# Bioavailability of Liposomal Vitamin C in Powder Form: A Randomized, Double-Blind, Cross-Over Trial
## 3 Results and Discussion
### Properties of liposomal Vitamin C in Powder Form
The cryo-TEM technique was not used to measure the size of the resulting liposomal particles in the powder because of its limited applicability in the quantitative analysis of the liposome carrier population [51]. A prerequisite for obtaining a reliable measurement of the size of liposomes is the study of a representative population of these carriers [56]. For cryo-TEM microscopy, this is difficult to achieve because a very small volume of a diluted sample is taken for measurement (3 mL in this experiment). Thus, only a small proportion of the total liposomes were analyzed. In addition, making quantitative measurements by cryo-TEM can be hampered by, among other things, the variable thickness of the liposome aqueous solution layer inside a hole of a holey carbon film, which leads to separation (also known as sorting) of vesicles according to their size. This means that larger liposome particles tend to accumulate close to the edge of a hole, while small liposomes are located in the thinnest area of the solution layer [51; 56]. Figure 2: Liposomal vitamin C in powder form: surface morphology in the SEM. Magnifications of (**A**) 1000\(\times\) and (**B**) 3000\(\times\); (**C**) particle size distribution in volume. The specificity of the sample preparation procedures for analysis and of the image analysis itself translates into a lack of consistency between liposome particle size measurement results obtained using the cryo-TEM technique and those obtained using other measurement techniques, e.g., dynamic light scattering (DLS) [56]. Taking into account the information above, dedicated STEP-Technology(r) applied in the LUMiSizer(r) (L.U.M GmbH, Berlin, Germany) was used to determine the particle size distribution (PSD) of the liposomes. The mean size of the lipid carriers measured in this way was 262 \(\pm\) 49.8 nm (Table 2) and fell within the lower range of standard sizes (25-2500 nm) for this type of carrier [36]. Figure 3: Cryogenic transmission electron microscopy (cryo-TEM)-imaged morphology of liposomes in the rehydrated powder. A relevant differentiator of the imaging technique applied is the possibility to assess the layered structure of the obtained liposomes. The photos show large unilamellar vesicles (LUVs) (1) and multivesicular vesicles (MVVs) (2). The latter contain non-concentrically arranged smaller internal vesicles entrapped inside a large carrier [52]. This is advantageous because any reduction in the particle size of liposomes results in an improved ratio of surface area to volume, and this in turn translates directly into increased bioavailability [57]. The size of the carrier is also a key parameter determining its half-life in the bloodstream: smaller liposomes generally last longer in the bloodstream than large liposomes [36]. The particle size distribution of liposomes is an important parameter for assessing both the quality (homogeneity) of the product and its flow behavior and predicting its efficacy in vivo [58, 59]. The narrow width of the distribution of liposomes in the obtained powder (span = 0.500) (Figure 4A) and their relatively small diameter prove that the drying process did not lead to the fusion and disintegration of the carriers. In contrast, the high absolute value of the zeta potential (-41.44 \(\pm\) 0.30 mV, that is \(>\)30 mV) obtained for the hydrated powder (Figure 4B) demonstrates the low tendency of liposomes to aggregate or flocculate, and thus the high stability of the resulting colloidal suspension [42, 58]. In other words, the process of spray drying used to transform liquid suspensions of liposomes into powder form did not adversely affect the quality of the carrier particles obtained. Comparison of Bioravailability of Non-Liposomal and Liposomal Ascorbic Acid in an In Vitro Cell Model  
Bioavailability is the basic pharmacokinetic term that defines the amount of an ingested dose of an active substance that reaches the bloodstream unchanged.