NOTE THAT YOU CAN NOT USE THIS AS A PACKAGE VIA LIBRARY() OR REQUIRE() AS OF YET, BUT WE ARE WORKING ON GETTING THERE. 

Another option to install is install_github within Hadley's devtools package. 

This set of functions/package will access full text articles from the Public Library of Science journals using their API. 

Get your PLoS API key here: 
http://api.plos.org/

Put your API key in your .Rprofile file using e.g., 
options(PlosApiKey = "YOURPLOSAPIKEY"), 
and the functions within this package will be able to use your API key without you having to enter it every time you run a search. 

rplos tutorial at rOpenSci website here: http://ropensci.org/tutorials/rplos-tutorial/