If you have been reading about Porous cake 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 2025-09-10. Where a claim depends on a specific study, the study is described rather than over-claimed.
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.
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.
Freeze-drying is distinct from simple evaporation and from spray drying. Evaporation removes water at temperatures above freezing, while spray drying rapidly dries droplets in a heated gas stream. Lyophilization avoids high temperatures, which can be useful for heat-sensitive materials such as proteins, vaccines, and some foods. The porous cake produced by sublimation dissolves or rehydrates more quickly than a dense dried mass. Not all materials tolerate freezing or the pH shifts that can occur as solutes concentrate during ice formation.
Lyophilization removes water from a frozen material by sublimation under reduced pressure. The process begins with freezing, which converts liquid water into ice and fixes the structure of the sample. After freezing, primary drying lowers pressure so ice changes directly to vapor without passing through a liquid phase. Secondary drying then removes bound water that remains after ice sublimation. The result is a dry, porous solid that often retains its original shape.
| Property | Value | Notes |
|---|---|---|
| Common name | Freeze-drying | Process removes water by sublimation under vacuum. |
| Typical primary drying shelf temperature | -40 C to -10 C | Set below the formulation's collapse temperature. |
| Typical chamber pressure | 0.05-0.3 mbar | Low pressure allows ice to sublime below its triple point. |
| Water content after drying | 0.5-3% by weight | Higher values may reduce storage stability for some materials. |
| Key thermal parameter | Collapse temperature | Measured by freeze-drying microscopy or differential scanning calorimetry. |
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.
Lyophilization is a dehydration process that removes water from a frozen material by sublimation under low pressure. The material is first frozen to convert liquid water into ice. Next, the pressure is reduced below the triple point of water so that ice changes directly into vapor without passing through a liquid phase. This step is called primary drying. The result is a porous solid or cake that retains the original shape of the frozen solution.
After primary drying, secondary drying removes water that remains bound to the material. This stage raises the shelf temperature while maintaining low pressure, which encourages desorption of unfrozen water. Residual moisture can be reduced to a low percentage, improving stability for many products. The process parameters, including freezing rate, shelf temperature, and chamber pressure, influence the final pore structure and reconstitution behavior. Control of these variables helps prevent collapse or meltback during drying.
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.
Secondary drying removes bound water that remains after ice sublimation. Shelf temperature is raised gradually while pressure remains low, reducing water content to a target range. Over-drying can cause brittleness or electrostatic issues, while under-drying affects stability. The endpoint is often judged by pressure rise tests, temperature measurements, or water content analysis. Scale-up depends on matching heat and mass transfer across equipment sizes. Small changes in shelf temperature or pressure can alter cycle length substantially.
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.
Lyophilization, also called freeze-drying, is a dehydration process in which a solvent, usually water, is frozen and then removed by sublimation under reduced pressure. The method preserves heat-sensitive materials that would degrade in conventional drying. Large-scale use grew during the mid-twentieth century for blood plasma and antibiotics, and it later expanded to vaccines, enzymes, foods, and advanced materials. The process produces a dry, porous solid that usually reconstitutes rapidly. It is distinct from simple evaporation because the solvent bypasses the liquid phase during primary removal.
The process generally proceeds in three stages: freezing, primary drying, and secondary drying. During freezing, controlled cooling converts water into ice and may also crystallize or vitrify solutes. In primary drying, the pressure is lowered below the triple point, and heat is supplied so ice sublimes directly to vapor. Secondary drying removes water that remains bound to the solid matrix, yielding a low final water content. Product temperature must stay below the collapse or glass transition temperature to maintain structure. Cycle design therefore balances shelf temperature, chamber pressure, and time.
Freeze-drying is used for materials whose activity or structure depends on low temperature and low water content. Examples include certain biologics, diagnostic reagents, starter cultures, coffee, and porous inorganic precursors. The dried product forms a cake whose porosity aids rapid wetting and dissolution. Main drawbacks are high energy use, long cycle times, and sensitivity to formulation and equipment variation. Questions remain about how freezing rates and ice morphology affect batch uniformity, especially when moving from laboratory to production scale.
