If you have been reading about Secondary drying and want a single page that covers the useful parts, this is it: definitions, context, how it is studied, and the questions that come up repeatedly.
Last reviewed on 2025-08-31. Where a claim depends on a specific study, the study is described rather than over-claimed.
Lyophilization, or freeze-drying, removes water from a material by freezing it and then lowering pressure so ice changes directly to vapor. The process relies on sublimation, the phase transition from solid to gas without an intervening liquid state. It is used for heat-sensitive materials that would degrade in conventional drying. The three stages are freezing, primary drying, and secondary drying, each with distinct temperature and pressure requirements. In practice, cycle design balances these variables.
Freezing determines ice crystal structure and pore size, which affect drying speed and product uniformity. Rapid freezing creates small crystals, while slow freezing creates larger crystals and often faster sublimation. During primary drying, chamber pressure is held below the vapor pressure of ice, and shelf temperature supplies heat for sublimation. The ice front recedes, leaving a porous matrix. Thermal limits such as collapse and eutectic temperatures set safe boundaries for formulation. These limits vary with solute composition and concentration.
A typical cycle begins with freezing, sometimes including an annealing step to control ice crystal size. Freezing conditions influence the pore network that later allows vapor escape. During primary drying, shelf temperature and chamber pressure are set so heat enters the product while its temperature stays below the collapse or eutectic point. Secondary drying then raises the shelf temperature to desorb bound water and lower residual moisture. Cycle design depends on formulation, fill volume, container type, and equipment capability.
The physics of lyophilization couples heat transfer, mass transfer, and phase behavior. Sublimation requires a vapor pressure difference between the ice front and the chamber, and the dried layer adds resistance to vapor flow. Amorphous formulations are characterized by a glass transition temperature of the maximally freeze-concentrated solute, often denoted Tg'. Crystalline bulking agents can provide structure, while amorphous excipients stabilize labile components. Open questions remain about spatial heterogeneity, edge effects, and how laboratory cycles scale to production.
| Property | Value | Notes |
|---|---|---|
| Process name | Lyophilization or freeze-drying | Both terms appear in technical standards and literature. |
| Phase transition | Sublimation | Solid ice becomes vapor without a liquid step. |
| Typical chamber pressure | 0.05-0.5 mbar | Range depends on product temperature and equipment. |
| Typical product temperature | -40 °C to -10 °C | Measured during primary drying; formulation sets limits. |
| Water content after drying | 0.5-3% w/w | Target varies by material and stability needs. |
The physics of freeze-drying couples heat transfer, mass transfer, and phase change. Heat supplied through the shelf must reach the sublimation front without melting the ice or degrading the product. Water vapor then travels through the already dried layer and leaves the chamber, where low pressure and cold traps keep it from returning. The dried layer acts as a resistance to vapor flow, so drying rate changes as the front recedes. Open questions remain about how pore structure and formulation heterogeneity affect drying uniformity at larger scales.
Lyophilization removes water from a frozen material by sublimation under reduced pressure. The process begins with freezing, which converts liquid water into ice and concentrates dissolved solids. Primary drying then lowers chamber pressure so ice changes directly into vapor without passing through a liquid phase. Secondary drying raises the shelf temperature to remove bound water that remains after ice sublimation. The result is a dry, porous structure that can be reconstituted later.
Formulation composition influences whether freeze-drying produces an intact cake or a collapsed mass. Excipients such as sugars and polymers can raise the collapse temperature and provide bulk during drying. The critical temperature for primary drying is often the collapse temperature or the glass transition temperature of the maximally concentrated phase. If the product temperature exceeds this threshold, the frozen matrix may soften and lose structure. Established practice therefore links shelf temperature and chamber pressure to the formulation's thermal properties.
A freeze-dryer consists of a vacuum chamber, temperature-controlled shelves, a condenser, and a vacuum pump. Vials, ampoules, or bulk trays hold the product during the cycle. The condenser traps water vapor as ice at a temperature lower than the product. Cycle development balances shelf temperature, chamber pressure, and time. Scale-up can be difficult because heat and mass transfer change with equipment size, so process analytical tools and conservative validation are often used.
Lyophilization is a dehydration technique in which a product is frozen and the solvent is removed under reduced pressure. The low pressure allows ice to sublimate directly into vapor without passing through a bulk liquid phase. This differs from conventional drying, where heat drives evaporation and can damage heat-sensitive structures. The process is used for biological materials, pharmaceutical formulations, and some foods. Its main advantage is preservation of porous structure and rapid reconstitution.
