Nowadays, drug delivery has an important role in medical therapy. The use of biopolymers in developing drug delivery systems (DDSs) is increasingly attracting attention due to their remarkable and numerous advantages, in contrast to conventional polymers. Biopolymers have many advantages (biodegradability, biocompatibility, renewability, affordability, and availability), which are extremely important for developing materials with applications in the biomedical field. Additionally, biopolymers are appropriate when they improve functioning and have a number of positive effects on human life. Therefore, this review presents the most used biopolymers for biomedical applications, especially in drug delivery. In addition, by combining different biopolymers DDSs with tailored functional properties (e.g., physical properties, biodegradability) can be developed. This review summarizes and provides data on the progress of research on biopolymers (chitosan, alginate, starch, cellulose, albumin, silk fibroin, collagen, and gelatin) used in DDSs, their preparation, and mechanism of action.

R₂BiOH (1) [R = 2-(Me₂NCH₂)C₆H₄] and (R₂Bi)₂O (2) are formed by hydrolysis of R₂BiCl with KOH. Single crystals of 2 were obtained by air oxidation of (R₂Bi)₂. The reaction of R₂BiCl and Na₂CO₃ leads to (R₂Bi)₂CO₃ (3). 3 is also formed by the absorption of CO₂ from the air in solutions of 1 or 2 in diethyl ether or toluene. (R₂Bi)₂S (4) is obtained from R₂BiCl and Na₂S or from (R₂Bi)₂ and S₈. Exchange reactions between R₂BiCl and KBr or NaI give R₂BiX [X = Br (5), I (6)]. The reaction of RBiCl₂ (7) with Na₂S and [W(CO)₅(THF)] gives cyclo-(RBiS)₂[W(CO)₅]₂ (8). cyclo-(R′BiS)₂ (9) [R′ = 2,6-(Me₂NCH₂)₂C₆H₃] is formed by reaction of R′BiCl₂ and Na₂S. The structures of 1–9 were determined by single-crystal X-ray diffraction.

The reaction of RBiCl₂ (1) [R = 2,6-(Me₂NCH₂)₂C₆H₃] with Na₂CO₃ or Ag₂SO₄ (1 : 1 molar ratio) gave RBiCO₃ (2) and RBiSO₄ (3), respectively. RBi(NO₃)₂ (4) was obtained from RBiCl₂ and AgNO₃ (1 : 2 molar ratio). The ionic complex [R₂Bi][W(CO)₅Cl] (6) was obtained from R₂BiCl (5) and W(CO)₅(THF), following an unusual chlorine transfer from bismuth to tungsten. Compounds 2–4 are partially soluble in water. The molecular structures of 2·0.5CH₂Cl₂, 3, 4·H₂O and 6 were established by single-crystal X-ray diffraction. The carbonate 2 and the sulfate 3 exhibit a polymeric structure based on bridging oxo anions, while for the compound 4 dimer associations are formed, with both bridging and terminal nitrate anions. Dimer associations, based on weak Cl⋯H interactions between the cation and the anion, were found in the crystal of 6.

The reaction of RLi [R = 2,6-{MeN(CH₂CH₂)₂NCH₂}₂C₆H₃] with BiCl₃ (1:1 molar ratio) afforded RBiCl₂ (2). RBiBr₂ (3) and RBiI₂ (4) were obtained by halogen-exchange reactions from 2. The organobismuth(III) dihalides containing the new steric demanding (N,C,N)-pincer ligand were characterized both in solution and solid state. The molecular structures of compounds 2–4 were established by single-crystal X-ray diffraction. They all show a T-shaped CBiX₂ core, stabilized by two strong intramolecular N→Bi interactions in trans positions to each other. The overall (N,C,N)BiX₂ core exhibits a distorted square-pyramidal coordination geometry. The NMR studies provided evidences for the presence of internal nitrogen-bismuth coordination in solution too. DFT calculations were performed on the related species [2,6-(R₂NCH₂)₂C₆H₃]BiX₂ (R = H, Me; X = Cl, Br, I) in order to elucidate the bond nature and vibrational spectroscopic features of these compounds.