==== Expiration date ==== The expiration date, required in several countries, specifies the date up to which the manufacturer guarantees the full potency and safety of a drug. In the United States, expiration dates are determined by regulations established by the FDA. The FDA advises consumers not to use products after their expiration dates. A study conducted by the U.S. Food and Drug Administration covered over 100 drugs, prescription and over-the-counter. The results showed that about 90% of them were safe and effective far past their original expiration date. At least one drug worked 15 years after its expiration date. Joel Davis, a former FDA expiration-date compliance chief, said that with a handful of exceptions—notably nitroglycerin, insulin, and some liquid antibiotics (outdated tetracyclines can cause Fanconi syndrome)—most expired drugs are probably effective. The American Medical Association issued a report and statement on Pharmaceutical Expiration Dates. The Harvard Medical School Family Health Guide notes that, with rare exceptions, "it's true the effectiveness of a drug may decrease over time, but much of the original potency still remains even a decade after the expiration date". The expiration date is the final day that the manufacturer guarantees the full potency and safety of a medication. Drug expiration dates exist on most medication labels, including prescription, over-the-counter and dietary supplements. U.S. pharmaceutical manufacturers are required by law to place expiration dates on prescription products prior to marketing.
== Conformation == Piperidine prefers a chair conformation, similar to cyclohexane. Unlike cyclohexane, piperidine has two distinguishable chair conformations: one with the N–H bond in an axial position, and the other in an equatorial position. After much controversy during the 1950s–1970s, the equatorial conformation was found to be more stable by 0.72 kcal/mol in the gas phase. In nonpolar solvents, a range between 0.2 and 0.6 kcal/mol has been estimated, but in polar solvents the axial conformer may be more stable. The two conformers interconvert rapidly through nitrogen inversion; the free energy activation barrier for this process, estimated at 6.1 kcal/mol, is substantially lower than the 10.4 kcal/mol for ring inversion. In the case of N-methylpiperidine, the equatorial conformation is preferred by 3.16 kcal/mol, which is much larger than the preference in methylcyclohexane, 1.74 kcal/mol.
Cremasco, V., Woodruff, M. C., Onder, L., Cupovic, J., Nieves-Bonilla, J. M., Schildberg, F. A., Chang, J., Harvey, C. J., Wucherpfennig, B., Ludewig, B., Carroll, M. C., & Turley, S. J. (2014). B cell homeostasis and follicle confines are governed by fibroblastic reticular cells. Nature Immunology, 15(8), 973–981. https://doi.org/10.1038/ni.2965 Li, L., Lu, Y., Zhou, L., Shi, S., Zhu, X., & Zhang, L. (2021). Lymph node fibroblastic reticular cells steer immune responses. Trends in Immunology. https://doi.org/10.1016/j.it.2021.06.002 Link, A., Vogt, T. K., Favre, S., Britschgi, M. R., Acha-Orbea, H., Hinz, B., Cyster, J. G., & Luther, S. A. (2007). Fibroblastic reticular cells in lymph nodes regulate the homeostasis of naïve T cells. Nature Immunology, 8(11), 1255–1265. https://doi.org/10.1038/ni1513 Lütge, M., Pikor, N. B., & Ludewig, B. (2021). Differentiation and activation of fibroblastic reticular cells. Immunological Reviews, 302(1), 32–46. https://doi.org/10.1111/imr.12981 Mueller, S. N., & Germain, R. N. (2015). Stromal cell contributions to the homeostasis and functionality of the immune system. Nature Reviews Immunology, 15(12), 729–740. https://doi.org/10.1038/nri3846 Onder, L., Papadopoulou, C., Lütge, A., Cheng, H.-W., Lütge, M., Perez-Shibayama, C., Gil-Cruz, C., De Martin, A., Kurz, L., Cadosch, N., Pikor, N. B., Rodriguez, R., Born, D., Jochum, W., Leskow, P., Dutly, A., Robinson, M. D., & Ludewig, B. (2025). Fibroblastic reticular cells generate protective intratumoral T cell environments in lung cancer. Cell, 188(2), 430–446.
