Biological preservation is the specialized process of stabilizing organic material to prevent decomposition, allowing for long-term study in science, medicine, and education. Stabilization is achieved by halting microbial decay and chemical breakdown within the tissues. Effective preservation requires liquids that penetrate the specimen quickly to stop enzymatic activity and maintain structural integrity. The primary methods rely on a two-step approach: an initial liquid to “fix” the tissues, and a second liquid for long-term storage. This article explores the various liquids used for both initial tissue setting and indefinite preservation.
The Role of Formalin in Anatomical Fixation
The liquid most commonly used for the initial setting of large biological specimens is formalin, an aqueous solution of formaldehyde gas. Formaldehyde is typically dissolved in water to a concentration of about 37% (referred to as 100% formalin), which is then usually diluted to a 10% working solution for tissue fixation. This chemical acts as a fixative, chemically altering the proteins within the specimen to make them rigid and stable against degradation.
The mechanism involves a process called cross-linking, which is the formation of chemical bridges between protein molecules. Formaldehyde molecules readily react with specific sites on amino acids, most notably the nitrogen atoms found in lysine residues, to form methylene bridges. This extensive network of cross-links locks the proteins into position, effectively hardening the tissue and preventing the enzymes responsible for decay from functioning. While formaldehyde penetrates the tissue relatively quickly, the complete cross-linking process necessary for full fixation can take several days or longer, especially for very dense or large specimens.
This fixation process is particularly valued in histology, the study of tissue structure, and for preparing large dissection specimens used in anatomy labs and medical schools. The stabilization of internal structures achieved by formalin ensures that the specimen’s morphology remains close to its living state for examination. Once fixation is complete, specimens are often transferred to a different liquid for indefinite storage to minimize the ongoing exposure to the fixative.
Storage Solutions The Use of Ethanol and Isopropanol
After a specimen is fixed, it is commonly transferred to a different liquid for long-term preservation, with various alcohols being the preferred choice. Ethanol (ethyl alcohol) and isopropanol (isopropyl alcohol) are the two primary liquids used for this purpose, particularly in natural history collections. Unlike formalin, which chemically cross-links, alcohols preserve by a physical mechanism of dehydration and protein denaturation.
These alcohols work by drawing water out of the cells and causing the proteins within the tissue to unfold, or denature, which halts both enzymatic activity and microbial growth. Ethanol is the superior choice for long-term storage because it causes less tissue shrinkage and fewer undesirable changes compared to isopropanol. The standard concentration for long-term storage of specimens in ethanol is 70% by volume, which is considered the most effective concentration for its bactericidal properties.
Isopropanol is also used, often in concentrations ranging from 55% to 70%, but it is generally reserved for situations where ethanol is unavailable or cost-prohibitive. For smaller specimens, such as invertebrates or plants that do not require the deep penetration of a chemical fixative, alcohol can serve as both the fixative and the storage solution. To prevent excessive shrinkage, specimens are often gradually introduced to the final concentration through a series of increasing alcohol concentrations.
Safer Preservation Methods and Modern Alternatives
Due to the known health risks associated with formaldehyde, particularly its classification as a human carcinogen, there has been a significant movement toward developing safer alternatives, especially for educational settings. These modern preservation liquids are designed to stabilize tissue structures while minimizing user exposure to volatile and toxic chemicals. Many of these alternative solutions are proprietary formulas that aim to mimic the tissue-stabilizing effects of traditional fixatives.
A common approach involves using propylene glycol-based formulas, which are significantly less toxic than formalin and do not require the specialized ventilation of a fume hood for safe handling. Another group of alternatives includes fixatives based on glyoxal, which is a small dialdehyde that can also cross-link proteins but without the high volatility and carcinogenic properties of formaldehyde. These replacement chemicals prioritize the safety of students and technicians while still providing specimens that are durable for dissection and morphological study.
Other non-toxic methods have explored natural substances, with some studies showing that sugar-based syrups, such as jaggery syrup or even honey, can function as effective tissue fixatives. While these alternatives may not fully replicate the structural preservation quality of formalin for highly detailed research, they represent a safer choice for general classroom use and have significantly reduced chemical exposure in educational environments.
Safety Protocols for Handling Preservative Liquids
Handling any chemical preservative requires strict adherence to safety protocols to protect against exposure, whether the liquid is formalin, alcohol, or a modern alternative. Proper ventilation is a primary control measure, and concentrated formalin solutions should always be handled inside a certified chemical fume hood to keep vapor concentrations below the permissible exposure limits. Alcohols, while less toxic, present a flammability hazard and must be kept away from heat and ignition sources.
Personal protective equipment is non-negotiable when working with these liquids. This typically includes a fully buttoned laboratory coat, chemical splash goggles or a face shield, and appropriate gloves, such as medium-to-heavyweight nitrile gloves. Gloves should be checked for chemical compatibility and changed immediately if they become contaminated or torn.
Storage protocols require that all preservative liquids be kept in tightly closed, clearly labeled containers. Larger volumes should utilize secondary containment to catch any spills. Concentrated alcohol solutions and formalin, which often contains methanol as a stabilizer, must be stored in approved flammable storage cabinets. In the event of a spill, specialized neutralizers, such as urea-based compounds for formalin, should be used for cleanup, and all contaminated materials must be disposed of as hazardous chemical waste.