Finding an air bubble in an IV line may be reason to panic. The introduction of air emboli in the blood stream can be fatal. But patients need not worry because in a growing number of IV sets, filters are there to keep air and foreign particles from entering the patient's bloodstream.

In its basic form, filtration is the separation of particles of one or more materials from a liquid or gas stream. In medical-device applications it usually involves one of the following:

• removing particles or microbial contaminants from liquids or gases
• separating gases from liquids
• removing liquids from air

Filtration can be divided into depth filters and surface filters. Depth filtration traps particles within the depth of the filter and at its surface. Depth filters are commonly used in gas and vacuum filtration to protect devices and environments from contamination, venting, hydrophobic barriers and smoke evacuation during surgery. Surface, or membrane filtration, primarily traps contaminants on the filter surface. The two are occasionally used in tandem, with a depth filter collecting larger particles and the surface filter capturing smaller particles. Surface filters are used in many different fluid-transfer applications, such as dispensing pins for the withdrawal or injection of medication from vials. Other filters provide pressure equalization and gas elimination in medical devices. This helps liquids flow into or out of a device, or regulate pressure within a device.

Depth filters

Depth filters trap particulate both within the random matrix of fibers found within the thickness of the media and on the surface. Depth filtration generally tolerates high flow rates and loading capacities. It predates surface-filtration techniques and is used in a wide array of medical, life science, industrial, and environmental monitoring applications. Common applications include cell harvesting, venting, wicking, pre-filtration to extend membrane filter life, bacterial-culture filtration, liquid scintillation counting, sample preparation, and clarification of protein solutions. “One of the most widely used applications for glass-microfiber depth filters is the prefiltration for membrane filtration,” says Jim Averso, General Manager, I.W. Tremont, (iwtremont.com), Hawthorne, N.J. “This prefiltration significantly extends membrane filter life and increases flow rates.”

The fibrous material within the company's filters is commonly glass or quartz microfiber, cellulose, and polypropylene, among others. Depth filters are categorized as either binder-free or containing binders. Binder-free, or binderless filters contain no additional strengthening agents commonly referred to as binders. Binders are materials such as acrylic resin that strengthen the matrix and hold fibers in place. Although added binders or compounds may add strength, the media is no longer pure glass and may react to heat or be susceptible to chemical incompatibilities. In the absence of binders, borosilicate or quartz microfiber is used. These inert materials further eliminate sample contamination.

Averso says a common mistake when choosing a depth filter is “targeting a retention rate smaller than necessary. This adversely affects flow and loading capacities resulting in short filter life or slow flow rates.” He suggests answering five basic questions to help choose the correct depth filter:

1. What level of filtration efficiency is required?

2. What is the target particle retention?

3. What level of loading is required?

4. What is the volume to be filtered?

5. Will there be any chemical or ambient incompatibilities with the media?

Depth filters demonstrate significantly higher flow rates and load capacities when compared to membrane filters. The combination of fast filtration and loading capacity makes depth filtration useful for large-volume applications. An obvious disadvantage to depth filtration for some applications is a particle-retention tolerance that results from the random positioning of fibers.

Surface filters

Surface, or membrane filtration, primarily traps particles on a membrane surface. Contaminants sufficiently smaller than the pores in the material pass through while larger particles are trapped. Membranes are generally used in medical devices that need venting and filtration.

Venting separates two environments, usually a moist or liquid environment from a gas environment. Venting membranes are used in disposable, single-use devices such as ostomy bags and IV infusion filters, where they don't actually filter the liquid in the device. They keep the liquid on one side of a membrane, while letting gases escape to the external environment on the other side of the membrane. “Expanded PTFE works well in this application by letting the two environments ‘communicate' and equalize pressure, while reliably retaining the liquid inside the device,” says Bryce Hartmann, Product Specialist, Gore Medical Membranes, W.L.Gore & Associates Inc., (gore.com), Elkton, Md. “For instance, if there is an increase of pressure on one side of the membrane, the membrane allows the pressures to equalize, while maintaining sterility inside the device.” Potential particles, bacteria, or viruses that may be in either the gas side or the liquid side are not allowed to cross the membrane. “However, it is important to select the proper membrane to get the right level of filtration and protection for the application,” says Hartmann.

Hartmann suggests focusing on the desired end result when choosing a membrane. The most common mistake he sees is designers who fixate on pore size. “We have several membranes with the same reference pore size but significantly different performance characteristics. A lot of customers say ‘I need a 0.2 micron membrane.' And really that's not what's important. What's important is what the membrane does, and more importantly, what the device and application need it to do. So start by determining required microbial barrier properties, liquid resistance, and airflow for the device. Then we'll select a membrane, which is often not the one they initially thought it would be.”

