Can I use a TENS Unit to help induce lactation?
The original purpose of using the TENS Unit is to stimulate the nerves and muscles in your body to “hide” pain such as lower back pain, etc. In this case, it is used to stimulate the nerves of your breast, mostly nerves in your nipple area, which promote the release of prolactin by your Pituitary Gland (in your brain). That induces growth and development of acini (milk-producing) tissues in your breasts and leads to lactation. From a BreastNotes.com article.
The original purpose of using the TENS Unit is to stimulate the nerves and muscles in your body to “hide” pain such as lower back pain, etc. In this case, it is used to stimulate the nerves of your breast... mostly nerves in your nipple area, which promote the release of prolactin by your Pituitary Gland (in your brain). That induces growth and development of acini (milk-producing) tissues in your breasts and leads to lactation. 
To use the TENS unit when you are at work or shopping, the idea is for the pads to be attached to your breasts before you get dressed. They can be used inside your bra (if you wear one) or simply worn under your outer clothing. The control unit can slip into your bra between your breasts, hang from a cord around your neck, clip to your belt or slip into your pocket. It makes no vibrations or noise, and can be running while you are working at your desk, in a board meeting, or having lunch with your friends. You merely turn it on when you want it to function and turn it off when you are finished with it. 
The intent of using this unit is not to replace your partner’s oral expression or your manual expression of your breasts, or your pumping, but to substitute in their place when you are not able to take time away from work to suckle, express or pump. More than that, however, it can be used in between the scheduled suckling, expressing or pumping sessions, for about ten minutes every two hours. 
All the nerves in the breast, when stimulated, promote the release of Prolactin by the pituitary gland. These include the sensory nerves of the nipple and areola and the nerves to the ducts under the areola that sense the compression and suckling actions. In addition to these, the nerves throughout the breast, in the alveoli and skin will promote Prolactin release but not as intensely as the nipple and areola. What this means is that virtually any type of playing with the breast whether it be fondling, squeezing, rubbing, suckling etc. is stimulating to the breast. 
TENS units come in a great variety of sizes and configurations. You can purchase one from multiple sites on the Internet. Buy a simple, small one, a 2-channel unit. It will probably have 2 modes of operation, which is all you need. 
Some couples practice ABF in order to strengthen their relationship. Read for yourself a few ANR testimonials and stories or watch videos of couples practicing adult breastfeeding. (warning:NSFW)
Reach out to others by using our ANR/ABF online communities list page to seek others who are in this lifestyle.
Frequently Asked Questions
About Inducing Lactation
Can I use a TENS Unit to help induce lactation? (warning:NSFW)
Are there health concerns from breastfeeding or letting someone drink my milk?
It's perfectly fine to try another person's breast milk if you know their health history. Breast milk is a body fluid and therefore it's possible to transmit pathogens. The next biggest concern is Mastitis (warning:NSFW) which can be lessened with Lecithin and a proper latch technique (warning:NSFW). The Piercings and Lactation page (warning:NSFW) addresses this concern as well.
- BreastNotes: How Do I Use a TENS Unit To Stimulate Lactation? http://breastnotes.com/breastfeeding/BrFd-ReLac-TENS.htm
- Gift of Milk: Using a TENS unit to Stimulate Lactation http://giftofmilk.com/drjim/using-a-tens-unit-to-stimulate-lactation