=== Backbone modifications === Nucleoside organothiophosphate (PS) analogs of nucleotides give oligonucleotides some beneficial properties. Key beneficial properties that PS backbones give nucleotides are diastereomer identification of each nucleotide and the ability to easily follow reactions involving the phosphorothioate nucleotides, which is useful in oligonucleotide synthesis. PS backbone modifications to oligonucleotides protects them against unwanted degradation by enzymes. Modifying the nucleotide backbone is widely used because it can be achieved with relative ease and accuracy on most nucleotides. Fluorescent modifications on 5' and 3' end of oligonucleotides was reported to evaluate the oligonucleotides structures, dynamics and interactions with respect to environment.
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== Structure == Like MHC class I molecules, class II molecules are also heterodimers, but in this case consist of two homogenous peptides, an α and β chain, both of which are encoded in the MHC. The subdesignation α1, α2, etc. refers to separate domains within the HLA gene; each domain is usually encoded by a different exon within the gene, and some genes have further domains that encode leader sequences, transmembrane sequences, etc. These molecules have both extracellular regions as well as a transmembrane sequence and a cytoplasmic tail. The α1 and β1 regions of the chains come together to make a membrane-distal peptide-binding domain, while the α2 and β2 regions, the remaining extracellular parts of the chains, form a membrane-proximal immunoglobulin-like domain. The antigen binding groove, where the antigen or peptide binds, is made up of two α-helixes walls and β-sheet. Because the antigen-binding groove of MHC class II molecules is open at both ends while the corresponding groove on class I molecules is closed at each end, the antigens presented by MHC class II molecules are longer, generally between 15 and 24 amino acid residues long.
An osteocyte, an oblate-shaped type of bone cell with dendritic processes, is the most commonly found cell in mature bone. It can live as long as the organism itself. The adult human body has about 42 billion of them. Osteocytes do not divide and have an average half life of 25 years. They are derived from osteoprogenitor cells, some of which differentiate into active osteoblasts (which may further differentiate to osteocytes). Osteoblasts/osteocytes develop in mesenchyme. In mature bones, osteocytes and their processes reside inside spaces called lacunae (Latin for a pit) and canaliculi, respectively. Osteocytes are simply osteoblasts trapped in the matrix that they secrete. They are networked to each other via long cytoplasmic extensions that occupy tiny canals called canaliculi, which are used for exchange of nutrients and waste through gap junctions. Although osteocytes have reduced synthetic activity and (like osteoblasts) are not capable of mitotic division, they are actively involved in the routine turnover of bony matrix, through various mechanosensory mechanisms. They destroy bone through a rapid, transient (relative to osteoclasts) mechanism called osteocytic osteolysis. Hydroxyapatite, calcium carbonate and calcium phosphate is deposited around the cell.
Sources: en.wikipedia.org
Prospects of Duterte running for president in the 2016 elections began in February 2015, during which Duterte, then outgoing Mayor of Davao City, re-entered the PDP-Laban party, who courted him to run for president. A month later, Duterte emerged as a preferred presidential candidate in a Pulse Asia poll; tied with Manila mayor and former president Joseph Estrada, Duterte ranked third behind Senator Grace Poe and frontrunner, Vice President Jejomar Binay. In the following months leading to October, however, Duterte repeatedly declined his supporters' clamors for him to run as president because of what he described as a "flawed government system", as well as lack of funds and opposition from his family members—notably, from his daughter Sara. Additionally, Duterte said he will retire from politics after his mayorship ends, but only if Sara runs as mayor. PDP-Laban standard bearer Martin Diño unexpectedly withdrew his candidacy on October 29 and named Duterte as his substitute. A month later, Duterte formally announced his presidential bid and accepted Alan Peter Cayetano's offer to be his running mate; Duterte also named Sara as his substitute for Mayor of Davao City.
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=== Birth control === A progesterone vaginal ring is available for birth control when breastfeeding in a number of areas of the world. An intrauterine device containing progesterone has also been marketed under the brand name Progestasert for birth control, including previously in the United States.
=== Second World War === The Special Air Service was a unit of the British Army during the Second World War that was formed in July 1941 by David Stirling and originally called "L" Detachment, Special Air Service Brigade – the "L" designation and Air Service name being a tie-in to a British disinformation campaign, trying to deceive the Axis into thinking there was a paratrooper regiment with numerous units operating in the area (the real SAS would "prove" to the Axis that the fake one existed). It was conceived as a commando force to operate behind enemy lines in the North African Campaign and initially consisted of five officers and 60 other ranks. Its first mission, in November 1941, was a parachute drop in support of the Operation Crusader offensive, codenamed Operation Squatter. Due to German resistance and adverse weather conditions, the mission was a disaster, with only 22 men, a third of the unit, making it back to base. The rest were either killed or captured. Its second mission was a major success. Transported by the Long Range Desert Group, it attacked three airfields in Libya, destroying 60 aircraft without loss. In September 1942, it was renamed 1st SAS, consisting at that time of four British squadrons, one Free French, one Greek, and the Folboat Section.
