Question: The default icon for toggling a `mat-expansion-panel` is `>`. Setting `hideToggle` true just hides the toggle icon. Is there any way to change it? I found nothing in the official documentation. I want to use `+` or `-` icon if the state is closed or opened respectively.

Answer: 
There is a class `.mat-expanded` which is applied to a `<mat-expansion-panel>`

So you can just **use CSS to change the icon**, no JS required.

* To change arrow down to up:

```css
.mat-expanded .mat-expansion-panel-header-title .material-icons{
  transform: rotate(180deg);
}

```

```
    <mat-panel-title>
      <span class="material-icons">expand_more</span>
      Panel with icon `transform`
    </mat-panel-title>

```

* To swap icons:

```css
.mat-expansion-panel-header-title .open,
.mat-expanded .mat-expansion-panel-header-title .close{
  display: inline-block;
}
.mat-expanded .mat-expansion-panel-header-title .open,
.mat-expansion-panel-header-title .close{
  display: none;
}

```

```
    <mat-panel-title>
      <span class="material-icons open">open_in_full</span>
      <span class="material-icons close">close_fullscreen</span>
      Panel with open/close
    </mat-panel-title>

```

Here is a link to a demo [https://stackblitz.com/edit/angular-e2fnlm](https://stackblitz.com/edit/angular-e2fnlm?file=src%2Fapp%2Fexpansion-overview-example.html)

CSS solution works with **multiple Material Expansion Panels**
For those interested, here is an example that works with multiple Expansion Panels:

**View:** 

```
<mat-accordion>
  <mat-expansion-panel *ngFor="let panel of panels; let i = index"
      (opened)="panelOpenState[i] = true" (closed)="panelOpenState[i] = false"
      hideToggle>
    <mat-expansion-panel-header>
      <mat-panel-title>
        {{panel.title}}
      </mat-panel-title>
      <span *ngIf="!panelOpenState[i]">Show</span>
      <span *ngIf="panelOpenState[i]">Hide</span>
    </mat-expansion-panel-header>

    Panel Content
  </mat-expansion-panel>
</mat-accordion>

```

**Component:**

```
export class PanelComponent implements OnInit {
  panels: [];
  panelOpenState: boolean[] = [];

  ngOnInit() {
    // loop to add panels
    for (let i = 0; i < 5; i++) {
      this.panels.push({ title: i });
      this.panelOpenState.push(false);
    }
  }
}

```