What is a Dendrimer

Dendrimers are a special class of polymers. (See http://www.helium.com/items/833616-what-is-a-polymer if you need a refresher on polymers.)
The typical polymer is a chain where each monomer unit connects with another monomer unit in sequence. When the monomers can only form bonds to each other at two sites apiece, the familiar chain structure is the result.
A dendrimer is a branched structure that results when the monomers are able to bond at three or more positions.

You can quickly illustrate the difference between two- and three-bond monomers. On paper, draw a dash. At each end of the dash, draw one more dash at an angle, with the ends connected.
Repeat, drawing new dashes only at ends which are not already connected. You should observe a zigzagging chain before long. This is basically how a traditional chain polymer works. Now try a variation by drawing a vee (v) on your paper. The rules are slightly different now. At each of the two ends (the “top”) of the vee, draw another vee, with the bottom point of the new vee attached to the end of the old vee. Repeat this process at any unoccupied ends. Very soon, your structure has expanded (exponentially) into a massive, branched structure. This is the basic idea behind a dendrimer. Dendrimers are branched polymers.

You probably noticed that the number of branches increases quickly with each cycle of growth. In the example we used, there are three bonding sites, and each level doubles the number of branches. In an arrangement with even more bonding positions available, each level can triple, quadruple, or more – up to a point. The exponential growth looks good on paper, but the reality of space is that there is never enough space, neither in the closet nor for a dendrimer. Because the branches increase so quickly, they crowd together, limiting the number of monomers that can attach to form the next level. This limit varies between dendrimers, and is dependent on the size (bulkiness) of the monomer as well as the number of branches it is capable of forming.

Dendrimers have not found too many applications as yet. Those which have been found are mainly dependant on one important fact: with all that branching, there is a lot of open space inside the three-dimensional structure of a dendrimer. You can envision a dendrimer as something akin to a sponge in this sense. The dendrimer can then carry anything that can fit into the empty spaces. The outermost ends of a dendrimer can be designed to have any desired chemical group, which then dictates the behavior of the dendrimer in its surroundings.

A dendrimer can be used to transport chemicals into the body which would otherwise go unabsorbed. For instance, a drug which fits into the spaces inside the dendrimer will go anywhere the dendrimer can go. If the dendrimer has hydrophilic ends, it will dissolve in water (most of your body, remember) taking the drugs inside along with it. This is useful if a drug is insoluble in water. Similarly, if you need to get a drug into a fatty area, and the drug won’t go on its own, a dendrimer which is fat-soluble can be used to transport it in.
Similarly, dendrimers can bypass cell walls and other obstacles, so long as they are designed properly.

In the same vein, the sponge-like property of dendrimers can be used to pick things up. For example, if toxic metals are found in water, a dendrimer which has spaces that attract the metal ions can “suck up” the metals.
They are then easily removed by filtration, since the dendrimers are much larger than the individual metal ions.

Other applications exist as well, capitalizing on either the porous nature of dendrimers, or on their very large surface area.

For more detailed reading, I recommend this source, which contributed to this material:

http://www.actabp.pl/pdf/1_2001/199-208s.pdf (“Dendrimers: properties and applications” by Klajnert and Bryszewska)