==== Fibrous astrocytes ==== The fibrous astrocytes are usually located within white matter, have relatively few organelles, and exhibit long unbranched cellular processes. This type often has astrocyte endfeet processes that physically connect the cells to the outside of capillary walls when they are in proximity to them.
Sources: en.wikipedia.org
== History == Acetone peroxide (specifically, triacetone triperoxide) was discovered in 1895 by a German chemist, Richard Wolffenstein. Wolffenstein combined acetone and hydrogen peroxide. Then he allowed the mixture to stand for a week at room temperature, during which time a small quantity of crystals precipitated, which had a melting point of 97 °C (207 °F). In 1899, Adolf von Baeyer and Victor Villiger described the first synthesis of the dimer and described use of acids for the synthesis of both peroxides. Baeyer and Villiger prepared the dimer by combining potassium persulfate in diethyl ether with acetone, under cooling. After separating the ether layer, the product was purified and found to melt at 132–133 °C (270–271 °F). They found that the trimer could be prepared by adding hydrochloric acid to a chilled mixture of acetone and hydrogen peroxide. By using the depression of freezing points to determine the molecular weights of the compounds, they also determined that the form of acetone peroxide that they had prepared via potassium persulfate was a dimer, whereas the acetone peroxide that had been prepared via hydrochloric acid was a trimer, like Wolffenstein's compound. Work on this methodology and on the various products obtained, was further investigated in the mid-20th century by Milas and Golubović.
After an initial or primary fermentation, beer is conditioned, matured or aged, in one of several ways, which can take from 2 to 4 weeks, several months, or several years, depending on the brewer's intention for the beer. The beer is usually transferred into a second container, so that it is no longer exposed to the dead yeast and other debris (also known as "trub") that have settled to the bottom of the primary fermenter. This prevents the formation of unwanted flavours and harmful compounds such as acetaldehyde.
In the Romanian general election of 1946, the Romanian Communist Party (PCR) employed widespread intimidation tactics and electoral fraud to obtain 80 percent of the vote and, thereafter, eliminated the role of the centrist parties and forced mergers, the result of which was that, by 1948, most non-Communist politicians were either executed, in exile or in prison. In the December 1945 Albanian election, the only effective ballot choices were those of the communist Democratic Front (Albania), led by Enver Hoxha. In 1946, Albania was declared the People's Republic of Albania. The defining characteristic of the Stalinist communism implemented in Eastern Bloc states was the unique symbiosis of the state with society and the economy, resulting in politics and economics losing their distinctive features as autonomous and distinguishable spheres. Initially, Stalin directed systems that rejected Western institutional characteristics of market economies, democratic governance (dubbed "bourgeois democracy" in Soviet parlance) and the rule of law subduing discretional intervention by the state. They were economically communist and depended upon the Soviet Union for significant amounts of materials. While in the first five years following World War II, massive emigration from these states to the West occurred, restrictions implemented thereafter stopped most East-West migration, except that under limited bilateral and other agreements.
=== Society and culture === The use of sovereign power, codes of conduct, religious and cultural practices and other dynamic processes in a society can be described as a part of an evolved homeostatic system of regularizing life and maintaining an overall equilibrium that protects the security of the whole from internal and external imbalances or dangers. Healthy civic cultures can be said to have achieved an optimal homeostatic balance between multiple contradictory concerns such as in the tension between respect for individual rights and concern for the public good, or that between governmental effectiveness and responsiveness to the interests of citizens.
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Sources: en.wikipedia.org
Conventional drying uses heat to evaporate liquid water, often at temperatures that can degrade sensitive materials. Lyophilization freezes the material first and then removes water by sublimation under vacuum. This avoids prolonged exposure to liquid water and high heat.
Lowering pressure reduces the boiling point of water and allows ice to sublimate at temperatures below freezing. Vacuum also limits convective heat transfer, so heat is usually supplied by shelves or radiation. The pressure must stay below the vapor pressure of ice at the product temperature.
The cycle typically includes freezing, primary drying, and secondary drying. Freezing solidifies water and sets the pore structure; primary drying removes bulk ice; secondary drying removes bound water. Some cycles add annealing or pre-freezing steps.
Freezing only converts liquid to solid. Lyophilization adds vacuum and controlled warming so frozen solvent sublimes, leaving a dry porous solid. The two steps are related but not interchangeable.