurs below Tg, but local adiabatic heating can raise the core temperature if dose delivery is rapid. Electron-beam sterilization at 25 kGy can produce comparable chain scission with shorter exposure time; however, oxidation at the polymer surface may be enhanced. Ethylene oxide sterilization is feasible when residual ethylene oxide and ethylene chlorohydrin are reduced below the limits specified in ISO 10993-7:2008; outgassing at 30–40 °C under vacuum is typically required. Moist heat sterilization is generally unsuitable because steam at 121 °C combined with water uptake accelerates hydrolysis and deforms amorphous devices above Tg.

R 207 S has been evaluated for long-acting injectable microspheres where release windows from 30 days to 90 days are required. The release profile from amorphous poly(D,L-lactide) microspheres is not controlled solely by surface erosion; instead, water uptake plastizes the glassy matrix, hydrolysis creates oligomers, and aqueous pores form after oligomer extraction. Initial release of drug near the particle surface occurs within 24 h, a lag phase follows, and a final erosion phase releases the remaining drug. In R 207 S, the ester-terminated structure delays the erosion phase relative to R 207 H when particle size, drug loading, and buffer conditions are matched. Protein and peptide formulations often require a solid-in-oil dispersion or a water-in-oil-in-water double emulsion to preserve native structure; trehalose, mannitol, and poloxamer 188 have been used as stabilizers in published studies. The polymer is soluble in dichloromethane, chloroform, tetrahydrofuran, and ethyl acetate; it is insoluble in water, methanol, ethanol, and aliphatic hydrocarbons. Water uptake at 37 °C reaches several weight percent within days and lowers the effective Tg toward body temperature, accelerating hydrolysis.

When Monomer and Residual Solvent Limits Influence Implant Design

Because R 207 S has a high melt viscosity, thin-wall micro-moulded features below 0.5 mm may require injection pressures above 800 bar unless processing temperature is raised toward 180 °C; this pressure limit interacts with the shear sensitivity of the polymer and the need to avoid degradation above 200 °C. In solvent-cast films, residual lactide monomer acts as a plasticizer and reduces the glass transition temperature; the specification limit of ≤0.5 wt% residual monomer is therefore relevant for dimensional stability above 25 °C. Residual dichloromethane must meet ICH Q3C limits, and chloroform is generally avoided in final implants due to toxicological constraints. Published data for this specific configuration is limited for long-term human implantation, so lot-to-lot consistency should be confirmed by size-exclusion chromatography with light scattering before scale-up, and degradation screening should be repeated after any change in sterilization dose or residual solvent profile.

Quality documentation for R 207 S is maintained under ISO 13485:2016, with product release testing for appearance, inherent viscosity, residual monomer, residual solvents, water content, and heavy metals. The grade is not supplied sterile; terminal sterilization must be selected and validated for the finished device. Handling and storage require exclusion of atmospheric moisture: unopened containers should be stored at −20 °C in a dry argon or nitrogen atmosphere, and after opening, the material should be equilibrated to ambient temperature before use to avoid condensation. Once processed, the polymer should be protected from humidity above 60% RH, because amorphous PDLLA absorbs water and may undergo premature molecular weight loss during storage.