of aluminum sandwiched between two layers of PEX. The metal layer serves as an oxygen barrier, stopping the oxygen diffusion through the polymer matrix, so it cannot dissolve into the water in the tube and corrode the metal components of the system. The aluminium layer is thin, typically 1 or 2 mm, and provides some rigidity to the tube such that when bent it retains the shape formed (normal PEX tube will spring back to straight). The aluminium layer also provides additional structural rigidity such that the tube will be suitable for higher safe operating temperatures and pressures. The use of AluPex tubing has grown greatly since 2010 onwards. It is easy to work and position. Curves may be easily formed by hand. Tube exists for use with both hot and cold water and also for gas. This product in Canada has been discontinued due to water infiltrating between the Layers resulting in premature failures. PEX tools There are two types of fitting that may be used. Crimped or compressive. Crimped connectors are less expensive but require a specialised crimping tool. Compression fittings are tightened with normal spanners and are designed to allow sections of the system to be easily disassembled, they are also popular for small works, esp. DIY, avoiding the need for extra tools. A PEX tool kit includes a number of basic tools required for making fittings and connections with PEX tubing. In most cases, such kits are either bought at a local hardware store, plumbing supply store or assembled by either a home owner or a contractor. PEX tools kits range from under $100 and can go up to $300+. A typical PEX tool kit includes crimp tools, an expander tool for joining, clamp tools, PEX cutters, rings, boards, and staplers. Other uses Artificial joints: Highly cross-linked polyethylene is used in artificial joints as a wear-resistant material. Cross-linked polyethylene is preferred in hip replacement because of its resistance to abrasive wear. Knee replacement, however, requires PE made with different parameters because cross-linking may affect mechanical strength and there is greater stress-concentration in knee joints due to lower geometric congruency of the bearing surfaces. Manufacturers start with ultra high molecular weight polyethylene, and crosslink with either electron beam or gamma irradiation. Dental applications: Some application of PEX has also been seen in dental restoration as a composite filling material. Watercraft: PEX is also used in many canoes