That same year, Hafezi was part of a research group that identified the visual cycle enzyme RPE65 as essential component of light-induced retinal degeneration. The article was published in Nature Genetics. In 2001, his work investigating the molecular pathways that underpin light-induced retinal photoreceptor apoptosis continued with a paper published in Cell Death & Differentiation showing that AP-1 mediated retinal photoreceptor apoptosis was independent of N-terminal phosphorylation of one of its components, c-Jun. Later that year, he and a team of researchers utilized a mouse strain with a single base change in codon 450 of the RPE65 gene (the Leu450Met variation) in which RPE65 regenerates rhodopsin at a far slower rate than wild-type animals; this mutation was observed to increase retinal resistance against light-induced degeneration, and demonstrating that the light damage susceptibility of the retina is tied to rhodopsin regeneration kinetics.
=== Synthetic scaffolds === The use of natural materials in scaffolds has its disadvantages. Usually, they are expensive, not available in large quantities and they have the risk of disease transmission. This has led to the development of synthetic scaffolds. When producing synthetic scaffolds there is full control over their properties. For example, they can be made to have good mechanical properties and the right biodegradability. When it comes to synthetic scaffolds thickness, porosity and pore size are important factors for controlling connective tissue formation. Examples of synthetic scaffolds are:
Sources: en.wikipedia.org
== Career == Baldwin began acting professionally as a teenager, landing the role of Ricky Linderman in the Chicago-set film My Bodyguard (1980). That same year he appeared in Robert Redford's Academy Award-winning drama Ordinary People as Kevin Stillman. In the early 1980s he continued with supporting roles in films including D.C. Cab (1983) and Reckless (1984). By the late 1980s he had moved into higher-profile studio productions, including Stanley Kubrick's Vietnam War film Full Metal Jacket (1987), in which he played the Marine nicknamed "Animal Mother," and the crime thriller Next of Kin (1989). In the 1990s he appeared in a run of mainstream features including Predator 2 (1990), the western biopic Wyatt Earp (1994) portraying Tom McLaury, and the science-fiction blockbuster Independence Day (1996). He later played Captain Wilkins in Roland Emmerich's Revolutionary War film The Patriot (2000). On television, Baldwin became a regular cast member of Joss Whedon's science-fiction series Firefly (2002–2003) as the mercenary Jayne Cobb, and reprised the role in the continuation film Serenity (2005). From 2007 to 2012 he played the National Security Agency agent John Casey on NBC's action-comedy series Chuck. His other television work includes a regular role on ABC's Day Break (2006–2007) and a lead role on the 1996–1997 drama series The Cape as Col. Jack Riles. He made a brief on-screen return to the schoolyard premise of My Bodyguard with a cameo in the comedy Drillbit Taylor (2008), noted as an explicit nod to his earlier film.
The Georgia Sustainment and Stability Operations Program (GSSOP) was a security assistance program designed to create an increased capability in the Georgian military to support Operation Iraqi Freedom stability missions. Launched in January 2005, GSSOP was also designed to help solidify the progress made during the Georgia Train and Equip Program (GTEP) of 2002–2004 and continue to assist in the implementation of western standards in the Georgian armed forces. The first phase of the program (GSSOP-I) lasted about 18 months and cost approximately $60 million. It ended in October 2006 to be succeeded by GSSOP-II, which lasted until June 2007. The training was conducted, primarily at the Krtsanisi National Training Centre near Tbilisi, by the United States Army Special Forces and United States Marine Corps Forces, Europe. The beneficiaries were the 22nd, 23rd, 31st, 32nd and 33rd Light Infantry Battalions, logistic battalions of the 1st, 2nd, and 3rd Infantry Brigades, the reconnaissance companies of the 2nd and 3rd Infantry Brigades, communication companies of the 2nd and 3rd Brigades, and an independent military police company. On August 31, 2009, the U.S. and Georgia inaugurated the Georgia Deployment Program—International Security Assistance Force (GDP—ISAF) In order to prepare the Georgian units for deployment in Afghanistan as part of the International Security Assistance Force. Originally planned as a two-year engagement, the success of past missions has extended the pairing as the Georgia Deployment Program—Resolute Support Mission (GDP—RSM) into 2020.