Hartmann suggests answering the following initial questions when selecting a membrane filter for a venting application:

1. What is the device and its application?

2. What does the device need the membrane to do?

3. What gases and liquids will be in contact with the membrane?

4. What are the maximum pressures of the liquids?

5. How long does the device have to work?

6. What is the device housing going to be made of? What plastic, film?

7. How will the membrane be incorporated or sealed into the housing?

8. What is the flow rate, if known?

9. Will the device be sterilized, and if so what is the mode of sterilization?

Some devices are sold clean and do not necessarily have to be sterile, but in most cases the device will be sterilized. Membranes work in all kinds of sterilization modes, but they depend on the application. The limiting factor may not be the membrane material, it's often the device's design or its manufacturing process. For instance, radiation sterilization modes such as gamma or e-beam have been used to sterilize devices with expanded PTFE membranes.

Gore recently introduced a combination depth and surface filter. The filter, for surgical smoke filtration, combines a microfiber-glass prefilter with an expanded PTFE membrane. “The microfiberglass prefilter primarily captures the smoke particles, while the expanded PTFE membrane provides a liquid and microbial barrier,” says Hartmann.

Hydrophilic vs hydrophobic

Both depth and surface filtration use hydrophilic and hydrophobic filters. Hydrophilic filters have an affinity for water. They can be wetted with almost any liquid and are often used with aqueous solutions. B. Braun OEM uses hydrophilic filters for IV delivery applications. “The inline filters remove any precipitant that could have formed in the solution container,” says Joel Bartholomew, Manager of OEM and R&D, B. Braun OEM, (www.bbraunoem.com), Bethlehem, Pa. “In addition, the added bonus of a hydrophilic filter is its ability to remove any air that might be in the line, which adds to greater patient safety.”

Hydrophobic filters on the other hand, will not wet in water, but will wet in low surface-tension liquids, such as organic solvents. After wetting, aqueous solutions pass through. These filters are best suited for gas filtration and venting applications. “In applications where we want to get air into a container, but don't want the water or solution that's in the container to exit, we use a hydrophobic filter on the air,” says Bartholomew.

Bio filter grabs particles from air

When it comes to cleaning air in medical schools, operating rooms, and labs, bio-hygienics technology competes with traditional filters and large-scale venting. Such devices from Air & Water Solutions, Clifton, N.J. (cleanairplant.com) sport three innovations, according to inventor Sam Sofer. “First, the spiral design of the water-washed element captures airborne particles as they bounce around the air passages. Air travels along the spiral so it has a low pressure drop. There is intimate gas-liquid contact and a particle in the air is pulled into a wall of bio film. The cartridge is continuously washed, so even smoke is removed.”

A second innovation is that immobilized enzymes in the element transform captured vapors such as formaldehyde, ammonia, Xylene, and phenols into carbon dioxide and water. “It does the same to viruses and pathogens,” says Sofer. And third, the smallest of the units runs on about 50 W and easily stands in a corner.

Most filters pass air through media in the hope it will trap airborne particles. “But some particles and vapor are on the order of 0.05 microns, so even HEPA filters let these pass. Also, odor-causing molecules and particles carry a charge. That means they are more influenced by electrical fields and less by the laws of flow mechanics,” says Sofer. Large particles that stir up like dust can be vented. But the ones that stay are small enough so they don't fall. And because they are electrically charged and stick to surfaces, they don't vent. Also their charge distributes them evenly throughout a room. “Nitrous oxide gas is like this, and a serious issue,” he adds. The electrical charge is why some lingering odors and pollution do not vent. The phenomena produces “sick building” syndrome.

Sofer's clean-air-plant units draw air into the middle of the unit and flings it out from a top duct, not straight up. “So air flow cleans walls, ceilings, and surfaces, creating a clean-air zone. And this zone, initially around the unit, eventually expands to fill a room.”

Also, by keeping the clean-air zone electrically grounded, the units “wash” the air with water that is grounded to the pump. “The air grounds particles, whether positively or negatively charged, and attracts them from far away. So even smaller units attract particles from 30 to 40 feet,” adds Sofer.

“There are more complicated molecules in hospital air, such as anesthetic gasses,” says Sofer. The bio-hygienic units digest those too, along with pesticides. He says pathogens and viruses are also destroyed by the bio film.

The University of Medicine and Dentistry of New Jersey uses several of Sofer's air cleaners. “They are in the gross anatomy lab where phenol is in the air, a nasty heavy and syrupy stuff. It's more like a solid,” says Sofer. He says his units also clean the lab air of formaldehyde, vapors, particulates, and ammonia.