how to use md-chips with ng-repeat (key, value)

I'm trying to use md-chips with ng-repeat when using (key, value) flavor here is an example of what I'm trying to do :

<md-content class="md-padding" layout="column" ng-repeat="(key,value) in items">
    <md-chips ng-model="???" name="fruitName" readonly="true" md-removable="" md-max-chips="5">
        <md-chip-template>
            <strong>{{key}} :{{value}}</strong>
        </md-chip-template>
    </md-chips>
</md-content>

(I don't know what to do in the ng-model).
thank you in advance
edit
here is my json data for example {'a':'1','b':'2','c':'3'}

<md-chips class="custom-chips" ng-model="ctrl.vegObjs" readonly="true">
        <md-chip-template>
            <span>
          <strong> {{$chip}} </strong>
        </span>
        </md-chip-template>
    </md-chips>

by using this code I can have {"a":"1"} {"b":"2"} {"c":"3"}, but it's not exactly what I want.

Try this

<md-chips ... ng-model="_key" ng-init="_key = [key]" ...></md-chips>

Working codepen