(2026) identify diagnostic Tasmanian devil specimens from the Juukan 2 Rock Shelter (Pilbara, Western Australia), providing direct evidence of presence of the species in the northwestern Australian mainland during the Late Pleistocene and Holocene, and study chewed and digested bones from the same site accumulated through human activity, interpreted as likely evidence of subsistence of the Tasmanian devils off human activity at the studied site. Watts et al. (2026) study the hindlimb morphology of Hadronomas puckridgi, and interpret its foot with a robust fifth digit as representing an intermediate stage in sthenurine evolution, preceding the appearance of astragalar specializations and loss of the fifth digit in later sthenurines. Evidence from the study of extant and extinct kangaroos, indicating that giant extinct kangaroos were mechanically capable of hopping but it may not have been their primary locomotor mode, is presented by Jones, Jones & Nudds (2026). Couzens, King & Prideaux (2026) report evidence from the study of the fossil record of kangaroos indicative of a progressive increase in tooth enamel thickness in grass-feeding taxa that coincided with expansion of arid habitats in Australia since the late Miocene. Koungoulos, Kerr & O'Connor (2026) describe a manual phalanx bone of a member of the genus Protemnodon from the Taora rockshelter (Papua New Guinea), providing evidence of local survival of members of this genus into the middle Holocene.
== Military career == After graduating from the Military Academy, al-Burhan worked in Khartoum, as part of the Sudanese army, and participated in the fighting fronts in the Darfur war and in the Second Sudanese Civil War in South Sudan and other regions. He was regional commander in Darfur. He later traveled to Egypt and then to Jordan to receive training courses in his military field until in 2018 he was appointed commander of the ground forces of the army. Al-Burhan held several positions throughout his career as he began as a soldier with the Border Guard Forces and later became commander of this force before becoming Deputy Chief of Staff of the Ground Forces Operations and then Chief of Staff of the Sudanese Army in February 2018 before he served as Inspector General of the Army for a period of time. By 26 February 2019, during the massive protests that swept the country and demanded the fall of Omar al-Bashir's regime, al-Burhan was elevated to the rank of lieutenant general.
Sources: en.wikipedia.org
Class A (or 1) (Rhodopsin-like) Class B (or 2) (Secretin receptor family) Class C (or 3) (Metabotropic glutamate/pheromone) Class D (or 4) (Fungal mating pheromone receptors) Class E (or 5) (Cyclic AMP receptors) Class F (or 6) (Frizzled/Smoothened) More recently, an alternative classification system called GRAFS (Glutamate, Rhodopsin, Adhesion, Frizzled/Taste2, Secretin) has been proposed for vertebrate GPCRs. They correspond to classical classes C, A, B2, F, and B. An early study based on available DNA sequence suggested that the human genome encodes roughly 750 G protein-coupled receptors, about 350 of which detect hormones, growth factors, and other endogenous ligands. Approximately 150 of the GPCRs found in the human genome have unknown functions. Some web-servers and bioinformatics prediction methods have been used for predicting the classification of GPCRs according to their amino acid sequence alone, by means of the pseudo amino acid composition approach. GPCRs are involved in a wide variety of physiological processes. Some examples of their physiological roles include:
These employ the same radioligands and have the same uses as SPECT scanning, but are able to provide even finer 3-D localization of high-uptake tissues, in cases where finer resolution is needed. An example is the sestamibi parathyroid scan which is performed using the 99mTc radioligand sestamibi, and can be done in either SPECT or SPECT/CT machines.
==== United Kingdom ==== Insulin, and all other medications, are supplied free of charge to people who use it to manage their diabetes by the National Health Services of the countries of the United Kingdom.
Sources: en.wikipedia.org
Primary drying removes ice by sublimation at low pressure and low shelf temperature. Secondary drying removes bound water by raising the shelf temperature, often under the same vacuum. The two stages differ in the water state being removed.
Freezing determines ice crystal size, pore structure, and the concentration of solutes in remaining liquid. Faster freezing generally creates smaller ice crystals and a denser dried matrix. These features affect drying rate and reconstitution behavior.
Lyophilization reduces water content but usually leaves a small amount of water in the dried material. Some water remains bound to solids or trapped in the dried matrix. Very low water targets can require extended secondary drying, which may alter product stability.
Lyophilization relies on sublimation, so water moves from solid ice to vapor without becoming liquid. The material is frozen, pressure is reduced, and controlled heat is supplied. Vapor is captured on a cold condenser, leaving a dry porous solid.