Some backpacks are specifically designed to carry certain items. Common examples include backpacks for small valuable items such as laptops and cameras; backpacks designed to hold laptop computers in particular generally have a padded compartment to hold the computer and medium-sized pockets and flaps to accommodate accessories such as charger cables and mice. These are especially common in college and university settings. In order to supply these devices with electricity, a few high-end backpacks are equipped with solar panels.
There are also single-strap packs that are essentially a hybrid between a backpack and a messenger bag.
Inexpensive, very simple packs that combine the drawstring and straps into a single piece of cloth or webbing are occasionally sold for use at sporting events and the like. Some high-end retailers use a similar design for their shopping bags as well.
Rolling backpacks are backpacks with wheels on the bottom and an extending handle. Because of their design, rolling backpacks reduce the strain on the user, though the shoulder straps may be used to carry the pack for short distances when the terrain is not suitable for wheels. Rolling backpacks are most commonly used while traveling by airplane or train.
Hydration backpacks are also available. These light daypacks are especially designed to hold water in a special water bladder (also known as reservoir), and their purpose is to allow the carrier constant fluid hydration handsfree, so that the carrier can focus on the mission ahead without having to stop for water breaks. This is possible because of the tube that leads from the bladder (which sits within the backpack), to the front strap of the bag, which is long enough to reach the mouth of the carrier. These packs are used in extreme conditions such as military operations, during performing extreme sports and also for different competitions, races (such as Spartan or marathon) and even music festivals.