Frameless Levonorgestrel IUD - FibroPlant™-LNG releases 14 μg of LNG per day, produced by Contrel, a Belgian company is still in the testing and is not yet commercially available but considering how ridiculously fast Skyla was FDA approved despite the class action lawsuit against Mirena, I suspect that it will be on the market soon. (she says bitterly, lamenting that more copper IUDs haven't been FDA approved).
Studies involving the FibroPlant LNG -
Assessment of menstrual blood loss in women with ideopathic menorrhagia using the frameless levonorgestrel-releasing intrauterine system
Menstrual blood loss in women using the frameless FibroPlant® LNG-IUS
New intrauterine technologies for contraception and treatment in nulliparous/adolescent and parous women
I would personally never use an LNG IUD but I know many here would be interested in a novel LNG IUD.