painless

形容词 painless:

1. requiring little hard work or exertion
2. not causing physical or psychological pain

1. An instrument that in a painless and noninvasive manner uses the differential transmission and reflection of ultrasonic waves to create a detailed visual image of the brain.
   超声波心动描记器一种用来显示详细的大脑图象的仪器，利用超声波发送和反射的差异来测试并无伤无痛

pain.less

adj.（形容词）
Free from complication or pain:
无痛的，无并发症的：
例句：
a painless operation.
无痛的手术

【引伸】
pain.lessly
adv.（副词）
pain.lessness
n.（名词）