mdb-ui-kit/scss/_radios.scss

.radio label,
label.radio-inline {
  position: relative;
  padding-left: $mdb-radio-size + $mdb-radio-label-padding; // absolutely positioned so add the radio size

  span {
    position: absolute;
    top: calc-top($line-height, $font-size-base, $mdb-radio-size); // vertical center of line-height
    left: 0;
    display: inline-block;
    width: $mdb-radio-size;
    height: $mdb-radio-size;
    transition-duration: 0.2s;

    &.mdb-radio-outer-circle {
      border: $mdb-radio-border solid $mdb-radio-color-off;
      border-radius: 50%;
      transition: border-color ease .28s;
    }
    &.mdb-radio-inner-circle {
      display: inline-block;
      background-color: $mdb-radio-color-on;
      border-radius: 50%;
      transition: transform ease .28s;
      transform: scale3d(0, 0, 0);

      // focus/press ripple
      &::after {
        position: absolute;
        top: -#{$mdb-radio-size};
        left: -#{$mdb-radio-size};
        z-index: 1;
        display: block;
        width: 50px;
        height: 50px;
        margin: 0;
        content: "";
        background-color: $gray-dark;
        border-radius: 100%;
        opacity: 0;
        transform: scale3d(1.5, 1.5, 1);
      }
    }
  }

  input[type=radio] {
    width: 0;
    height: 0;
    overflow: hidden;
    opacity: 0;

    &:checked {
      @include mdb-radio-color($mdb-radio-color-on);

      ~ .mdb-radio-inner-circle {
        transform: scale3d(0.55, 0.55, 1);
        &::after {
          animation: rippleOn 500ms;
        }
      }
    }

    &:not(:checked) ~ .mdb-radio-inner-circle::after {
      animation: rippleOff 500ms;
    }

    &[disabled],
    fieldset[disabled] & {
      @include mdb-radio-color($mdb-radio-color-disabled);

      .theme-dark & {
        @include mdb-radio-color($mdb-radio-color-disabled-dark);
      }
    }
  }
}

@keyframes rippleOn {
  0% {
    opacity: 0;
  }
  50% {
    opacity: 0.2;
  }
  100% {
    opacity: 0;
  }
}

@keyframes rippleOff {
  0% {
    opacity: 0;
  }
  50% {
    opacity: 0.2;
  }
  100% {
    opacity: 0;
  }
}