Angular 4,

I have a checkbox in HTML as below and want to have two-way binding with an variable in ts:

...
        </md-grid-tile>
        <md-grid-tile class="grid-box-row"> 
            <md-checkbox ng-model="item.exportable">
            </md-checkbox>
        </md-grid-tile>        
    </div>
    </ng-container>
</md-grid-list>

item.exportable is a boolean defined in ts:

exportable: boolean;

it is shown like this:

it's not checked even if the value is true. And click on the checkbox on UI doesn't affect the value displayed after the checkbox. Pleaes help. Thanks.